Biochemistry Concepts

Biochemistry

Rate-limiting enzymes, vitamins, urea cycle, molecular techniques, glycogen and lysosomal storage diseases, lipoproteins and porphyrias — taught with the logic behind them.

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Name the committed step of each major pathway and its physiological regulator.

Match a vitamin deficiency to its enzyme defect and clinical syndrome.

Explain hyperammonaemia from the position of the urea cycle block.

Choose the right molecular technique for a given diagnostic question.

Predict the biochemical picture of each glycogen storage disease from the missing enzyme.

Match lysosomal storage disorders to their enzyme and distinguishing signs.

Describe the lipoprotein pathways and the familial dyslipidaemias.

Explain the porphyrias and lead poisoning from the haem synthesis pathway.

1Rate-limiting enzymes and metabolic control6 min read+15 XP

Glycolysis, with PFK-1 as the committed stepWikimedia Commons ↗

Most metabolism questions reduce to naming the committed step and its regulator.

Why this is true

A pathway is controlled at the first irreversible step, because regulating anything later would waste intermediates already committed. That step is almost always allosterically sensitive to the pathway's end product or to the cell's energy charge. Phosphofructokinase-1 is the textbook case: ATP inhibits it (energy is plentiful), AMP activates it (energy is scarce), and fructose-2,6-bisphosphate overrides both to link glycolysis to insulin signalling. Once you know the regulator, you can predict the direction of flux in any hormonal state without memorising a table.

Key points

Fed versus fasting state

State	Dominant hormone	Pathways on	Pathways off
Fed	Insulin	Glycolysis, glycogenesis, lipogenesis	Gluconeogenesis, lipolysis
Fasting	Glucagon	Glycogenolysis, gluconeogenesis, lipolysis	Glycolysis, lipogenesis
Starvation	Glucagon + cortisol	Ketogenesis, proteolysis	Glycogenolysis (stores gone)

Common traps

Fructose-2,6-bisphosphate is not an intermediate of glycolysis — it is purely a regulator.
Hexokinase is inhibited by its product; glucokinase (liver) is not and has a high Km, so the liver only takes up glucose when it is abundant.

Clinical case

A child presents with microcytic anaemia and basophilic stippling. Blood lead is elevated.

High-yield

Lead inhibits ALA dehydratase and ferrochelatase → basophilic stippling on the smear.