Biochemistry
Core framework
Biochemistry questions cluster around metabolism, its regulation, and the diseases caused by enzyme defects. Master glycolysis, the citric acid cycle, oxidative phosphorylation, gluconeogenesis, glycogen metabolism, the pentose phosphate pathway, fatty acid oxidation and ketogenesis, amino acid metabolism and the urea cycle, and nucleotide metabolism. Then attach vitamins and cofactors to the reactions that require them, and attach enzyme deficiencies to the accumulated metabolite.
Concepts you must own
Rate-limiting enzymes and their allosteric and hormonal regulation. Which pathways are active in the fed versus fasted state. The cofactors derived from vitamins. The metabolic consequences of a blocked step: accumulation upstream, deficiency downstream.
Traps
- Deficiency versus accumulation: identify which side of the block produces the clinical picture.
- Tissue specificity: the same enzyme deficiency may spare one tissue because of an isoenzyme.
- Fasting state: do not apply fed-state regulation to a fasting stem.
Worked rule of thumb
For any enzyme defect, trace the pathway in the direction of flow. Substrate and its byproducts accumulate proximal to the block; products distal to the block are deficient. In a glycogen storage disease, this tells you whether hypoglycemia, hepatomegaly, or lactate elevation predominates, which usually matches the answer option.