01Core principlesThe concepts and mechanisms needed to understand the subject.
Biochemistry becomes clinically useful when reactions are placed in a person, organ and time. Flux through a pathway depends on substrate, cofactors, enzyme quantity, allosteric regulation, covalent modification and compartment. A high plasma concentration may reflect increased production, reduced clearance, redistribution or cell leakage. Conversely, normal concentration can coexist with abnormal flux if production and consumption change together. Before interpreting a marker, identify what was sampled, after how long without food, during which treatment and with what renal or hepatic function.
Glucose metabolism changes with state. After feeding, insulin favours cellular glucose uptake in responsive tissues, glycogen synthesis, glycolysis and lipid storage. Between meals, hepatic glycogenolysis supplies glucose; with longer fasting, gluconeogenesis uses lactate, glycerol and glucogenic amino-acid carbon. Red cells depend on glycolysis because they lack mitochondria and return lactate to liver through the Cori cycle. Pyruvate can become lactate to regenerate cytosolic NAD+ or enter mitochondria as acetyl-CoA. The direction depends on redox state, oxygen-linked mitochondrial capacity and tissue needs, not oxygen presence alone.
Fatty acids undergo mitochondrial beta-oxidation after activation and, for long-chain species, carnitine-dependent entry. Each cycle shortens the chain and generates acetyl-CoA plus reduced cofactors for oxidative phosphorylation. Liver converts excess acetyl-CoA to ketone bodies when carbohydrate availability or insulin action is low and oxaloacetate is directed toward gluconeogenesis. Peripheral tissues can oxidise ketones; liver cannot consume the ketones it makes. Fat cannot yield net glucose from even-chain fatty acids because acetyl-CoA carbons are lost through the cycle, although glycerol and odd-chain products contribute gluconeogenic substrate.
Amino-acid nitrogen must be handled separately from its carbon skeleton. Transamination moves amino groups, while oxidative deamination can release ammonia. Glutamine and alanine transport nitrogen between tissues. The hepatic urea cycle converts ammonia and aspartate nitrogen into urea at energetic cost, spanning mitochondrial and cytosolic reactions. Renal excretion completes disposal. Hyperammonaemia can therefore arise from hepatic dysfunction, shunting, excessive nitrogen load or inherited enzyme defects. Metabolic interpretation should trace the first blocked or overloaded step and predict upstream accumulation, downstream deficiency and alternative-route products.
Key points
- Metabolic pathways are regulated networks: substrate availability, enzyme activity, compartment and hormonal state determine flux more than memorising an isolated reaction list.
- Glycolysis converts glucose to pyruvate in cytosol and can generate ATP without oxygen; mitochondrial oxidation yields more ATP when oxygen delivery and respiratory machinery are adequate.
- Glycogen buffers short-term glucose supply, hepatic gluconeogenesis supports fasting plasma glucose, and adipose lipolysis plus hepatic ketogenesis become increasingly important with prolonged insulin deficiency or fasting.
- Acetyl-CoA links carbohydrate, fat and amino-acid metabolism but cannot provide net glucose in humans; anaplerotic reactions replenish tricarboxylic-acid-cycle intermediates.
- Ammonia generated by amino-acid metabolism is transported and converted to urea mainly in liver; hepatic failure or urea-cycle defects allow neurotoxic accumulation.
- Interpret a laboratory concentration only after checking specimen, units, timing, nutritional state, organ clearance and whether the marker reflects pool size, flux or cellular injury.
02Mechanisms and patternsImportant relationships and how to distinguish them.
A regulated step responds to energy state, substrate and hormones; pathway flux cannot be inferred solely from the amount of one enzyme.
Higher insulin-to-glucagon signalling promotes glucose use and storage while suppressing lipolysis, ketogenesis and hepatic glucose output.
Glycogen supplies early glucose, then gluconeogenesis and fat-derived fuel become more prominent as stores and hormonal signals change.
NADH and NAD+ balance influences lactate-pyruvate direction and mitochondrial oxidation, connecting oxygen-linked electron disposal to cytosolic reactions.
Amino-acid catabolism creates ammonia that must be transported, converted to urea and excreted; failure at any stage changes the biochemical pattern.
An intracellular enzyme in plasma may indicate cell injury and release, but tissue distribution, clearance and timing determine diagnostic usefulness.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
State and compartment map - Why
- Place each reaction in cytosol, mitochondrion, liver, muscle, adipose tissue or circulating blood.
- Interpretation and limitations
- A proposed pathway is impossible if its required enzyme or substrate is absent from that compartment, even when the reaction is chemically plausible.
- 02
Carbon tracing - Why
- Follow substrate carbon through glucose, pyruvate, acetyl-CoA and cycle intermediates.
- Interpretation and limitations
- Count what enters and leaves; carbon loss as carbon dioxide explains why even-chain fatty acids cannot create net glucose.
- 03
Redox accounting - Why
- Determine how NADH and FADH2 production links metabolism to electron transport.
- Interpretation and limitations
- Reduced cofactors must be reoxidised for pathways to continue; impaired disposal shifts reactions such as pyruvate toward lactate.
- 04
ATP balance - Why
- Compare direct substrate-level phosphorylation with oxidative ATP production and pathway energy costs.
- Interpretation and limitations
- State the convention used because shuttle and coupling assumptions alter exact totals; qualitative energy dependence is often more robust than a memorised number.
- 05
Upstream-downstream prediction - Why
- Infer the consequences of an enzyme block or organ failure.
- Interpretation and limitations
- Expect substrate or alternative metabolites to accumulate and products to fall, modified by parallel pathways, intake and excretion.
- 06
Pre-analytical review - Why
- Decide whether a biochemical result represents the patient's physiology reliably.
- Interpretation and limitations
- Sampling delay, haemolysis, posture, tourniquet, feeding, drugs and units can distort a result before disease mechanisms are considered.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked exampleExplain rising lactate during impaired oxidative metabolismA model tissue has ongoing glycolysis, adequate glucose, impaired mitochondrial reoxidation of NADH and a rising cytosolic NADH-to-NAD+ ratio; no additional clinical signs are assumed.+
- 1Glycolysis requires oxidised NAD+ at the glyceraldehyde-3-phosphate step, so continued ATP generation needs a route that regenerates NAD+.
- 2Impaired mitochondrial redox disposal prevents normal transfer of reducing equivalents into oxidative phosphorylation and raises cytosolic reduction pressure.
- 3Lactate dehydrogenase converts pyruvate to lactate while oxidising NADH back to NAD+, allowing glycolysis to continue temporarily.
- 4The final predicted pattern is increased lactate relative to pyruvate with limited ATP yield compared with complete mitochondrial oxidation.
- 5Verify the mechanism by checking redox balance: if NAD+ were not regenerated, glycolytic flux would stall despite abundant glucose.
02Carbon reasoningTest whether a fuel can make net glucoseAn even-chain fatty acid is completely converted to acetyl-CoA and enters the tricarboxylic acid cycle.+
- 1Trace each acetyl-CoA as two carbon atoms entering the cycle.
- 2Recognise that cycle turnover releases carbon dioxide while regenerating oxaloacetate rather than expanding its pool from acetyl-CoA.
- 3Identify that gluconeogenesis requires a net supply of suitable carbon intermediates.
- 4Conclude that even-chain fatty-acid carbon cannot yield net glucose, while glycerol or odd-chain products can contribute.
03Enzyme-block reasoningPredict a urea-cycle defect patternA mitochondrial urea-cycle step cannot process its substrate effectively.+
- 1Locate the failed reaction and identify the nitrogen-containing substrate immediately upstream.
- 2Predict reduced forward urea formation and diversion into available alternative compounds.
- 3Connect retained nitrogen to elevated ammonia and neurological toxicity risk.
- 4Check how feeding state, catabolism, hepatic function and renal removal could modify measured concentrations.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Check pathway answers by tracing atoms or electrons rather than relying on a remembered arrow alone.
- State nutritional and hormonal context whenever predicting direction of glycogen, gluconeogenic, lipolytic or ketone flux.
- Review units and specimen conditions before interpreting a concentration as production, clearance or tissue damage.
- Use a blocked-step table—substrate, product, alternative route and organ compartment—to verify metabolic predictions.
- Recalculate energy claims when the question changes shuttle assumptions, substrate length or whether activation costs are included.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Concentration is not flux
A stable blood level can hide simultaneous high production and high removal; dynamic tests or contextual markers may reveal the turnover.
Lactate is a circulating fuel
Lactate is not merely waste: it transfers carbon and redox equivalents and can be oxidised or recycled to glucose in other tissues.
Acetyl-CoA is a junction
It receives carbon from several fuels and supports oxidation, lipid synthesis or ketogenesis, but its fate depends on energy and oxaloacetate availability.
Compartment protects regulation
Separating opposing pathways between organelles and tissues prevents futile cycling and permits selective hormonal control.
Biochemical patterns evolve
Timing after injury, fasting or treatment changes substrate, marker release and clearance, so one sample represents a phase rather than a permanent state.
07Common pitfallsFrequent interpretation and management errors.
- 01
Memorising a pathway in one direction without considering feeding, hormones, energy charge and reversible versus regulated steps.
- 02
Treating lactate production as proof that no oxygen is present, despite redox and clearance mechanisms that can raise it.
- 03
Claiming that all fatty-acid carbon can become glucose because acetyl-CoA enters the citric-acid cycle.
- 04
Interpreting an enzyme concentration without accounting for tissue distribution, sampling time and organ clearance.
- 05
Predicting a metabolic block without identifying its compartment, immediate substrate and plausible alternative routes.