1.1 Meaning of carbohydrate metabolism

Carbohydrate metabolism is the integrated set of biochemical reactions by which cells digest, transform, store, mobilise, oxidise and synthesize carbohydrates. It links nutrient availability with ATP production, biosynthesis, redox balance and maintenance of blood and cellular glucose.

1.2 Chemical foundation

Carbohydrates contain carbon, hydrogen and oxygen and range from simple monosaccharides to complex polysaccharides. Their carbonyl and hydroxyl groups permit cyclisation, glycosidic bond formation, oxidation, reduction and phosphorylation.

1.3 Monosaccharides

Glucose is a major metabolic fuel, but fructose, galactose and other monosaccharides enter metabolism through enzyme-specific routes. Aldoses and ketoses differ in carbonyl position, while stereochemistry strongly influences enzyme recognition.

1.4 Metabolic roles

Carbohydrates provide immediate fuel, stored energy, biosynthetic carbon, ribose for nucleotides, reducing power through the pentose phosphate pathway and structural materials such as cellulose and glycosaminoglycans.

1.5 Catabolism and anabolism

Catabolic pathways release usable energy and intermediates from carbohydrate breakdown. Anabolic pathways consume energy and reducing equivalents to construct glucose, glycogen and other carbohydrate-containing molecules.

1.6 Compartmentation

Pathways occur in defined cellular locations. Glycolysis is cytosolic, whereas pyruvate oxidation and the TCA cycle in eukaryotic cells are primarily mitochondrial. Compartmentation permits regulation and prevents incompatible processes from operating indiscriminately.

2.1 Disaccharides

Disaccharides consist of two monosaccharide residues joined by a glycosidic bond. Maltose contains glucose residues, lactose contains galactose and glucose, and sucrose contains glucose and fructose.

2.2 Reducing properties

A sugar is reducing when it retains a free anomeric carbon capable of opening to a reactive carbonyl form. Glycosidic linkage at both anomeric carbons, as in sucrose, removes this reducing property.

2.3 Oligosaccharides

Oligosaccharides contain a small number of monosaccharide residues and commonly occur attached to proteins and lipids, where they participate in recognition, trafficking and cell-surface interactions.

2.4 Starch

Plant starch contains amylose, which is mainly linear with alpha(1→4) glucose linkages, and amylopectin, which contains alpha(1→4) chains plus alpha(1→6) branch points.

2.5 Glycogen

Glycogen is a highly branched glucose-storage polysaccharide in animals and many microorganisms. Frequent branching creates many non-reducing ends, allowing rapid synthesis and mobilisation.

2.6 Cellulose

Cellulose consists of beta(1→4)-linked glucose residues. Its extended chains form strong fibres through hydrogen bonding. Humans lack cellulase and therefore cannot digest cellulose to glucose.

2.7 Functional comparison

The biological function of a polysaccharide follows from monomer identity, linkage type, branching, molecular size and association with other molecules.

3.1 Isolation

Polysaccharide isolation begins with selection and disruption of biological material, followed by removal of insoluble debris and extraction under conditions suited to the target polymer.

3.2 Removal of contaminants

Proteins, lipids, pigments, nucleic acids and small molecules can interfere with analysis and are removed by precipitation, solvent partition, dialysis, enzymatic treatment or chromatographic methods as appropriate.

3.3 Precipitation

Some polysaccharides can be concentrated by changing solvent composition, ionic conditions or temperature. Precipitation is useful for bulk recovery but rarely proves chemical purity.

3.4 Chromatographic purification

Size-exclusion, ion-exchange and affinity-based approaches separate polysaccharides according to molecular size, charge or specific interactions.

3.5 Molecular-weight determination

Polysaccharides are heterogeneous macromolecules, so molecular mass may be reported as an average distribution. Size-exclusion chromatography, light scattering and related physical methods can provide estimates.

3.6 Monosaccharide composition

Hydrolysis followed by chromatographic or spectrometric analysis can identify constituent monosaccharides and their relative amounts.

3.7 Linkage analysis

Determining which carbon atoms participate in glycosidic bonds is essential for reconstructing polysaccharide architecture. Chemical derivatisation, hydrolysis and instrumental analysis can reveal linkage patterns.

3.8 Spectroscopic methods

NMR spectroscopy and mass spectrometry provide structural information about residue identity, sequence, branching and linkage environment when interpreted with complementary evidence.

3.9 Purity and identity

A single analytical signal does not necessarily establish a complete polysaccharide structure; robust characterisation integrates composition, molecular size, linkage and spectroscopic evidence.

4.1 Dietary substrates

Major digestible carbohydrates include starch, sucrose and lactose. Cellulose and several other fibres resist human digestive enzymes and reach the large intestine.

4.2 Salivary digestion

Salivary alpha-amylase begins hydrolysis of internal alpha(1→4) linkages in starch. Its action is limited by food transit and the acidic gastric environment.

4.3 Pancreatic digestion

Pancreatic alpha-amylase continues starch digestion in the small intestine, generating maltose, maltotriose and alpha-limit dextrins.

4.4 Brush-border enzymes

Maltase-glucoamylase, sucrase-isomaltase, lactase and related activities convert oligosaccharides and disaccharides into absorbable monosaccharides.

4.5 Glucose and galactose uptake

At the intestinal apical membrane, sodium-dependent transport supports uptake of glucose and galactose, while facilitated transport contributes to exit across the basolateral membrane.

4.6 Fructose uptake

Fructose is absorbed mainly by facilitated transport through a transporter with different substrate specificity from the sodium-glucose cotransporter.

4.7 Fibre

Undigested carbohydrate can be fermented by colonic microorganisms to products including short-chain fatty acids, while also affecting stool bulk and intestinal physiology.

4.8 Clinical significance

Deficiency of a digestive enzyme such as lactase leaves substrate in the intestinal lumen, where osmotic effects and microbial fermentation can produce symptoms.

5.1 Purpose

Glycolysis converts glucose to pyruvate through ten enzyme-catalysed reactions in the cytosol. It can generate ATP without requiring molecular oxygen directly.

5.2 Glucose phosphorylation

Hexokinase or glucokinase converts glucose to glucose-6-phosphate using ATP. Phosphorylation traps the carbon skeleton in the cell and prepares it for metabolism.

5.3 Isomerisation

Phosphoglucose isomerase converts glucose-6-phosphate to fructose-6-phosphate, repositioning the carbonyl group for later symmetrical cleavage.

5.4 Committed phosphorylation

Phosphofructokinase-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate using ATP. This strongly regulated reaction commits carbon to the glycolytic sequence.

5.5 Cleavage

Aldolase splits fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

5.6 Triose interconversion

Triose phosphate isomerase converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate, so both three-carbon fragments proceed through the payoff phase.

5.7 Energy investment

Two ATP equivalents are consumed per glucose before energy-generating reactions begin.

6.1 Glyceraldehyde-3-phosphate oxidation

Glyceraldehyde-3-phosphate dehydrogenase oxidises its substrate while reducing NAD+ to NADH and incorporating inorganic phosphate into a high-energy intermediate.

6.2 First substrate-level phosphorylation

Phosphoglycerate kinase transfers phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP.

6.3 Rearrangement

Phosphoglycerate mutase relocates phosphate from carbon 3 to carbon 2.

6.4 Formation of PEP

Enolase removes water from 2-phosphoglycerate to form phosphoenolpyruvate, a compound with high phosphoryl-transfer potential.

6.5 Second substrate-level phosphorylation

Pyruvate kinase transfers phosphate from PEP to ADP, generating ATP and pyruvate.

6.6 Net yield

Per glucose, glycolysis consumes two ATP and produces four ATP, giving a net gain of two ATP. It also produces two NADH and two pyruvate.

6.7 Overall equation

A simplified net expression is: glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O.

6.8 Fate of NADH

Continued glycolysis requires regeneration of NAD+. Under aerobic conditions cytosolic reducing equivalents can be transferred toward mitochondrial oxidation; under anaerobic conditions fermentation regenerates NAD+ through reduction of an organic acceptor.

7.1 Control points

Major regulatory reactions are catalysed by hexokinase/glucokinase, phosphofructokinase-1 and pyruvate kinase because these steps are strongly displaced from equilibrium in cells.

7.2 Hexokinase

Many hexokinase isoenzymes have high affinity for glucose and can be inhibited by glucose-6-phosphate, coupling glucose phosphorylation to downstream use.

7.3 Glucokinase

Liver glucokinase has kinetic and regulatory properties suited to handling increased glucose availability after a carbohydrate-rich meal.

7.4 PFK-1

Phosphofructokinase-1 is a major control point. High ATP and selected metabolic signals reduce its activity, whereas AMP and fructose-2,6-bisphosphate favour glycolytic flux.

7.5 Pyruvate kinase

Pyruvate kinase responds to metabolic signals and can be hormonally regulated in liver, helping coordinate glycolysis with whole-body nutritional state.

7.6 Reciprocal regulation

Cells avoid wasteful simultaneous operation of glycolysis and gluconeogenesis by regulating opposing irreversible steps in coordinated directions.

7.7 Flux concept

Pathway regulation is distributed. No single enzyme independently determines metabolic flux under all physiological conditions.

8.1 Pyruvate as a branch point

Pyruvate can be reduced to lactate, converted to acetyl-CoA, carboxylated to oxaloacetate or used in other biosynthetic reactions depending on tissue and metabolic state.

8.2 Lactate formation

Lactate dehydrogenase reduces pyruvate to lactate while oxidising NADH to NAD+, permitting glycolysis to continue when mitochondrial oxidation cannot meet cytosolic NAD+ regeneration needs.

8.3 Pyruvate dehydrogenase

The mitochondrial pyruvate dehydrogenase complex converts pyruvate to acetyl-CoA, producing CO2 and NADH. The reaction links glycolysis to the TCA cycle.

8.4 Cofactor requirement

The complex uses thiamine pyrophosphate, lipoamide, coenzyme A, FAD and NAD+, illustrating integration of vitamin-derived cofactors in metabolism.

8.5 Regulation

Pyruvate dehydrogenase is inhibited when energy-rich products accumulate and is regulated through reversible phosphorylation and metabolic signals.

8.6 Anaplerosis

Pyruvate carboxylase converts pyruvate to oxaloacetate, replenishing TCA-cycle intermediates and supporting gluconeogenesis.

9.1 Central role

The TCA cycle oxidises acetyl groups to carbon dioxide while conserving energy as NADH, FADH2 and GTP or ATP. It also supplies intermediates for biosynthesis.

9.2 Citrate formation

Citrate synthase condenses acetyl-CoA with oxaloacetate to form citrate.

9.3 Isomerisation

Aconitase converts citrate through cis-aconitate to isocitrate.

9.4 First oxidative decarboxylation

Isocitrate dehydrogenase converts isocitrate to alpha-ketoglutarate, producing CO2 and NADH.

9.5 Second oxidative decarboxylation

Alpha-ketoglutarate dehydrogenase converts alpha-ketoglutarate to succinyl-CoA, producing another CO2 and NADH.

9.6 Substrate-level phosphorylation

Succinyl-CoA synthetase uses thioester energy to form GTP or ATP while producing succinate.

9.7 FAD reduction

Succinate dehydrogenase oxidises succinate to fumarate and reduces FAD to FADH2. The enzyme is also part of the mitochondrial electron-transport chain.

9.8 Hydration

Fumarase hydrates fumarate to malate.

9.9 Oxaloacetate regeneration

Malate dehydrogenase oxidises malate to oxaloacetate, producing NADH and completing the cycle.

9.10 Yield per acetyl-CoA

One turn yields three NADH, one FADH2, one GTP or ATP and two CO2, while oxaloacetate is regenerated.

9.11 Amphibolic nature

The TCA cycle is amphibolic because it participates in both degradation and biosynthesis. Intermediates withdrawn for biosynthesis must be replenished by anaplerotic reactions.

10.1 Metabolic convergence

Acetyl-CoA derived from carbohydrate, fatty-acid and some amino-acid metabolism enters the TCA cycle, making the pathway a major point of convergence.

10.2 Energy status

High ratios of ATP/ADP and NADH/NAD+ generally signal adequate energy and suppress key oxidative steps, whereas increased ADP favours energy production.

10.3 Citrate synthase control

Citrate synthase responds to substrate availability and inhibitory metabolic signals, linking entry of acetyl units to cycle status.

10.4 Isocitrate dehydrogenase control

Isocitrate dehydrogenase is activated or inhibited by energy-related signals and contributes importantly to cycle flux.

10.5 Alpha-ketoglutarate dehydrogenase

This enzyme is inhibited by products and high-energy signals and is sensitive to availability of its substrates and cofactors.

10.6 Biosynthetic withdrawals

Citrate contributes carbon for lipid synthesis, alpha-ketoglutarate and oxaloacetate contribute to amino-acid metabolism, and succinyl-CoA participates in porphyrin biosynthesis.

10.7 Anaplerotic reactions

Anaplerotic reactions replenish intermediates removed for biosynthesis. Pyruvate carboxylase formation of oxaloacetate is particularly important in mammals.

11.1 Purpose

The pentose phosphate pathway oxidises glucose-6-phosphate to generate NADPH and produces ribose-5-phosphate and interchangeable sugar phosphates.

11.2 Oxidative phase

Glucose-6-phosphate is oxidised and decarboxylated, yielding ribulose-5-phosphate, CO2 and NADPH.

11.3 NADPH

NADPH supplies reducing power for fatty-acid and cholesterol synthesis, maintenance of reduced glutathione and several reductive or protective reactions.

NADPH should be distinguished from NADH. Both carry reducing equivalents, but cellular enzyme specificity and compartmentation give them different dominant roles: NADH is strongly associated with energy-yielding oxidation, whereas NADPH is strongly associated with reductive biosynthesis and antioxidant defence.

11.4 Non-oxidative phase

Sugar phosphates are rearranged through reversible carbon-transfer reactions involving transketolase and transaldolase.

11.5 Ribose requirement

Cells requiring nucleotides can direct pathway intermediates toward ribose-5-phosphate.

11.6 Reducing-power requirement

Cells needing more NADPH than ribose can recycle carbon through non-oxidative reactions and glycolytic intermediates.

11.7 Transketolase

Transketolase transfers two-carbon units and requires thiamine pyrophosphate, linking the pathway to vitamin B1 biochemistry.

11.8 Physiological significance

The pathway is especially important in tissues engaged in reductive biosynthesis and in cells that require strong antioxidant defence.

12.1 Definition

Gluconeogenesis is synthesis of glucose from non-carbohydrate precursors, especially lactate, glycerol and glucogenic amino-acid carbon skeletons.

12.2 Physiological role

It helps maintain glucose availability during fasting or prolonged exercise for tissues with substantial glucose requirements.

12.3 Relationship to glycolysis

Most reversible glycolytic reactions are used in reverse, but the three strongly irreversible glycolytic steps require distinct bypass reactions.

12.4 Pyruvate carboxylase

Pyruvate is carboxylated to oxaloacetate in mitochondria using ATP and biotin.

12.5 PEP carboxykinase

Oxaloacetate carbon is converted toward phosphoenolpyruvate through phosphoenolpyruvate carboxykinase with GTP expenditure and CO2 release.

12.6 Fructose-1,6-bisphosphatase

This enzyme bypasses phosphofructokinase-1 by hydrolysing fructose-1,6-bisphosphate to fructose-6-phosphate.

12.7 Glucose-6-phosphatase

Glucose-6-phosphate is hydrolysed to free glucose in tissues capable of exporting glucose.

12.8 Energy cost

Gluconeogenesis is energetically expensive, consuming ATP/GTP and reducing equivalents. This cost helps make simultaneous unrestricted glycolysis and gluconeogenesis undesirable.

12.9 Regulation

Hormonal and allosteric mechanisms coordinate gluconeogenesis with glycolysis and whole-body nutritional state.

13.1 Definition

Glycogenesis is the synthesis of glycogen from glucose-derived units for compact, rapidly mobilisable carbohydrate storage.

13.2 Glucose activation

Glucose-6-phosphate is converted to glucose-1-phosphate, which reacts with UTP to form UDP-glucose, an activated glucosyl donor.

13.3 Glycogenin

Glycogenin provides a protein primer on which initial glucose residues can be assembled.

13.4 Glycogen synthase

Glycogen synthase extends chains by forming alpha(1→4) glycosidic bonds using UDP-glucose.

13.5 Branching enzyme

Branching enzyme transfers chain segments to create alpha(1→6) branch points.

13.6 Importance of branching

Branching increases solubility and creates numerous non-reducing ends, allowing rapid simultaneous addition or removal of glucose residues.

13.7 Regulation

Glycogen synthase is controlled by phosphorylation state and allosteric signals, coordinating storage with hormonal and energy status.

14.1 Definition

Glycogenolysis is the mobilisation of glycogen to release phosphorylated glucose units for metabolism or, in liver, contribution to blood glucose.

14.2 Glycogen phosphorylase

Phosphorylase cleaves alpha(1→4) bonds by phosphorolysis, producing glucose-1-phosphate from non-reducing ends.

14.3 Debranching

Debranching enzyme resolves branch structures through transferase and alpha(1→6)-glucosidase activities.

14.4 Phosphoglucomutase

Glucose-1-phosphate is converted to glucose-6-phosphate.

14.5 Liver

Liver contains glucose-6-phosphatase and can release free glucose into blood, supporting systemic glucose homeostasis.

14.6 Muscle

Skeletal muscle uses glucose-6-phosphate largely within its own glycolytic metabolism because it does not function as a major exporter of free glucose.

14.7 Hormonal regulation

Glucagon strongly promotes hepatic glycogen mobilisation during low blood glucose, while adrenaline can stimulate glycogen breakdown in liver and muscle according to physiological demand.

14.8 Reciprocal control

Phosphorylation cascades tend to coordinate activation of glycogen breakdown with inhibition of glycogen synthesis, reducing futile cycling.

15.1 Definition

The Cori cycle describes transfer of lactate from tissues producing it to the liver, where lactate carbon is used for gluconeogenesis and newly formed glucose can return to peripheral tissues.

15.2 Muscle contribution

During intense activity, glycolytic ATP demand can exceed oxidative handling of pyruvate and cytosolic NADH, favouring lactate formation and NAD+ regeneration.

15.3 Red blood cells

Erythrocytes lack mitochondria and therefore depend on glycolysis, producing lactate continuously.

15.4 Liver role

Hepatic lactate dehydrogenase converts lactate to pyruvate, which can enter gluconeogenesis.

15.5 Energy transfer

The cycle shifts part of the energetic cost of glucose regeneration to the liver. Conversion of two lactate molecules back to glucose requires substantial ATP/GTP expenditure.

15.6 Physiological meaning

Lactate is not simply a waste product; it is a circulating metabolic intermediate that can be oxidised or used for gluconeogenesis.

16.1 Dietary fructose

Fructose occurs naturally in fruits and honey and is also obtained from sucrose after intestinal hydrolysis.

16.2 Hepatic pathway

In liver, fructokinase converts fructose to fructose-1-phosphate, and aldolase B cleaves it to dihydroxyacetone phosphate and glyceraldehyde.

16.3 Entry into central metabolism

The triose products can enter glycolysis, gluconeogenesis or lipid-related pathways depending on nutritional and hormonal state.

16.4 Extrahepatic route

Hexokinase can phosphorylate fructose to fructose-6-phosphate in some tissues, although glucose is often the preferred substrate.

16.5 Regulatory implication

The hepatic fructose-1-phosphate route enters glycolytic metabolism below the phosphofructokinase-1 control step, giving it a different regulatory context from glucose.

16.6 Essential fructosuria

Deficiency of fructokinase can cause a generally benign inability to efficiently trap fructose in the hepatic pathway.

16.7 Hereditary fructose intolerance

Aldolase B deficiency causes accumulation of fructose-1-phosphate and can disrupt phosphate and energy metabolism after fructose ingestion; this is metabolically distinct from essential fructosuria.

17.1 Overview

The polyol pathway converts glucose to sorbitol and then, in tissues with sufficient sorbitol dehydrogenase activity, to fructose.

17.2 Aldose reductase

Aldose reductase reduces glucose to sorbitol using NADPH.

17.3 Sorbitol dehydrogenase

Sorbitol dehydrogenase oxidises sorbitol to fructose using NAD+. Its activity differs among tissues.

17.4 Hyperglycaemia

When intracellular glucose is high, increased flux through aldose reductase can increase sorbitol production and consume NADPH.

17.5 Osmotic effects

Sorbitol can accumulate where its further metabolism or diffusion is limited, contributing to osmotic stress.

17.6 Redox effects

Consumption of NADPH can influence antioxidant capacity because NADPH is needed to maintain reduced glutathione through glutathione reductase.

17.7 Clinical relevance

Increased polyol-pathway flux has been implicated in tissue complications associated with chronic hyperglycaemia, although disease mechanisms are multifactorial.

18.1 Definition

The glyoxylate cycle is a modified TCA-related pathway that permits net conversion of acetyl-CoA carbon into four-carbon intermediates without losing both acetyl carbons as CO2.

18.2 Occurrence

It is found in plants, many bacteria, fungi and some other organisms but is absent as a complete pathway in humans.

18.3 Isocitrate lyase

Isocitrate lyase cleaves isocitrate to succinate and glyoxylate, bypassing an oxidative decarboxylation step of the TCA cycle.

18.4 Malate synthase

Malate synthase condenses glyoxylate with another acetyl-CoA to form malate.

18.5 Net carbon conservation

By bypassing the CO2-producing steps, organisms can use acetyl-CoA derived from fatty acids or acetate to generate carbon skeletons for carbohydrate synthesis.

18.6 Seed germination

In germinating oil-rich seeds, the cycle contributes to conversion of stored lipid carbon into carbohydrate needed before photosynthesis is fully established.

18.7 Metabolic integration

Succinate and malate connect the cycle with mitochondrial metabolism and gluconeogenic processes.

19.1 Purpose

The Calvin-Benson cycle fixes inorganic carbon dioxide into organic carbon in photosynthetic organisms, using ATP and NADPH generated by the light reactions.

19.2 Carboxylation

Ribulose-1,5-bisphosphate carboxylase/oxygenase, Rubisco, adds CO2 to ribulose-1,5-bisphosphate, producing an unstable six-carbon intermediate that yields two 3-phosphoglycerate molecules.

19.3 Reduction

ATP and NADPH are used to convert 3-phosphoglycerate toward glyceraldehyde-3-phosphate.

19.4 Product formation

A fraction of triose phosphate exits the cycle and contributes to synthesis of carbohydrates and other cellular compounds.

19.5 Regeneration

Most triose-phosphate carbon is rearranged through a sequence of reactions to regenerate ribulose-1,5-bisphosphate.

19.6 Energy requirement

Carbon fixation is energetically demanding because reduction of CO2 requires both ATP and reducing power.

19.7 Rubisco oxygenase activity

Rubisco can also react with O2, initiating photorespiration and reducing carbon-fixation efficiency under some conditions.

19.8 Relationship to metabolism

The Calvin cycle is anabolic and should not be confused with the TCA cycle, which has a major oxidative role despite also providing biosynthetic intermediates.

20.1 Fed state

After carbohydrate intake, increased insulin signalling favours glucose uptake in responsive tissues, glycolysis, glycogen synthesis and other pathways of nutrient storage and utilisation.

20.2 Fasting state

Falling insulin and increased counter-regulatory signals promote hepatic glycogenolysis and gluconeogenesis to maintain circulating glucose.

20.3 Allosteric control

ATP, AMP, citrate, acetyl-CoA and fructose-2,6-bisphosphate provide rapid information about cellular energy and metabolite status.

20.4 Covalent modification

Phosphorylation and dephosphorylation alter activities of key enzymes in glycogen and carbohydrate metabolism.

20.5 Gene expression

Longer-term nutritional and hormonal changes alter abundance of metabolic enzymes through transcriptional and post-transcriptional mechanisms.

20.6 Tissue specialization

Liver buffers blood nutrient levels, skeletal muscle supports contraction, adipose tissue stores and releases fuels, brain has substantial glucose demand, and erythrocytes depend completely on glycolysis for ATP.

20.7 Pathway integration

Glycolysis, gluconeogenesis, glycogen metabolism, the pentose phosphate pathway and the TCA cycle share metabolites but are regulated so that carbon is directed according to physiological need.

21.1 Lactose intolerance

Reduced intestinal lactase activity prevents complete lactose hydrolysis. Unabsorbed lactose retains water osmotically and is fermented by intestinal microorganisms.

21.2 Galactosaemia

Defects in enzymes of galactose metabolism can cause accumulation of galactose-related metabolites. Classical galactosaemia is associated with deficiency of galactose-1-phosphate uridylyltransferase.

21.3 Hereditary fructose intolerance

Aldolase B deficiency disrupts fructose metabolism and can cause serious metabolic effects after fructose or sucrose exposure.

21.4 Glycogen storage diseases

Inherited defects in glycogen synthesis or breakdown produce tissue-specific abnormalities depending on the affected enzyme and its expression in liver, muscle or other tissues.

21.5 Pyruvate dehydrogenase deficiency

Reduced conversion of pyruvate to acetyl-CoA can divert pyruvate toward lactate and impair oxidative energy metabolism.

21.6 Glucose-6-phosphate dehydrogenase deficiency

Reduced G6PD activity limits NADPH generation through the oxidative pentose phosphate pathway, particularly affecting erythrocyte protection against oxidative stress.

21.7 Diabetes mellitus

Diabetes mellitus involves chronic hyperglycaemia resulting from inadequate insulin secretion, impaired insulin action or both. The metabolic consequences extend beyond carbohydrate metabolism to lipid and protein metabolism.

21.8 Hypoglycaemia

Abnormally low blood glucose can arise through several mechanisms, including excessive glucose utilisation, impaired hepatic glucose production, hormonal disturbances or medication effects.

21.9 Metabolic reasoning

A metabolic disorder should be understood by identifying the blocked or altered reaction, predicting substrate accumulation and product deficiency, and then connecting these biochemical changes to tissue physiology.

22.1 Measurement strategy

Biochemical investigation may quantify substrates, products, enzyme activities, isotopic fluxes or pathway intermediates. The method chosen must answer the biological question.

22.2 Glucose assays

Enzymatic assays can exploit glucose oxidase or hexokinase-linked reactions to generate measurable products proportional to glucose concentration.

22.3 Chromatography

Chromatographic methods separate sugars and sugar derivatives based on chemical interactions with stationary and mobile phases.

22.4 Spectrophotometry

Coupled enzyme assays often monitor NADH or NADPH because these cofactors absorb strongly near 340 nm in their reduced forms.

22.5 Isotopic tracers

Stable or radioactive isotopes can trace movement of carbon through pathways, distinguishing concentration from actual metabolic flux.

22.6 Metabolomics

Modern mass spectrometry and NMR approaches can measure many metabolites simultaneously, providing broad views of metabolic state.

22.7 Interpretation

Metabolite concentration is not identical to pathway flux. A metabolite can accumulate because its production increases, its consumption falls, transport changes or compartmentation is altered.

ATP Accounting in Glycolysis

TCA Product Accounting

Glycogen Branching Reasoning

Metabolic Block Reasoning

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