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.
Methionine cycle & homocysteine's two fates
Cystathionine
Made from HOMOCYSTEINE — the fork by CYSTATHIONINE β-SYNTHASE (CBS) — transsulfuration, Fate 2
CYSTATHIONINE β-SYNTHASE (CBS) — transsulfuration, Fate 2 · irreversible — Methionine cycle & homocysteine's two fates
Cofactors: Serine, PLP (vitamin B₆)
Deficiency: HOMOCYSTINURIA (AR) — high plasma and urinary homocysteine, HIGH methionine (it backs up behind the fork), and LOW cysteine (the downstream product is starved). Lens dislocation DOWNWARD, marfanoid habitus, intellectual disability, and thrombosis — which is the leading cause of death. Treat with methionine restriction plus folate and B12 to push the remethylation escape route; some forms respond to high-dose B6
Homocysteine condenses with SERINE. This is the committed step of transsulfuration and the only route by which the body makes cysteine — which is why methionine is 'conditionally essential': you need a lot of it when dietary cysteine is short.
Condition cards:Homocystinuria (cystathionine β-synthase deficiency) →
Becomes CYSTEINE via Cystathionase (cystathionine γ-lyase) — releases cysteine
Cystathionase (cystathionine γ-lyase) — releases cysteine — Methionine cycle & homocysteine's two fates
Cofactors: PLP (vitamin B₆)
Deficiency: CYSTATHIONINURIA — cystathionine accumulates and spills into urine. Generally BENIGN and often an incidental finding; the contrast with homocystinuria one step above is a deliberate board trap
Both transsulfuration enzymes are B6-dependent, which is why B6 deficiency raises homocysteine.
Condition cards:Cystathioninuria →
Becomes α-Ketobutyrate via Cystathionase (γ-lyase) — and the α-ketobutyrate half
Cystathionase (γ-lyase) — and the α-ketobutyrate half — Methionine cycle & homocysteine's two fates
Cofactors: PLP (vitamin B₆)
The same cleavage produces two things. Cysteine is the point of the pathway; α-ketobutyrate is the carbon skeleton being handed on.