Learn / Metabolism
Metabolic pathways
Every FMK pathway as a clickable map. Pick one pathway to hammer it — next step, enzyme, regulator, cofactor, and the facts that get tested (NADPH and who needs it, rate-limiting steps, deficiencies) — or set it to all pathways for a mixed drill. Every answer says which enzyme and which pathway.
Glycogen synthesis & breakdown
Limit dextrin (4 residues from branch)
Made from Branched glycogen by Glycogen phosphorylase ★
Glycogen phosphorylase ★ rate-limiting — Glycogen synthesis & breakdown
Cofactors: PLP (B₆), Pi
Activated by: Glucagon (liver) & epinephrine (via cAMP → PKA → phosphorylase kinase → phosphorylase a); AMP (muscle b form); Ca²⁺-calmodulin (muscle, via phosphorylase kinase)
Inhibited by: Insulin (PP1 → phosphorylase b); ATP; Glucose-6-phosphate; Free glucose (liver only)
Deficiency: McArdle (GSD V, muscle isoform): exercise cramps, myoglobinuria, 'second wind', normal blood glucose. Hers (GSD VI, liver isoform): mild hypoglycemia, hepatomegaly, benign
Rate-limiting step of breakdown. Cleaves α(1→4) bonds phosphorolytically (no ATP needed) and stops 4 residues from a branch.
Condition cards:McArdle disease (glycogen storage disease type V) →Hers disease (GSD Type VI) →
Becomes Free glucose (from α1→6 bond) via Debranching enzyme — α(1→6) glucosidase activity
Debranching enzyme — α(1→6) glucosidase activity — Glycogen synthesis & breakdown
Deficiency: Cori disease (GSD III) — milder fasting hypoglycemia, hepatomegaly, limit dextrin accumulates; gluconeogenesis intact
After the 4:4 transferase moves three residues, the last α(1→6) glucose is hydrolysed as FREE glucose (~10% of output).
Condition cards:Cori disease (GSD Type III) →
Becomes Glucose-1-phosphate (released) via Debranching enzyme — 4:4 transferase activity
Debranching enzyme — 4:4 transferase activity — Glycogen synthesis & breakdown
Moves three residues to the neighbouring chain so phosphorylase can continue.