BCH 304
METABOLISM OF AMINO ACIDS & PROTEINS
At the end of the course, students should be able to illustrate why and how proteins are broken in cellular systems; explain molecular-weight determination of proteins; recognise relationships between the urea cycle and other pathways of protein metabolism; distinguish ketogenic from glucogenic amino acids; and identify the role of inorganic nitrogen in protein synthesis and breakdown.
Amino acid; zwitterion; peptide bond; protein structure; proteolysis; ubiquitin; proteasome; chromatography; electrophoresis; Edman degradation; molecular mass; transamination; oxidative deamination; glutamate; glutamine; ammonia; urea cycle; glucogenic; ketogenic; tetrahydrofolate; one-carbon unit; nitrogen fixation; polyamine.
Amino Acids as the Building Blocks of Proteins
1.1 General structure
An alpha-amino acid contains an amino group, a carboxyl group, a hydrogen atom and a distinctive side chain attached to the alpha carbon. At physiological pH most free amino acids exist predominantly as zwitterions, carrying both positive and negative charges.
1.2 Side-chain chemistry
The side chain determines polarity, charge, hydrophobicity, chemical reactivity and the interactions an amino-acid residue can form inside a protein. Non-polar residues tend to favour hydrophobic environments, while polar and charged residues often participate in hydrogen bonding, ionic interactions and catalysis.
1.3 Acid-base behaviour
Amino and carboxyl groups can donate or accept protons. Ionisable side chains add further acid-base complexity. The net charge of an amino acid therefore changes with pH.
1.4 Isoelectric point
The isoelectric point is the pH at which a molecule has zero net charge. This property influences solubility and electrophoretic behaviour and is exploited in protein separation.
When the pH is below an amino acid or protein's isoelectric point, protonation tends to make its net charge more positive. Above the isoelectric point, deprotonation tends to make net charge more negative. At the isoelectric point the average net charge is zero, often reducing electrostatic repulsion and sometimes decreasing solubility.
1.5 Essentiality
Essential amino acids cannot be synthesised in sufficient amounts to meet physiological needs and must be obtained from the diet. Non-essential amino acids can be synthesised from metabolic intermediates, although nutritional and physiological circumstances can alter requirements.
1.6 Metabolic significance
Amino acids are not stored in a dedicated polymeric reserve comparable with glycogen or triacylglycerol. Excess amino acids are therefore used for synthesis of other nitrogenous compounds or degraded, with nitrogen disposed of safely and carbon skeletons entering central metabolism.
At 300 level, amino acids should be understood simultaneously as chemical species, protein residues and metabolic substrates. Their ionisation controls analytical behaviour; their side chains control protein folding and catalysis; and their carbon and nitrogen atoms connect protein turnover with central metabolism. This three-level view prevents the subject from becoming a list of twenty structures.
Chapter Summary
- General structure: An alpha-amino acid contains an amino group, a carboxyl group, a hydrogen atom and a distinctive side chain attached to the alpha carbon.
- Side-chain chemistry: The side chain determines polarity, charge, hydrophobicity, chemical reactivity and the interactions an amino-acid residue can form inside a protein.
- Acid-base behaviour: Amino and carboxyl groups can donate or accept protons.
- Isoelectric point: The isoelectric point is the pH at which a molecule has zero net charge.
- Essentiality: Essential amino acids cannot be synthesised in sufficient amounts to meet physiological needs and must be obtained from the diet.
- Metabolic significance: Amino acids are not stored in a dedicated polymeric reserve comparable with glycogen or triacylglycerol.
Review and Examination Questions
- Explain the major biochemical principles involved in amino acids as the building blocks of proteins.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on amino acids as the building blocks of proteins.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Peptide Bond and Levels of Protein Structure
2.1 Peptide bond
The peptide bond links the alpha-carboxyl group of one amino acid residue to the alpha-amino group of another. Resonance gives the C–N bond partial double-bond character, restricting rotation and making the peptide group relatively planar.
2.2 Primary structure
Primary structure is the linear amino-acid sequence of a polypeptide. Sequence determines the chemical possibilities for folding, modification, binding and catalysis.
2.3 Secondary structure
Alpha helices, beta sheets and turns arise mainly from regular hydrogen bonding involving peptide backbone groups. Side-chain properties influence which conformations are favoured.
2.4 Tertiary structure
Tertiary structure describes the overall three-dimensional fold of a single polypeptide chain, including packing of secondary structural elements and long-range side-chain interactions.
2.5 Quaternary structure
Proteins containing multiple polypeptide subunits possess quaternary structure. Subunit association can create cooperativity, regulation and functional specialisation.
2.6 Structure-function relationship
Protein function depends on a correctly organised three-dimensional structure. Sequence changes, chemical modification, pH, temperature and solvent conditions can therefore alter function by changing conformation.
The peptide backbone has directionality from the amino terminus to the carboxyl terminus. Protein sequences are conventionally written in that direction, and ribosomes synthesize chains by adding residues to the carboxyl end. Because the peptide bond is planar, most conformational freedom comes from rotation around adjacent backbone bonds rather than free rotation through the peptide C–N bond.
Chapter Summary
- Peptide bond: The peptide bond links the alpha-carboxyl group of one amino acid residue to the alpha-amino group of another.
- Primary structure: Primary structure is the linear amino-acid sequence of a polypeptide.
- Secondary structure: Alpha helices, beta sheets and turns arise mainly from regular hydrogen bonding involving peptide backbone groups.
- Tertiary structure: Tertiary structure describes the overall three-dimensional fold of a single polypeptide chain, including packing of secondary structural elements and long-range side-chain interactions.
- Quaternary structure: Proteins containing multiple polypeptide subunits possess quaternary structure.
- Structure-function relationship: Protein function depends on a correctly organised three-dimensional structure.
Review and Examination Questions
- Explain the major biochemical principles involved in peptide bond and levels of protein structure.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on peptide bond and levels of protein structure.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Forces Stabilising Protein Structure
3.1 Hydrophobic effect
Non-polar side chains tend to become buried away from water during folding. The hydrophobic effect is a major driving force in formation of compact globular structures.
3.2 Hydrogen bonds
Hydrogen bonds stabilise backbone secondary structures and many side-chain interactions. Their contribution depends on geometry and the surrounding environment.
3.3 Ionic interactions
Oppositely charged side chains can form salt bridges. Their strength depends strongly on solvent exposure, pH and local dielectric environment.
3.4 Van der Waals forces
Close packing permits numerous weak van der Waals interactions. Individually small, their collective contribution can be substantial.
3.5 Disulfide bonds
Oxidation of two cysteine side chains can form a covalent disulfide bond. Such bonds are particularly important in many extracellular proteins.
3.6 Denaturation
Heat, extreme pH, organic solvents, detergents or chaotropic agents can disrupt non-covalent interactions and unfold proteins. Denaturation generally leaves peptide bonds intact but destroys higher-order structure.
3.7 Renaturation
Some proteins can regain native structure when denaturing conditions are removed, demonstrating that sequence contains essential information for folding; however, many proteins require cellular chaperones or cannot fully refold after aggregation.
Protein stability is the net result of many favourable and unfavourable interactions rather than one dominant covalent scaffold. Hydrophobic burial, hydrogen bonds, electrostatic interactions and close packing work together. A mutation can destabilise a protein even when it does not remove a catalytic residue because changing packing or local charge can shift the folding equilibrium.
Chapter Summary
- Hydrophobic effect: Non-polar side chains tend to become buried away from water during folding.
- Hydrogen bonds: Hydrogen bonds stabilise backbone secondary structures and many side-chain interactions.
- Ionic interactions: Oppositely charged side chains can form salt bridges.
- Van der Waals forces: Close packing permits numerous weak van der Waals interactions.
- Disulfide bonds: Oxidation of two cysteine side chains can form a covalent disulfide bond.
- Denaturation: Heat, extreme pH, organic solvents, detergents or chaotropic agents can disrupt non-covalent interactions and unfold proteins.
- Renaturation: Some proteins can regain native structure when denaturing conditions are removed, demonstrating that sequence contains essential information for folding; however, many proteins require cellular chaperones or cannot fully refold after aggregation.
Review and Examination Questions
- Explain the major biochemical principles involved in forces stabilising protein structure.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on forces stabilising protein structure.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Protein Turnover and Cellular Protein Breakdown
4.1 Dynamic proteome
Cellular proteins have finite lifetimes. Continuous synthesis and degradation permit removal of damaged proteins and rapid adjustment of enzyme, receptor and regulatory-protein abundance.
4.2 Ubiquitin
In eukaryotic cells, many proteins destined for proteasomal degradation are tagged with ubiquitin through an ATP-dependent enzyme cascade.
4.3 Proteasome
The proteasome recognises many polyubiquitinated substrates, unfolds them and degrades them into peptides within a proteolytic core.
4.4 Lysosomal degradation
Lysosomes degrade extracellular material taken up by endocytosis and intracellular components delivered by autophagic pathways.
4.5 Autophagy
Autophagy encloses portions of cytoplasm or organelles for lysosomal degradation. It contributes to nutrient recycling, organelle quality control and adaptation to starvation.
4.6 Protease specificity
Proteolysis is controlled by enzyme localisation, substrate recognition and protease inhibitors. Unregulated proteolysis would destroy essential cellular proteins.
4.7 Amino-acid recycling
Products of proteolysis re-enter the intracellular amino-acid pool and can be reused for protein synthesis or directed toward catabolism.
Protein breakdown is selective and biologically purposeful. Short-lived regulatory proteins may be degraded rapidly to switch signals off, while damaged or misfolded proteins are removed for quality control. During starvation, proteolysis also supplies amino acids. Thus turnover is simultaneously a regulatory, protective and nutritional process.
Chapter Summary
- Dynamic proteome: Cellular proteins have finite lifetimes.
- Ubiquitin: In eukaryotic cells, many proteins destined for proteasomal degradation are tagged with ubiquitin through an ATP-dependent enzyme cascade.
- Proteasome: The proteasome recognises many polyubiquitinated substrates, unfolds them and degrades them into peptides within a proteolytic core.
- Lysosomal degradation: Lysosomes degrade extracellular material taken up by endocytosis and intracellular components delivered by autophagic pathways.
- Autophagy: Autophagy encloses portions of cytoplasm or organelles for lysosomal degradation.
- Protease specificity: Proteolysis is controlled by enzyme localisation, substrate recognition and protease inhibitors.
- Amino-acid recycling: Products of proteolysis re-enter the intracellular amino-acid pool and can be reused for protein synthesis or directed toward catabolism.
Review and Examination Questions
- Explain the major biochemical principles involved in protein turnover and cellular protein breakdown.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on protein turnover and cellular protein breakdown.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Protein Isolation and Fractionation
5.1 Extraction
Protein isolation begins by disrupting cells or tissues under conditions that preserve the target protein. Buffers control pH and ionic strength, while low temperature and protease inhibitors can reduce unwanted degradation.
5.2 Differential solubility
Proteins differ in solubility as ionic strength, pH and solvent conditions change. These differences permit fractionation.
5.3 Salting out
High concentrations of salts such as ammonium sulfate compete for water and can reduce protein solubility. Different proteins precipitate over different saturation ranges.
5.4 Isoelectric precipitation
A protein is often least soluble near its isoelectric point because net electrostatic repulsion is reduced. Carefully adjusting pH can therefore aid fractionation.
5.5 Dialysis
A semipermeable membrane can retain proteins while small solutes diffuse away, making dialysis useful for removing salts after precipitation.
5.6 Centrifugation
Centrifugation separates material according to sedimentation behaviour and is commonly used to clarify extracts or isolate cellular fractions before protein purification.
5.7 Recovery
Every purification step trades purity against yield. A successful scheme therefore monitors both the amount of target protein recovered and removal of contaminants.
Purification begins with a biological question: which measurable property identifies the desired protein? Without an activity assay, antibody or other marker, increasing total purity does not prove that the correct protein has been isolated. Each fractionation step should therefore be accompanied by quantitative tracking of the target.
Chapter Summary
- Extraction: Protein isolation begins by disrupting cells or tissues under conditions that preserve the target protein.
- Differential solubility: Proteins differ in solubility as ionic strength, pH and solvent conditions change.
- Salting out: High concentrations of salts such as ammonium sulfate compete for water and can reduce protein solubility.
- Isoelectric precipitation: A protein is often least soluble near its isoelectric point because net electrostatic repulsion is reduced.
- Dialysis: A semipermeable membrane can retain proteins while small solutes diffuse away, making dialysis useful for removing salts after precipitation.
- Centrifugation: Centrifugation separates material according to sedimentation behaviour and is commonly used to clarify extracts or isolate cellular fractions before protein purification.
- Recovery: Every purification step trades purity against yield.
Review and Examination Questions
- Explain the major biochemical principles involved in protein isolation and fractionation.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on protein isolation and fractionation.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Chromatographic Purification of Proteins
6.1 Ion-exchange chromatography
Ion-exchange matrices carry charged groups and separate proteins according to net surface charge. Bound proteins are commonly eluted by changing salt concentration or pH.
6.2 Size-exclusion chromatography
Porous beads separate proteins according to hydrodynamic size. Large molecules are excluded from more pores and generally elute earlier than smaller molecules.
6.3 Affinity chromatography
An immobilised ligand selectively binds the target protein or a protein class. High selectivity can provide large purification in a single step.
6.4 Hydrophobic interaction chromatography
Proteins interact with mildly hydrophobic matrix groups, often under high-salt conditions, and are separated according to exposed hydrophobic surfaces.
6.5 HPLC
High-performance liquid chromatography uses controlled high-pressure flow and efficient stationary phases to improve speed and resolution.
6.6 Purification table
A purification scheme is evaluated by total protein, total activity, specific activity, percentage yield and purification fold.
Question: A crude extract contains 12,000 enzyme units in 600 mg protein. After chromatography, 7,200 units remain in 60 mg protein. Calculate specific activities, purification fold and yield.
Formula: Specific activity = total activity ÷ total protein.
Solution: Initial = 12,000/600 = 20 U mg−1. Final = 7,200/60 = 120 U mg−1. Purification fold = 120/20 = 6. Yield = (7,200/12,000)×100 = 60%.
Answer: 6-fold purification with 60% activity yield.
6.7 Method selection
The best sequence of methods exploits independent protein properties while minimising irreversible loss of activity.
Chromatography works because proteins differ in physical properties. Ion exchange asks about charge, gel filtration about hydrodynamic size, affinity methods about selective recognition and hydrophobic interaction about exposed non-polar surfaces. Combining orthogonal properties usually produces better purification than repeatedly exploiting the same property.
Chapter Summary
- Ion-exchange chromatography: Ion-exchange matrices carry charged groups and separate proteins according to net surface charge.
- Size-exclusion chromatography: Porous beads separate proteins according to hydrodynamic size.
- Affinity chromatography: An immobilised ligand selectively binds the target protein or a protein class.
- Hydrophobic interaction chromatography: Proteins interact with mildly hydrophobic matrix groups, often under high-salt conditions, and are separated according to exposed hydrophobic surfaces.
- HPLC: High-performance liquid chromatography uses controlled high-pressure flow and efficient stationary phases to improve speed and resolution.
- Purification table: A purification scheme is evaluated by total protein, total activity, specific activity, percentage yield and purification fold.
- Method selection: The best sequence of methods exploits independent protein properties while minimising irreversible loss of activity.
Review and Examination Questions
- Explain the major biochemical principles involved in chromatographic purification of proteins.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on chromatographic purification of proteins.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Protein Characterisation and Electrophoretic Techniques
7.1 Native PAGE
Native electrophoresis separates proteins while preserving much of their native charge and conformation. Migration depends on charge, size and shape.
7.2 SDS-PAGE
Sodium dodecyl sulfate unfolds many proteins and gives polypeptides a broadly similar charge-to-mass relationship, making migration primarily dependent on molecular size.
| Method | Main separation basis | Typical information |
|---|---|---|
| Native PAGE | Charge, size and shape | Native mobility/complex state |
| SDS-PAGE | Primarily polypeptide size | Approximate subunit molecular mass |
| Isoelectric focusing | Isoelectric point | Protein pI/charge variants |
| 2-D electrophoresis | pI then size | Complex protein-mixture resolution |
7.3 Reducing conditions
Reducing agents such as beta-mercaptoethanol or dithiothreitol break disulfide bonds, allowing disulfide-linked polypeptide chains to separate.
7.4 Isoelectric focusing
Proteins migrate through a pH gradient until they reach the pH corresponding to their isoelectric point.
7.5 Two-dimensional electrophoresis
Isoelectric focusing can be followed by SDS-PAGE, separating complex protein mixtures first by isoelectric point and then by molecular mass.
7.6 Western blotting
Proteins separated by electrophoresis can be transferred to a membrane and detected using specific antibodies.
7.7 Activity assays
For enzymes, catalytic activity is a crucial characterisation parameter because a highly purified but inactive preparation may be biologically useless.
Electrophoresis illustrates the importance of experimental conditions. A native protein may migrate according to charge, size and shape, while SDS largely removes native shape and normalises charge. Reducing SDS-PAGE adds another variable by breaking disulfide bonds. The band pattern must therefore always be interpreted in light of sample treatment.
Chapter Summary
- Native PAGE: Native electrophoresis separates proteins while preserving much of their native charge and conformation.
- SDS-PAGE: Sodium dodecyl sulfate unfolds many proteins and gives polypeptides a broadly similar charge-to-mass relationship, making migration primarily dependent on molecular size.
- Reducing conditions: Reducing agents such as beta-mercaptoethanol or dithiothreitol break disulfide bonds, allowing disulfide-linked polypeptide chains to separate.
- Isoelectric focusing: Proteins migrate through a pH gradient until they reach the pH corresponding to their isoelectric point.
- Two-dimensional electrophoresis: Isoelectric focusing can be followed by SDS-PAGE, separating complex protein mixtures first by isoelectric point and then by molecular mass.
- Western blotting: Proteins separated by electrophoresis can be transferred to a membrane and detected using specific antibodies.
- Activity assays: For enzymes, catalytic activity is a crucial characterisation parameter because a highly purified but inactive preparation may be biologically useless.
Review and Examination Questions
- Explain the major biochemical principles involved in protein characterisation and electrophoretic techniques.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on protein characterisation and electrophoretic techniques.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Amino-Acid Analysis of Peptides and Proteins
8.1 Hydrolysis
Amino-acid composition can be investigated after hydrolysing peptide bonds to release constituent amino acids. Hydrolysis conditions must be considered because some residues can be altered or destroyed.
8.2 Separation
Released amino acids can be separated chromatographically based on charge, hydrophobicity or other chemical properties.
8.3 Detection
Amino acids may be detected after reaction with chromogenic or fluorescent reagents, or directly by modern instrumental methods.
8.4 Composition
Amino-acid analysis reports the types and relative amounts of residues but does not by itself reveal their sequence.
8.5 Special residues
Tryptophan can be destroyed under strong acid hydrolysis, while asparagine and glutamine can be converted to their acidic counterparts, so specialised approaches may be required.
8.6 Quantitative use
Composition data can support identity testing, nutritional evaluation, peptide analysis and comparison with a sequence predicted from a gene.
8.7 Limitations
A composition such as five alanines and three glycines does not reveal their order; sequence determination requires additional methods.
Amino-acid analysis and sequencing answer different questions. Composition tells how many residues of each type are present; sequencing tells their order. Two peptides can have identical composition but completely different sequences and biological properties. This distinction is fundamental when interpreting analytical data.
Chapter Summary
- Hydrolysis: Amino-acid composition can be investigated after hydrolysing peptide bonds to release constituent amino acids.
- Separation: Released amino acids can be separated chromatographically based on charge, hydrophobicity or other chemical properties.
- Detection: Amino acids may be detected after reaction with chromogenic or fluorescent reagents, or directly by modern instrumental methods.
- Composition: Amino-acid analysis reports the types and relative amounts of residues but does not by itself reveal their sequence.
- Special residues: Tryptophan can be destroyed under strong acid hydrolysis, while asparagine and glutamine can be converted to their acidic counterparts, so specialised approaches may be required.
- Quantitative use: Composition data can support identity testing, nutritional evaluation, peptide analysis and comparison with a sequence predicted from a gene.
- Limitations: A composition such as five alanines and three glycines does not reveal their order; sequence determination requires additional methods.
Review and Examination Questions
- Explain the major biochemical principles involved in amino-acid analysis of peptides and proteins.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on amino-acid analysis of peptides and proteins.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Determination of Amino-Acid Sequence
9.1 N-terminal identification
Identifying the N-terminal residue establishes one end of a peptide and provides an anchor for sequence analysis.
9.2 Edman degradation
Edman chemistry sequentially labels and removes the N-terminal residue under conditions designed to leave remaining peptide bonds intact, allowing stepwise identification of residues.
9.3 Proteolytic cleavage
Specific proteases generate peptide fragments at predictable residue types. Different cleavage reagents produce overlapping fragments that help reconstruct a complete sequence.
9.4 Chemical cleavage
Chemical reagents with residue selectivity can complement proteolytic digestion and provide a second fragment map.
9.5 Mass spectrometry
Peptide mass spectrometry and tandem mass spectrometry infer sequence information from peptide masses and fragmentation patterns.
9.6 Overlapping peptides
Fragments from different cleavage methods are aligned through overlaps, allowing the original sequence to be reconstructed.
9.7 Genomic prediction
DNA sequence can predict a protein's primary amino-acid sequence, but direct protein analysis remains important for identifying processing, cleavage and post-translational modifications.
Classical sequence chemistry and modern mass spectrometry are complementary. Edman degradation provides sequential N-terminal information, whereas tandem mass spectrometry can analyse complex peptide mixtures rapidly. Proteolytic specificity, fragment overlap and database comparison allow protein identity and modification state to be reconstructed.
Chapter Summary
- N-terminal identification: Identifying the N-terminal residue establishes one end of a peptide and provides an anchor for sequence analysis.
- Edman degradation: Edman chemistry sequentially labels and removes the N-terminal residue under conditions designed to leave remaining peptide bonds intact, allowing stepwise identification of residues.
- Proteolytic cleavage: Specific proteases generate peptide fragments at predictable residue types.
- Chemical cleavage: Chemical reagents with residue selectivity can complement proteolytic digestion and provide a second fragment map.
- Mass spectrometry: Peptide mass spectrometry and tandem mass spectrometry infer sequence information from peptide masses and fragmentation patterns.
- Overlapping peptides: Fragments from different cleavage methods are aligned through overlaps, allowing the original sequence to be reconstructed.
- Genomic prediction: DNA sequence can predict a protein's primary amino-acid sequence, but direct protein analysis remains important for identifying processing, cleavage and post-translational modifications.
Review and Examination Questions
- Explain the major biochemical principles involved in determination of amino-acid sequence.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on determination of amino-acid sequence.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Molecular-Weight Determination of Proteins
10.1 SDS-PAGE estimation
Migration of standards of known molecular mass can be used to construct a calibration relationship and estimate the mass of an unknown polypeptide.
10.2 Size-exclusion estimation
Elution behaviour in a calibrated size-exclusion column can estimate native hydrodynamic size, although molecular shape influences the result.
10.3 Analytical ultracentrifugation
Sedimentation behaviour provides information about molecular mass, shape and association state.
10.4 Mass spectrometry
Modern mass spectrometry can determine molecular masses with high precision and resolve different proteoforms or peptide species.
10.5 Subunit composition
A native oligomer may have a molecular mass much greater than each SDS-PAGE band because SDS-PAGE dissociates non-covalently associated subunits.
10.6 Calibration
Molecular-weight estimates depend on suitable standards and method assumptions. Anomalous migration or unusual shape can produce discrepancies.
10.7 Interpretation
Using complementary methods helps distinguish polypeptide-chain mass from native oligomeric mass.
Apparent molecular mass is method-dependent. SDS-PAGE estimates the mass of denatured polypeptide chains, size-exclusion chromatography reflects native hydrodynamic volume, and mass spectrometry measures molecular ions. Agreement among independent approaches strengthens structural conclusions; disagreement can reveal oligomerisation, unusual shape or modification.
Chapter Summary
- SDS-PAGE estimation: Migration of standards of known molecular mass can be used to construct a calibration relationship and estimate the mass of an unknown polypeptide.
- Size-exclusion estimation: Elution behaviour in a calibrated size-exclusion column can estimate native hydrodynamic size, although molecular shape influences the result.
- Analytical ultracentrifugation: Sedimentation behaviour provides information about molecular mass, shape and association state.
- Mass spectrometry: Modern mass spectrometry can determine molecular masses with high precision and resolve different proteoforms or peptide species.
- Subunit composition: A native oligomer may have a molecular mass much greater than each SDS-PAGE band because SDS-PAGE dissociates non-covalently associated subunits.
- Calibration: Molecular-weight estimates depend on suitable standards and method assumptions.
- Interpretation: Using complementary methods helps distinguish polypeptide-chain mass from native oligomeric mass.
Review and Examination Questions
- Explain the major biochemical principles involved in molecular-weight determination of proteins.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on molecular-weight determination of proteins.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Protein Biosynthesis: Translation
11.1 Information flow
Protein biosynthesis translates the nucleotide sequence of messenger RNA into an amino-acid sequence according to the genetic code.
11.2 Aminoacyl-tRNA formation
Aminoacyl-tRNA synthetases attach specific amino acids to their corresponding tRNAs using ATP. This charging step is a major determinant of translational accuracy.
11.3 Ribosome
Ribosomes coordinate mRNA decoding, tRNA positioning and peptide-bond formation. The ribosomal RNA contributes directly to catalytic activity.
11.4 Initiation
Translation begins when initiation factors assemble the ribosomal machinery at the correct start codon with initiator tRNA.
11.5 Elongation
Aminoacyl-tRNAs enter according to codon-anticodon recognition, peptide bonds form, and the ribosome translocates along the mRNA.
11.6 Termination
Stop codons are recognised by release factors rather than ordinary aminoacyl-tRNAs, promoting release of the completed polypeptide.
11.7 Post-translational fate
New polypeptides fold, may be chemically modified, and can be directed to specific cellular compartments.
11.8 Energetic cost
Protein synthesis requires substantial energy for amino-acid activation, tRNA charging and ribosomal elongation, reflecting the high fidelity demanded by cellular protein production.
Question: How many ATP molecules and high-energy phosphate bonds are consumed per urea formed?
Given: Carbamoyl phosphate synthetase I uses 2 ATP → ADP; argininosuccinate synthetase uses ATP → AMP + PPi.
Solution: Three ATP molecules are consumed. ATP → AMP uses the equivalent of two high-energy phosphate bonds, so total high-energy phosphate equivalents = 2 + 2 = 4.
Answer: 3 ATP molecules, equivalent to 4 high-energy phosphate bonds.
Translation connects nucleic-acid information to the protein metabolic system. Fidelity depends not only on codon-anticodon pairing but also on aminoacyl-tRNA synthetases choosing the correct amino acid. Once synthesis ends, folding, cleavage, targeting and covalent modification create the mature functional protein, so translation is necessary but not sufficient for proteome formation.
Chapter Summary
- Information flow: Protein biosynthesis translates the nucleotide sequence of messenger RNA into an amino-acid sequence according to the genetic code.
- Aminoacyl-tRNA formation: Aminoacyl-tRNA synthetases attach specific amino acids to their corresponding tRNAs using ATP.
- Ribosome: Ribosomes coordinate mRNA decoding, tRNA positioning and peptide-bond formation.
- Initiation: Translation begins when initiation factors assemble the ribosomal machinery at the correct start codon with initiator tRNA.
- Elongation: Aminoacyl-tRNAs enter according to codon-anticodon recognition, peptide bonds form, and the ribosome translocates along the mRNA.
- Termination: Stop codons are recognised by release factors rather than ordinary aminoacyl-tRNAs, promoting release of the completed polypeptide.
- Post-translational fate: New polypeptides fold, may be chemically modified, and can be directed to specific cellular compartments.
- Energetic cost: Protein synthesis requires substantial energy for amino-acid activation, tRNA charging and ribosomal elongation, reflecting the high fidelity demanded by cellular protein production.
Review and Examination Questions
- Explain the major biochemical principles involved in protein biosynthesis: translation.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on protein biosynthesis: translation.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Digestion and Absorption of Dietary Proteins
12.1 Gastric digestion
Protein digestion begins substantially in the stomach, where acid denatures proteins and pepsin cleaves peptide bonds.
12.2 Pancreatic proteases
The small intestine receives proteolytic zymogens from the pancreas. Their controlled activation produces enzymes such as trypsin and chymotrypsin.
12.3 Zymogen protection
Proteases are secreted as inactive precursors to limit digestion of the cells and ducts that produce them.
12.4 Brush-border digestion
Peptidases at the intestinal surface and within enterocytes complete digestion of oligopeptides.
12.5 Amino-acid transport
Free amino acids use several transporter systems with different substrate specificities. Di- and tripeptides can also be absorbed and hydrolysed intracellularly.
12.6 Portal delivery
Absorbed amino acids enter portal circulation and reach the liver, where they contribute to protein synthesis, nitrogen metabolism and systemic distribution.
12.7 Amino-acid pool
Dietary absorption, tissue protein turnover and biosynthesis contribute to the free amino-acid pool, while protein synthesis and catabolic pathways remove amino acids from it.
Protein digestion is controlled proteolysis. Gastric acid exposes peptide bonds, pancreatic enzymes are activated in the intestinal lumen rather than inside pancreatic cells, and membrane and intracellular peptidases finish digestion. Multiple transport systems then prevent absorption from depending on one universal amino-acid carrier.
Chapter Summary
- Gastric digestion: Protein digestion begins substantially in the stomach, where acid denatures proteins and pepsin cleaves peptide bonds.
- Pancreatic proteases: The small intestine receives proteolytic zymogens from the pancreas.
- Zymogen protection: Proteases are secreted as inactive precursors to limit digestion of the cells and ducts that produce them.
- Brush-border digestion: Peptidases at the intestinal surface and within enterocytes complete digestion of oligopeptides.
- Amino-acid transport: Free amino acids use several transporter systems with different substrate specificities.
- Portal delivery: Absorbed amino acids enter portal circulation and reach the liver, where they contribute to protein synthesis, nitrogen metabolism and systemic distribution.
- Amino-acid pool: Dietary absorption, tissue protein turnover and biosynthesis contribute to the free amino-acid pool, while protein synthesis and catabolic pathways remove amino acids from it.
Review and Examination Questions
- Explain the major biochemical principles involved in digestion and absorption of dietary proteins.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on digestion and absorption of dietary proteins.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Transamination and Amino-Group Transfer
13.1 Principle
Transamination transfers an amino group from an amino acid to an alpha-keto acid without releasing free ammonia in the transfer step.
13.2 Aminotransferases
Aminotransferases commonly use pyridoxal phosphate, derived from vitamin B6, as a coenzyme that temporarily carries amino groups.
13.3 Alpha-ketoglutarate
Many amino groups are collected on alpha-ketoglutarate to form glutamate, making glutamate a central hub of nitrogen metabolism.
13.4 ALT
Alanine aminotransferase transfers the amino group of alanine to alpha-ketoglutarate, forming pyruvate and glutamate.
13.5 AST
Aspartate aminotransferase transfers the amino group of aspartate to alpha-ketoglutarate, forming oxaloacetate and glutamate.
13.6 Reversibility
Transamination reactions are generally reversible, allowing them to participate in both amino-acid degradation and synthesis.
13.7 Clinical application
Activities of aminotransferases in blood can be used as biochemical indicators of tissue injury, although interpretation requires clinical context.
Transamination conserves nitrogen by moving amino groups without immediately releasing ammonia. Alpha-ketoglutarate and glutamate form a central collecting pair because many aminotransferases use them. The carbon skeleton left behind is simultaneously redirected toward energy metabolism, glucose synthesis or other biosynthetic routes.
Chapter Summary
- Principle: Transamination transfers an amino group from an amino acid to an alpha-keto acid without releasing free ammonia in the transfer step.
- Aminotransferases: Aminotransferases commonly use pyridoxal phosphate, derived from vitamin B6, as a coenzyme that temporarily carries amino groups.
- Alpha-ketoglutarate: Many amino groups are collected on alpha-ketoglutarate to form glutamate, making glutamate a central hub of nitrogen metabolism.
- ALT: Alanine aminotransferase transfers the amino group of alanine to alpha-ketoglutarate, forming pyruvate and glutamate.
- AST: Aspartate aminotransferase transfers the amino group of aspartate to alpha-ketoglutarate, forming oxaloacetate and glutamate.
- Reversibility: Transamination reactions are generally reversible, allowing them to participate in both amino-acid degradation and synthesis.
- Clinical application: Activities of aminotransferases in blood can be used as biochemical indicators of tissue injury, although interpretation requires clinical context.
Review and Examination Questions
- Explain the major biochemical principles involved in transamination and amino-group transfer.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on transamination and amino-group transfer.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Oxidative Deamination and Ammonia Handling
14.1 Glutamate dehydrogenase
Glutamate can undergo oxidative deamination, producing alpha-ketoglutarate and ammonium while transferring reducing equivalents to NAD(P)+.
14.2 Transdeamination
Many amino acids first transfer nitrogen to glutamate by transamination; glutamate then releases ammonium. This combined process is often termed transdeamination.
14.3 Glutamine
Glutamine safely carries ammonia nitrogen in many tissues. Glutamine synthetase incorporates ammonium into glutamate at an ATP cost.
14.4 Glutaminase
Glutaminase releases ammonium from glutamine, notably in tissues such as liver and kidney according to metabolic need.
14.5 Alanine transport
Muscle can transfer nitrogen to pyruvate to form alanine, which travels to liver. This participates in the glucose-alanine cycle.
14.6 Ammonia toxicity
High ammonia is especially harmful to the nervous system. Disturbance of glutamate/glutamine and TCA-cycle-related metabolism contributes to impaired cerebral energy and neurotransmitter balance.
A useful biochemical model is that excess ammonia drives glutamate toward glutamine formation and perturbs nitrogen and carbon balance in brain cells. Because alpha-ketoglutarate is linked to the TCA cycle, severe hyperammonaemia can interfere with energy metabolism as well as neurotransmitter homeostasis. The neurological effects therefore arise from interconnected disturbances rather than from a single toxic reaction.
14.7 Safe disposal
The liver converts a large fraction of waste nitrogen to urea, a less toxic water-soluble compound that can be excreted by the kidneys.
Free ammonia is kept low because nitrogen is transported mainly in safer molecular forms. Glutamine packages ammonia into an amide, while alanine couples muscle nitrogen export to pyruvate carbon. Liver and kidney then process these carriers according to whole-body nitrogen and acid-base requirements.
Chapter Summary
- Glutamate dehydrogenase: Glutamate can undergo oxidative deamination, producing alpha-ketoglutarate and ammonium while transferring reducing equivalents to NAD(P)+.
- Transdeamination: Many amino acids first transfer nitrogen to glutamate by transamination; glutamate then releases ammonium.
- Glutamine: Glutamine safely carries ammonia nitrogen in many tissues.
- Glutaminase: Glutaminase releases ammonium from glutamine, notably in tissues such as liver and kidney according to metabolic need.
- Alanine transport: Muscle can transfer nitrogen to pyruvate to form alanine, which travels to liver.
- Ammonia toxicity: High ammonia is especially harmful to the nervous system.
- Safe disposal: The liver converts a large fraction of waste nitrogen to urea, a less toxic water-soluble compound that can be excreted by the kidneys.
Review and Examination Questions
- Explain the major biochemical principles involved in oxidative deamination and ammonia handling.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on oxidative deamination and ammonia handling.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Urea Cycle
15.1 Purpose
The urea cycle converts toxic nitrogen into urea in the liver. It spans mitochondrial and cytosolic compartments.
15.2 Carbamoyl phosphate
Carbamoyl phosphate synthetase I uses ammonium, bicarbonate and two ATP to form carbamoyl phosphate in mitochondria.
15.3 Citrulline formation
Ornithine transcarbamylase transfers the carbamoyl group to ornithine, forming citrulline, which moves to the cytosol.
15.4 Argininosuccinate synthesis
Citrulline combines with aspartate in an ATP-dependent reaction catalysed by argininosuccinate synthetase.
15.5 Fumarate release
Argininosuccinate lyase cleaves argininosuccinate to arginine and fumarate.
15.6 Urea formation
Arginase hydrolyses arginine to urea and ornithine. Ornithine returns to mitochondria for another cycle.
15.7 Nitrogen sources
The two nitrogen atoms of urea arise from free ammonium and aspartate.
15.8 Energetic cost
The cycle consumes three ATP molecules but four high-energy phosphate bonds because one ATP is converted to AMP.
Question: How many ATP molecules and high-energy phosphate bonds are consumed per urea formed?
Given: Carbamoyl phosphate synthetase I uses 2 ATP → ADP; argininosuccinate synthetase uses ATP → AMP + PPi.
Solution: Three ATP molecules are consumed. ATP → AMP uses the equivalent of two high-energy phosphate bonds, so total high-energy phosphate equivalents = 2 + 2 = 4.
Answer: 3 ATP molecules, equivalent to 4 high-energy phosphate bonds.
15.9 Regulation
Carbamoyl phosphate synthetase I requires N-acetylglutamate as an essential activator, linking urea production to amino-acid catabolic state.
The urea cycle solves a chemical toxicity problem at significant energetic cost. Compartmentation between mitochondria and cytosol organises the reactions, while aspartate supplies the second nitrogen. The cycle's connection to fumarate, malate and oxaloacetate demonstrates that nitrogen disposal is embedded in central carbon metabolism.
Chapter Summary
- Purpose: The urea cycle converts toxic nitrogen into urea in the liver.
- Carbamoyl phosphate: Carbamoyl phosphate synthetase I uses ammonium, bicarbonate and two ATP to form carbamoyl phosphate in mitochondria.
- Citrulline formation: Ornithine transcarbamylase transfers the carbamoyl group to ornithine, forming citrulline, which moves to the cytosol.
- Argininosuccinate synthesis: Citrulline combines with aspartate in an ATP-dependent reaction catalysed by argininosuccinate synthetase.
- Fumarate release: Argininosuccinate lyase cleaves argininosuccinate to arginine and fumarate.
- Urea formation: Arginase hydrolyses arginine to urea and ornithine.
- Nitrogen sources: The two nitrogen atoms of urea arise from free ammonium and aspartate.
- Energetic cost: The cycle consumes three ATP molecules but four high-energy phosphate bonds because one ATP is converted to AMP.
- Regulation: Carbamoyl phosphate synthetase I requires N-acetylglutamate as an essential activator, linking urea production to amino-acid catabolic state.
Review and Examination Questions
- Explain the major biochemical principles involved in urea cycle.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on urea cycle.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Relationship of the Urea Cycle to Central Metabolism
16.1 Aspartate-argininosuccinate shunt
Fumarate produced by the urea cycle can be converted to malate and oxaloacetate, linking nitrogen disposal to TCA-cycle intermediates.
16.2 Aspartate supply
Oxaloacetate can accept an amino group by transamination to form aspartate, which returns nitrogen to the urea cycle.
16.3 Carbon skeletons
After removal of nitrogen, amino-acid carbon skeletons enter pathways such as glycolysis, the TCA cycle, ketogenesis or gluconeogenesis.
16.4 Energy relationship
Urea synthesis costs energy, but associated oxidation of amino-acid carbon skeletons can generate reducing equivalents and ATP.
16.5 Liver specialisation
Hepatic compartmentation and enzyme expression permit the liver to receive nitrogen from peripheral tissues and convert it efficiently to urea.
16.6 Kidney role
The kidney excretes urea and also participates in acid-base-related ammonium handling, particularly through glutamine metabolism.
16.7 Integrated view
Protein catabolism therefore cannot be separated conceptually into 'nitrogen metabolism' and 'energy metabolism'; the two are connected through shared carbon intermediates.
The link between urea and TCA metabolism is sometimes called the aspartate-argininosuccinate shunt. Fumarate produced during urea synthesis can become malate and oxaloacetate; oxaloacetate can be transaminated to aspartate, which re-enters urea synthesis. This creates a biochemical bridge between disposal of nitrogen and handling of carbon skeletons.
Chapter Summary
- Aspartate-argininosuccinate shunt: Fumarate produced by the urea cycle can be converted to malate and oxaloacetate, linking nitrogen disposal to TCA-cycle intermediates.
- Aspartate supply: Oxaloacetate can accept an amino group by transamination to form aspartate, which returns nitrogen to the urea cycle.
- Carbon skeletons: After removal of nitrogen, amino-acid carbon skeletons enter pathways such as glycolysis, the TCA cycle, ketogenesis or gluconeogenesis.
- Energy relationship: Urea synthesis costs energy, but associated oxidation of amino-acid carbon skeletons can generate reducing equivalents and ATP.
- Liver specialisation: Hepatic compartmentation and enzyme expression permit the liver to receive nitrogen from peripheral tissues and convert it efficiently to urea.
- Kidney role: The kidney excretes urea and also participates in acid-base-related ammonium handling, particularly through glutamine metabolism.
- Integrated view: Protein catabolism therefore cannot be separated conceptually into 'nitrogen metabolism' and 'energy metabolism'; the two are connected through shared carbon intermediates.
Review and Examination Questions
- Explain the major biochemical principles involved in relationship of the urea cycle to central metabolism.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on relationship of the urea cycle to central metabolism.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Glucogenic and Ketogenic Amino Acids
17.1 Glucogenic concept
A glucogenic amino acid yields pyruvate or a TCA-cycle intermediate capable of contributing carbon to gluconeogenesis.
17.2 Ketogenic concept
A ketogenic amino acid yields acetyl-CoA or acetoacetate carbon that can contribute to ketone bodies or lipid synthesis but cannot provide net glucose in humans.
17.3 Exclusively ketogenic amino acids
Leucine and lysine are classically regarded as exclusively ketogenic.
17.4 Both glucogenic and ketogenic
Several amino acids, including phenylalanine, tyrosine, tryptophan, threonine and isoleucine, yield products belonging to both categories.
17.5 TCA entry points
Amino-acid carbon skeletons can enter metabolism as pyruvate, acetyl-CoA/acetoacetate, alpha-ketoglutarate, succinyl-CoA, fumarate or oxaloacetate.
| Carbon-skeleton entry | Metabolic implication |
|---|---|
| Pyruvate | Can support gluconeogenesis or acetyl-CoA formation |
| Acetyl-CoA / acetoacetate | Ketogenic carbon; no net glucose in humans |
| Alpha-ketoglutarate | TCA intermediate; potentially glucogenic |
| Succinyl-CoA | TCA intermediate; potentially glucogenic |
| Fumarate | TCA intermediate; potentially glucogenic |
| Oxaloacetate | Directly connected to gluconeogenesis |
17.6 Physiological significance
During fasting, glucogenic amino acids can support hepatic glucose production, while ketogenic carbon can support ketone-body formation.
17.7 Conceptual caution
The classification refers to carbon fate after nitrogen removal; it does not mean an amino acid participates in only one biochemical reaction.
Glucogenic and ketogenic classification is best derived rather than memorised. Ask what carbon product remains after the amino group is removed. Entry as pyruvate or a TCA intermediate supports net glucose synthesis; entry solely as acetyl-CoA or acetoacetate does not. Leucine and lysine provide the important exclusively ketogenic examples.
Chapter Summary
- Glucogenic concept: A glucogenic amino acid yields pyruvate or a TCA-cycle intermediate capable of contributing carbon to gluconeogenesis.
- Ketogenic concept: A ketogenic amino acid yields acetyl-CoA or acetoacetate carbon that can contribute to ketone bodies or lipid synthesis but cannot provide net glucose in humans.
- Exclusively ketogenic amino acids: Leucine and lysine are classically regarded as exclusively ketogenic.
- Both glucogenic and ketogenic: Several amino acids, including phenylalanine, tyrosine, tryptophan, threonine and isoleucine, yield products belonging to both categories.
- TCA entry points: Amino-acid carbon skeletons can enter metabolism as pyruvate, acetyl-CoA/acetoacetate, alpha-ketoglutarate, succinyl-CoA, fumarate or oxaloacetate.
- Physiological significance: During fasting, glucogenic amino acids can support hepatic glucose production, while ketogenic carbon can support ketone-body formation.
- Conceptual caution: The classification refers to carbon fate after nitrogen removal; it does not mean an amino acid participates in only one biochemical reaction.
Review and Examination Questions
- Explain the major biochemical principles involved in glucogenic and ketogenic amino acids.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on glucogenic and ketogenic amino acids.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
One-Carbon Metabolism
18.1 One-carbon units
Single-carbon fragments at different oxidation states are transferred in metabolism rather than allowed to react nonspecifically.
18.2 Tetrahydrofolate
Tetrahydrofolate accepts and carries one-carbon units in several oxidation states and supports nucleotide and amino-acid metabolism.
18.3 Serine and glycine
Serine hydroxymethyltransferase transfers a one-carbon unit from serine to tetrahydrofolate while producing glycine.
18.4 Methionine cycle
Methionine is converted to S-adenosylmethionine, a major biological methyl donor. After methyl transfer, homocysteine is formed through subsequent reactions.
18.5 Remethylation
Homocysteine can be remethylated to methionine using folate-dependent one-carbon chemistry and vitamin B12-dependent methionine synthase.
18.6 Transsulfuration
Homocysteine can also enter the transsulfuration pathway toward cysteine through vitamin B6-dependent reactions.
18.7 Biosynthetic significance
One-carbon metabolism supports purine synthesis, thymidylate synthesis, methylation and interconversion of selected amino acids.
18.8 Vitamin integration
Folate, vitamin B12 and vitamin B6 deficiencies can therefore disturb interconnected pathways rather than a single isolated reaction.
One-carbon metabolism demonstrates how vitamins act as parts of coenzymes rather than as vague dietary factors. Folate derivatives carry carbon fragments, vitamin B12 supports methionine regeneration, and vitamin B6 supports transsulfuration. A deficiency can therefore alter nucleotide synthesis, methylation and amino-acid metabolism at the same time.
Chapter Summary
- One-carbon units: Single-carbon fragments at different oxidation states are transferred in metabolism rather than allowed to react nonspecifically.
- Tetrahydrofolate: Tetrahydrofolate accepts and carries one-carbon units in several oxidation states and supports nucleotide and amino-acid metabolism.
- Serine and glycine: Serine hydroxymethyltransferase transfers a one-carbon unit from serine to tetrahydrofolate while producing glycine.
- Methionine cycle: Methionine is converted to S-adenosylmethionine, a major biological methyl donor.
- Remethylation: Homocysteine can be remethylated to methionine using folate-dependent one-carbon chemistry and vitamin B12-dependent methionine synthase.
- Transsulfuration: Homocysteine can also enter the transsulfuration pathway toward cysteine through vitamin B6-dependent reactions.
- Biosynthetic significance: One-carbon metabolism supports purine synthesis, thymidylate synthesis, methylation and interconversion of selected amino acids.
- Vitamin integration: Folate, vitamin B12 and vitamin B6 deficiencies can therefore disturb interconnected pathways rather than a single isolated reaction.
Review and Examination Questions
- Explain the major biochemical principles involved in one-carbon metabolism.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on one-carbon metabolism.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Biosynthesis of Non-Essential Amino Acids
19.1 General principle
Many non-essential amino acids are synthesised by adding nitrogen to carbon skeletons derived from glycolysis, the TCA cycle or other central pathways.
19.2 Glutamate
Alpha-ketoglutarate can acquire amino nitrogen to form glutamate, which then donates amino groups in numerous transamination reactions.
19.3 Glutamine
Glutamine synthetase amidates glutamate using ammonium and ATP, producing glutamine, an important nitrogen donor and transporter.
19.4 Alanine
Alanine is formed readily by transamination of pyruvate.
19.5 Aspartate
Aspartate is formed by transamination of oxaloacetate.
19.6 Asparagine
Asparagine synthetase converts aspartate to asparagine using an amide nitrogen donor and ATP.
19.7 Serine
Serine can be synthesised from the glycolytic intermediate 3-phosphoglycerate through oxidation, transamination and dephosphorylation.
19.8 Glycine and cysteine
Glycine can be formed from serine, while cysteine synthesis uses serine carbon and sulfur derived from methionine metabolism in humans.
19.9 Conditional requirements
Rapid growth, disease or metabolic immaturity can make the endogenous supply of some normally non-essential amino acids insufficient.
Amino-acid biosynthesis shows how carbon and nitrogen metabolism converge. Central pathways first provide carbon skeletons; transamination or amidation then introduces nitrogen. Glutamate and glutamine occupy privileged positions because they distribute amino and amide nitrogen to numerous biosynthetic reactions.
Chapter Summary
- General principle: Many non-essential amino acids are synthesised by adding nitrogen to carbon skeletons derived from glycolysis, the TCA cycle or other central pathways.
- Glutamate: Alpha-ketoglutarate can acquire amino nitrogen to form glutamate, which then donates amino groups in numerous transamination reactions.
- Glutamine: Glutamine synthetase amidates glutamate using ammonium and ATP, producing glutamine, an important nitrogen donor and transporter.
- Alanine: Alanine is formed readily by transamination of pyruvate.
- Aspartate: Aspartate is formed by transamination of oxaloacetate.
- Asparagine: Asparagine synthetase converts aspartate to asparagine using an amide nitrogen donor and ATP.
- Serine: Serine can be synthesised from the glycolytic intermediate 3-phosphoglycerate through oxidation, transamination and dephosphorylation.
- Glycine and cysteine: Glycine can be formed from serine, while cysteine synthesis uses serine carbon and sulfur derived from methionine metabolism in humans.
- Conditional requirements: Rapid growth, disease or metabolic immaturity can make the endogenous supply of some normally non-essential amino acids insufficient.
Review and Examination Questions
- Explain the major biochemical principles involved in biosynthesis of non-essential amino acids.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on biosynthesis of non-essential amino acids.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Metabolism of Inorganic Nitrogen
20.1 Nitrogen cycle perspective
Atmospheric nitrogen is abundant but chemically inert. Biological nitrogen fixation converts N2 into ammonia that can enter biosynthetic pathways.
20.2 Nitrogen fixation
Specialised microorganisms use nitrogenase to reduce atmospheric nitrogen to ammonia. The process requires substantial energy and occurs under conditions that protect the oxygen-sensitive enzyme system.
20.3 Ammonium assimilation
Microorganisms and plants incorporate inorganic ammonium principally through glutamate and glutamine, which distribute nitrogen to other biomolecules.
20.4 GS-GOGAT system
Glutamine synthetase and glutamate synthase provide an efficient route for ammonium assimilation, especially when environmental ammonium concentration is low.
20.5 Nitrate assimilation
Plants and many microorganisms can reduce nitrate to nitrite and then to ammonium before incorporation into organic compounds.
20.6 Human dependence
Humans cannot fix atmospheric nitrogen and depend ultimately on dietary organic nitrogen whose origin traces back to microbial and plant assimilation.
20.7 Protein synthesis link
Inorganic nitrogen enters the biosphere as amino nitrogen and amide nitrogen, which then supplies amino-acid and protein biosynthesis across food webs.
Inorganic nitrogen metabolism explains the ultimate origin of biological nitrogen. Human metabolism can redistribute amino nitrogen but cannot reduce atmospheric N2. Nitrogen-fixing microorganisms perform that energetically difficult chemistry, while plants and microbes assimilate ammonia or nitrate-derived nitrogen into glutamate and glutamine, ultimately supporting animal protein nutrition.
Chapter Summary
- Nitrogen cycle perspective: Atmospheric nitrogen is abundant but chemically inert.
- Nitrogen fixation: Specialised microorganisms use nitrogenase to reduce atmospheric nitrogen to ammonia.
- Ammonium assimilation: Microorganisms and plants incorporate inorganic ammonium principally through glutamate and glutamine, which distribute nitrogen to other biomolecules.
- GS-GOGAT system: Glutamine synthetase and glutamate synthase provide an efficient route for ammonium assimilation, especially when environmental ammonium concentration is low.
- Nitrate assimilation: Plants and many microorganisms can reduce nitrate to nitrite and then to ammonium before incorporation into organic compounds.
- Human dependence: Humans cannot fix atmospheric nitrogen and depend ultimately on dietary organic nitrogen whose origin traces back to microbial and plant assimilation.
- Protein synthesis link: Inorganic nitrogen enters the biosphere as amino nitrogen and amide nitrogen, which then supplies amino-acid and protein biosynthesis across food webs.
Review and Examination Questions
- Explain the major biochemical principles involved in metabolism of inorganic nitrogen.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on metabolism of inorganic nitrogen.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Amino-Acid Derivatives and Specialised Products
21.1 Creatine
Arginine, glycine and methionine contribute to creatine synthesis. Phosphocreatine provides a rapidly exchangeable high-energy phosphate reserve in muscle and other tissues.
21.2 Catecholamines
Tyrosine is a precursor of dopamine, noradrenaline and adrenaline through hydroxylation, decarboxylation and subsequent modifications.
21.3 Serotonin
Tryptophan is converted through hydroxylation and decarboxylation to serotonin, an important signalling molecule.
21.4 Histamine
Histidine decarboxylation produces histamine, which participates in immune, gastric and neural functions.
21.5 Nitric oxide
Arginine is a substrate for nitric-oxide synthases, which generate nitric oxide as a signalling molecule.
21.6 Porphyrins
Glycine contributes nitrogen and carbon to haem biosynthesis through condensation with succinyl-CoA.
21.7 Melanin
Tyrosine supplies carbon for melanin synthesis through tyrosinase-dependent reactions.
21.8 Metabolic integration
Amino-acid metabolism therefore supplies not only protein building blocks and energy but also hormones, neurotransmitters, pigments and specialised cofactors.
Specialised amino-acid products demonstrate why amino-acid metabolism extends beyond protein synthesis. Decarboxylation, hydroxylation, methyl transfer and other reactions convert amino-acid carbon skeletons into signalling molecules and cofactors. The physiological effect of an amino-acid deficiency can therefore involve both proteins and non-protein products.
Chapter Summary
- Creatine: Arginine, glycine and methionine contribute to creatine synthesis.
- Catecholamines: Tyrosine is a precursor of dopamine, noradrenaline and adrenaline through hydroxylation, decarboxylation and subsequent modifications.
- Serotonin: Tryptophan is converted through hydroxylation and decarboxylation to serotonin, an important signalling molecule.
- Histamine: Histidine decarboxylation produces histamine, which participates in immune, gastric and neural functions.
- Nitric oxide: Arginine is a substrate for nitric-oxide synthases, which generate nitric oxide as a signalling molecule.
- Porphyrins: Glycine contributes nitrogen and carbon to haem biosynthesis through condensation with succinyl-CoA.
- Melanin: Tyrosine supplies carbon for melanin synthesis through tyrosinase-dependent reactions.
- Metabolic integration: Amino-acid metabolism therefore supplies not only protein building blocks and energy but also hormones, neurotransmitters, pigments and specialised cofactors.
Review and Examination Questions
- Explain the major biochemical principles involved in amino-acid derivatives and specialised products.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on amino-acid derivatives and specialised products.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Polyamine Metabolism
22.1 Polyamines
Putrescine, spermidine and spermine are small polycationic molecules associated with nucleic acids, membranes and processes of cell growth.
22.2 Ornithine decarboxylase
Ornithine decarboxylase converts ornithine to putrescine and is an important regulated step in polyamine synthesis.
22.3 Decarboxylated SAM
S-adenosylmethionine can be decarboxylated to provide aminopropyl groups for conversion of putrescine to spermidine and spermine.
22.4 Functions
Polyamines influence DNA and RNA interactions, translation, ion channels, membrane properties and cellular proliferation.
22.5 Regulation
Because excessive or deficient polyamine concentrations can disturb growth, synthesis, degradation, uptake and export are tightly regulated.
22.6 Catabolism
Oxidative reactions participate in polyamine interconversion and breakdown, generating products that must be controlled because reactive species may arise.
22.7 Clinical and experimental relevance
Rapidly proliferating cells often show altered polyamine metabolism, making the pathway important in research on growth and disease.
Polyamine synthesis intersects with both ornithine metabolism and S-adenosylmethionine chemistry. Because polyamines bind negatively charged cellular molecules, their concentration influences nucleic-acid organisation and growth. Rapid regulation of ornithine decarboxylase helps keep this potentially powerful growth-related pathway under control.
Chapter Summary
- Polyamines: Putrescine, spermidine and spermine are small polycationic molecules associated with nucleic acids, membranes and processes of cell growth.
- Ornithine decarboxylase: Ornithine decarboxylase converts ornithine to putrescine and is an important regulated step in polyamine synthesis.
- Decarboxylated SAM: S-adenosylmethionine can be decarboxylated to provide aminopropyl groups for conversion of putrescine to spermidine and spermine.
- Functions: Polyamines influence DNA and RNA interactions, translation, ion channels, membrane properties and cellular proliferation.
- Regulation: Because excessive or deficient polyamine concentrations can disturb growth, synthesis, degradation, uptake and export are tightly regulated.
- Catabolism: Oxidative reactions participate in polyamine interconversion and breakdown, generating products that must be controlled because reactive species may arise.
- Clinical and experimental relevance: Rapidly proliferating cells often show altered polyamine metabolism, making the pathway important in research on growth and disease.
Review and Examination Questions
- Explain the major biochemical principles involved in polyamine metabolism.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on polyamine metabolism.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Disorders of Amino-Acid Metabolism
23.1 Phenylketonuria
Phenylketonuria commonly results from deficient phenylalanine hydroxylase activity, limiting conversion of phenylalanine to tyrosine. Accumulation of phenylalanine and alternative metabolites can damage the developing nervous system if untreated.
23.2 Alkaptonuria
Deficiency of homogentisate oxidase in tyrosine degradation causes homogentisic acid accumulation and excretion, with long-term pigment deposition and tissue effects.
23.3 Maple syrup urine disease
Deficiency of the branched-chain alpha-ketoacid dehydrogenase complex impairs degradation of branched-chain amino acids and their ketoacids, producing serious neurological toxicity.
23.4 Homocystinuria
Several defects can cause excessive homocysteine or related metabolites. Classical homocystinuria often involves cystathionine beta-synthase deficiency.
23.5 Tyrosinaemias
Different enzyme defects in tyrosine degradation produce distinct tyrosinaemias, demonstrating that disorders within one pathway can have different biochemical signatures.
23.6 Urea-cycle disorders
Deficiency of a urea-cycle enzyme reduces nitrogen disposal and can cause hyperammonaemia. The exact metabolite pattern depends on the blocked step.
23.7 Diagnostic reasoning
Interpretation begins with the blocked reaction, followed by predicted substrate accumulation, product deficiency, alternative metabolite formation and tissue consequences.
23.8 Management principle
Treatment strategies vary but may include restriction of offending substrates, provision of essential products or cofactors, alternative nitrogen-disposal approaches and prevention of catabolic stress.
Inborn errors are biochemical experiments produced by nature. A missing enzyme exposes pathway direction because upstream metabolites accumulate while downstream products fall or alternative routes become prominent. Understanding this logic makes phenylketonuria, alkaptonuria and branched-chain amino-acid disorders easier to reason through than memorising isolated symptom lists.
Chapter Summary
- Phenylketonuria: Phenylketonuria commonly results from deficient phenylalanine hydroxylase activity, limiting conversion of phenylalanine to tyrosine.
- Alkaptonuria: Deficiency of homogentisate oxidase in tyrosine degradation causes homogentisic acid accumulation and excretion, with long-term pigment deposition and tissue effects.
- Maple syrup urine disease: Deficiency of the branched-chain alpha-ketoacid dehydrogenase complex impairs degradation of branched-chain amino acids and their ketoacids, producing serious neurological toxicity.
- Homocystinuria: Several defects can cause excessive homocysteine or related metabolites.
- Tyrosinaemias: Different enzyme defects in tyrosine degradation produce distinct tyrosinaemias, demonstrating that disorders within one pathway can have different biochemical signatures.
- Urea-cycle disorders: Deficiency of a urea-cycle enzyme reduces nitrogen disposal and can cause hyperammonaemia.
- Diagnostic reasoning: Interpretation begins with the blocked reaction, followed by predicted substrate accumulation, product deficiency, alternative metabolite formation and tissue consequences.
- Management principle: Treatment strategies vary but may include restriction of offending substrates, provision of essential products or cofactors, alternative nitrogen-disposal approaches and prevention of catabolic stress.
Review and Examination Questions
- Explain the major biochemical principles involved in disorders of amino-acid metabolism.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on disorders of amino-acid metabolism.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Integrated Protein and Amino-Acid Metabolism
24.1 Fed state
Dietary amino acids support tissue protein synthesis and synthesis of specialised nitrogenous compounds. Excess amino acids are not stored as a dedicated reserve and are therefore catabolised.
24.2 Fasting
During fasting, protein breakdown contributes amino acids for gluconeogenesis and other essential functions, although prolonged adaptation seeks to conserve body protein.
24.3 Muscle-liver interaction
Muscle exports nitrogen particularly as alanine and glutamine. Liver handles much of the resulting nitrogen through urea synthesis while using carbon skeletons according to metabolic state.
24.4 Glucose-alanine cycle
Muscle transfers amino nitrogen to pyruvate to form alanine. Liver converts alanine carbon to glucose and nitrogen to urea, linking peripheral nitrogen disposal with glucose homeostasis.
24.5 Carbon-nitrogen balance
Amino-acid metabolism always involves two linked questions: what happens to nitrogen, and what happens to the carbon skeleton.
24.6 Hormonal regulation
Insulin promotes protein synthesis in nutrient abundance, while fasting hormones and cellular energy signals favour mobilisation and oxidation according to tissue needs.
24.7 Clinical integration
Liver failure, renal dysfunction, malnutrition, inherited enzyme defects and severe catabolic states can all disturb amino-acid and nitrogen balance through different mechanisms.
Whole-body amino-acid metabolism changes with nutritional state. After feeding, insulin and substrate availability favour protein synthesis. In fasting, amino acids released particularly from muscle support hepatic gluconeogenesis and essential synthesis, but prolonged starvation increases ketone utilisation and thereby reduces the glucose demand that would otherwise require continued protein breakdown.
Chapter Summary
- Fed state: Dietary amino acids support tissue protein synthesis and synthesis of specialised nitrogenous compounds.
- Fasting: During fasting, protein breakdown contributes amino acids for gluconeogenesis and other essential functions, although prolonged adaptation seeks to conserve body protein.
- Muscle-liver interaction: Muscle exports nitrogen particularly as alanine and glutamine.
- Glucose-alanine cycle: Muscle transfers amino nitrogen to pyruvate to form alanine.
- Carbon-nitrogen balance: Amino-acid metabolism always involves two linked questions: what happens to nitrogen, and what happens to the carbon skeleton.
- Hormonal regulation: Insulin promotes protein synthesis in nutrient abundance, while fasting hormones and cellular energy signals favour mobilisation and oxidation according to tissue needs.
- Clinical integration: Liver failure, renal dysfunction, malnutrition, inherited enzyme defects and severe catabolic states can all disturb amino-acid and nitrogen balance through different mechanisms.
Review and Examination Questions
- Explain the major biochemical principles involved in integrated protein and amino-acid metabolism.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on integrated protein and amino-acid metabolism.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Techniques and Quantitative Reasoning in Protein Biochemistry
25.1 Protein concentration
Protein concentration can be estimated using absorbance or colorimetric assays. Each method depends on particular chemical properties and may be affected by interfering substances.
25.2 Specific activity
For an enzyme preparation, specific activity equals total enzyme activity divided by total protein and usually rises as contaminants are removed.
25.3 Purification fold
Purification fold is calculated by dividing specific activity at a purification step by the initial specific activity.
25.4 Percentage yield
Percentage yield equals total activity remaining after a step divided by initial total activity, multiplied by 100.
25.5 Electrophoretic calibration
The logarithm of molecular mass can be related to migration distance or relative mobility for suitable protein standards in SDS-PAGE.
25.6 Method validation
Accuracy, precision, sensitivity, specificity and recovery must be considered when interpreting analytical results.
25.7 Complementary evidence
No single protein technique answers every question. Composition, sequence, mass, charge, structure and activity often require different but complementary methods.
Quantitative protein biochemistry requires distinguishing concentration, amount and activity. A fraction can contain less total protein yet be more useful because its target enzyme is enriched. Specific activity measures that enrichment, percentage yield tracks recovery, and purification fold describes improvement relative to the starting material.
Chapter Summary
- Protein concentration: Protein concentration can be estimated using absorbance or colorimetric assays.
- Specific activity: For an enzyme preparation, specific activity equals total enzyme activity divided by total protein and usually rises as contaminants are removed.
- Purification fold: Purification fold is calculated by dividing specific activity at a purification step by the initial specific activity.
- Percentage yield: Percentage yield equals total activity remaining after a step divided by initial total activity, multiplied by 100.
- Electrophoretic calibration: The logarithm of molecular mass can be related to migration distance or relative mobility for suitable protein standards in SDS-PAGE.
- Method validation: Accuracy, precision, sensitivity, specificity and recovery must be considered when interpreting analytical results.
- Complementary evidence: No single protein technique answers every question.
Review and Examination Questions
- Explain the major biochemical principles involved in techniques and quantitative reasoning in protein biochemistry.
- Identify the important molecules, enzymes or techniques described in this chapter.
- Describe the sequence of events or reactions where a pathway is involved.
- Explain the regulation or experimental basis of the topic.
- Compare two related concepts from the chapter.
- Predict the consequence of disrupting one important reaction or technique.
- Write a structured examination essay on techniques and quantitative reasoning in protein biochemistry.
- Construct a labelled pathway, table or experimental scheme appropriate to this topic.
Integrated Metabolic Problems and Worked Examples
Nitrogen Balance
Question: A student consumes protein containing 14 g nitrogen per day and loses 12 g nitrogen through urine, faeces and other routes. Determine nitrogen balance.
Formula: Nitrogen balance = nitrogen intake − nitrogen loss.
Solution: 14 − 12 = +2 g nitrogen/day.
Answer: +2 g/day, a positive nitrogen balance under the assumptions given.
Protein Purification
Question: An enzyme preparation begins with 5,000 units in 250 mg protein. A later fraction contains 3,000 units in 25 mg. Find initial and final specific activities, purification fold and percentage yield.
Solution: Initial specific activity = 5,000/250 = 20 U/mg. Final = 3,000/25 = 120 U/mg. Fold = 120/20 = 6. Yield = 3,000/5,000 × 100 = 60%.
Answer: 20 U/mg initially; 120 U/mg finally; 6-fold purification; 60% yield.
Carbon-Skeleton Reasoning
If degradation of an amino acid produces oxaloacetate, its carbon skeleton can support gluconeogenesis because oxaloacetate can enter the gluconeogenic sequence. If degradation produces only acetyl-CoA, those carbons cannot yield net glucose in humans. This is the biochemical basis of the glucogenic-versus-ketogenic classification.
Hyperammonaemia Reasoning
When urea synthesis is impaired, nitrogen cannot be disposed of efficiently. Ammonia rises, glutamine formation may increase, cerebral nitrogen and osmotic balance are disturbed, and central nervous system function can deteriorate. The exact biochemical pattern helps locate the defective urea-cycle step.
Protein Molecular-Mass Interpretation
Suppose a native protein behaves as approximately 120 kDa by size-exclusion chromatography but produces one 40 kDa band on reducing SDS-PAGE. A reasonable structural hypothesis is a homotrimer containing three similar 40 kDa subunits. This conclusion remains a hypothesis until supported by additional evidence because shape and anomalous migration can influence apparent masses.
Comprehensive Examination Revision
- Explain amino-acid structure, ionisation and classification and relate side-chain chemistry to protein function.
- Describe the peptide bond and explain its influence on protein conformation.
- Discuss primary, secondary, tertiary and quaternary protein structure.
- Explain the forces that stabilise proteins and the biochemical meaning of denaturation.
- Describe ubiquitin-proteasome and lysosomal/autophagic protein degradation.
- Discuss extraction, salting out, dialysis and isoelectric precipitation in protein isolation.
- Compare ion-exchange, size-exclusion, affinity and hydrophobic-interaction chromatography.
- Explain how a protein purification table is constructed and interpreted.
- Compare native PAGE, SDS-PAGE, isoelectric focusing and two-dimensional electrophoresis.
- Describe amino-acid analysis and explain why composition does not determine sequence.
- Discuss Edman degradation, selective cleavage and mass spectrometry in protein sequencing.
- Explain methods for molecular-weight determination and distinguish native mass from subunit mass.
- Describe protein biosynthesis from aminoacyl-tRNA formation through termination.
- Explain digestion and absorption of dietary proteins.
- Describe transamination and the role of pyridoxal phosphate.
- Explain oxidative deamination and the central role of glutamate in nitrogen metabolism.
- Discuss glutamine and alanine as nitrogen-transport molecules.
- Describe ammonia toxicity and mechanisms for safe nitrogen disposal.
- Write all reactions of the urea cycle and state their cellular compartments.
- Explain the energetic cost and regulation of the urea cycle.
- Discuss the relationship between the urea cycle and the TCA cycle.
- Differentiate glucogenic and ketogenic amino acids and state important examples.
- Discuss tetrahydrofolate-mediated one-carbon metabolism and the methionine cycle.
- Explain biosynthesis of major non-essential amino acids from central metabolic intermediates.
- Describe biological nitrogen fixation and assimilation of inorganic nitrogen.
- Discuss the formation of selected amino-acid derivatives including creatine, catecholamines, serotonin, histamine and nitric oxide.
- Explain biosynthesis and functions of polyamines.
- Discuss the biochemical basis of phenylketonuria, alkaptonuria, maple syrup urine disease and homocystinuria.
- Explain how urea-cycle defects produce hyperammonaemia.
- Integrate amino-acid metabolism in fed and fasting states, including the glucose-alanine cycle.
- Calculate specific activity, purification fold and percentage yield from a protein-purification data set.
Selected References
- National Universities Commission. Core Curriculum and Minimum Academic Standards for the Nigerian University System: Sciences. BCH 304: Metabolism of Amino Acids & Proteins.
- Nelson, D. L. and Cox, M. M. Lehninger Principles of Biochemistry.
- Berg, J. M., Tymoczko, J. L., Gatto, G. J. and Stryer, L. Biochemistry.
- Voet, D., Voet, J. G. and Pratt, C. W. Fundamentals of Biochemistry.
- Rodwell, V. W. et al. Harper's Illustrated Biochemistry.
- Devlin, T. M. Textbook of Biochemistry with Clinical Correlations.