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)
Citrulline (matrix)
Made from Carbamoyl phosphate (mitochondrial) by Ornithine transcarbamylase (OTC)
Ornithine transcarbamylase (OTC) · irreversible — Urea cycle (+ the CPS-II collision)
Deficiency: OTC deficiency — the MOST COMMON urea cycle disorder and the ONLY X-LINKED one (every other is autosomal recessive). Hyperammonemia + LOW citrulline + HIGH orotic acid. Classically a full-term male newborn, well at birth (the mother's liver cleared his nitrogen in utero), lethargic and TACHYPNEIC by 24–48 h once he metabolises his own protein, then seizing
Carbamoyl phosphate + ornithine → citrulline. This is the gate that fails in the commonest urea cycle disorder.
Condition cards:Ornithine transcarbamylase (OTC) deficiency →
Made from Ornithine (matrix) by Ornithine transcarbamylase (OTC)
Ornithine transcarbamylase (OTC) — Urea cycle (+ the CPS-II collision)
Ornithine is the other substrate — carbamoyl phosphate is transferred onto it.
Condition cards:Ornithine transcarbamylase (OTC) deficiency →
Becomes Citrulline (cytosol) via Ornithine/citrulline antiporter (inner mitochondrial membrane)
Ornithine/citrulline antiporter (inner mitochondrial membrane) — Urea cycle (+ the CPS-II collision)
Citrulline is the molecule that crosses into the cytosol; the same carrier brings a fresh ornithine back in for the next turn (s12).