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)
N-acetylglutamate (NAG)
Made from Acetyl-CoA + glutamate by N-ACETYLGLUTAMATE SYNTHASE (NAGS)
N-ACETYLGLUTAMATE SYNTHASE (NAGS) — Urea cycle (+ the CPS-II collision)
Activated by: Arginine
Deficiency: NAG SYNTHASE DEFICIENCY — hyperammonemia clinically indistinguishable from CPS-I deficiency, because CPS-I cannot work without its activator. Treated with N-carbamylglutamate, a NAG analogue that activates CPS-I directly
NAG is built from acetyl-CoA and glutamate, and arginine activates the synthase. That is the whole short-term control of the urea cycle: an amino-acid-rich meal raises acetyl-CoA, glutamate and arginine, NAG rises, CPS-I switches on, and urea output climbs within minutes (s16).
Condition cards:N-acetylglutamate synthase deficiency →
Becomes Carbamoyl phosphate (mitochondrial) via NAG — obligate allosteric activator of CPS-I
NAG — obligate allosteric activator of CPS-I — Urea cycle (+ the CPS-II collision)
Without NAG, CPS-I is essentially inactive (s12). This is why the cell bothers with an obligate cofactor at all: it creates one dedicated lever that turns urea synthesis on and off with protein intake, without having to change enzyme levels.