Biochemistry: kinetics, cofactors and metabolic logic
What is tested
Biochemistry questions cluster around enzyme kinetics, the pathways that generate and store energy, cofactor–vitamin relationships, nitrogen disposal, nucleotide and lipid metabolism, and the molecular biology of replication, transcription and translation.
Core rules
- Michaelis–Menten. At [S] = Km the velocity is half Vmax; a low Km means high affinity.
- Inhibition patterns. Competitive: Km rises, Vmax unchanged. Non-competitive: Vmax falls, Km unchanged. Uncompetitive: both fall.
- Cofactors. Thiamine pyrophosphate for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and transketolase; NAD and NADP from niacin; FAD and FMN from riboflavin; pyridoxal phosphate for transamination and ALA synthase; biotin for carboxylases; cobalamin for methionine synthase and methylmalonyl-CoA mutase.
- Rate-limiting steps are the regulated steps: phosphofructokinase-1 in glycolysis, isocitrate dehydrogenase in the TCA cycle, carbamoyl phosphate synthetase I in the urea cycle.
Worked example
An inhibitor raises the Km for substrate from 2 mM to 6 mM while Vmax stays at 100 units/mg. Because only Km changed, the inhibition is competitive and can be overcome by raising the substrate concentration. Had Vmax fallen to 50 units/mg with Km unchanged, the pattern would be non-competitive instead.
Traps that cost marks
- Saying a high Km means high affinity - it means the opposite.
- Mixing the vitamin with the cofactor name, for example writing that riboflavin forms NAD.
- Forgetting that the urea cycle costs ATP and takes its two nitrogen atoms from ammonia and aspartate.
- Quoting a deficiency without the clinical clue: thiamine deficiency gives beriberi and Wernicke encephalopathy, not pellagra.