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.
Catecholamines, serotonin & the tyrosine tree
Homogentisic acid
Made from 4-Hydroxyphenylpyruvate by 4-Hydroxyphenylpyruvate dioxygenase
4-Hydroxyphenylpyruvate dioxygenase — Catecholamines, serotonin & the tyrosine tree
Cofactors: Vitamin C (ascorbate)
Deficiency: Tyrosinemia type III — rare, neurologic
This is the enzyme NITISINONE blocks on purpose, to treat tyrosinemia type I — you stop the pathway ABOVE the toxic metabolite and accept a high tyrosine instead.
Becomes Maleylacetoacetate via Homogentisate oxidase (homogentisate 1,2-dioxygenase)
Homogentisate oxidase (homogentisate 1,2-dioxygenase) — Catecholamines, serotonin & the tyrosine tree
Cofactors: O₂, Fe²⁺
Deficiency: ALKAPTONURIA — homogentisic acid accumulates; urine darkens on standing, and over DECADES the pigment deposits in cartilage and connective tissue (ochronosis) causing arthritis that shows up around age 40. Blocked far downstream, so the damage is slow and mechanical — contrast PKU, blocked at the top, which harms the developing brain early
Ring-opening. The product is MALEYLacetoacetate — the cis isomer — not fumarylacetoacetate; that takes one more step (slides 5 and 10).
Condition cards:Alkaptonuria →