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.
Urea cycle (+ the CPS-II collision)
Argininosuccinate
Made from Citrulline (cytosol) by Argininosuccinate synthetase
Argininosuccinate synthetase · irreversible — Urea cycle (+ the CPS-II collision)
Cofactors: ATP → AMP + PPi, Aspartate — the SECOND nitrogen
Deficiency: Citrullinemia type I (AR) — the block is AFTER citrulline is made, so citrulline piles up to VERY HIGH levels. Sky-high citrulline means look downstream
The second urea nitrogen enters here, and it comes from ASPARTATE, not free ammonia. Knowing the two nitrogens have two different sources is one of the most commonly tested single facts about this cycle (s13).
Condition cards:Citrullinemia (argininosuccinate synthetase deficiency) →
Becomes Arginine via Argininosuccinate lyase (argininosuccinase)
Argininosuccinate lyase (argininosuccinase) — Urea cycle (+ the CPS-II collision)
Deficiency: Argininosuccinic aciduria (AR) — argininosuccinate accumulates in blood and urine; often with trichorrhexis nodosa (brittle hair)
Cleaves argininosuccinate into arginine and fumarate.
Becomes Fumarate → TCA via Argininosuccinate lyase — the other product
Argininosuccinate lyase — the other product — Urea cycle (+ the CPS-II collision)
The aspartate–argininosuccinate shunt: fumarate feeds the TCA cycle, becomes oxaloacetate, is transaminated back to aspartate, and re-donates its amino group on the next turn. This is the direct link between the urea cycle and the TCA cycle.