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.

Pathway

Methionine cycle & homocysteine's two fates · Cytosol of most cells; transsulfuration is strongest in liver, kidney, pancreas and brain. Enzymes sit beside the arrows, ★ = rate-limiting, ⊕/⊖ = activators/inhibitors (toggle), −ATP +NADH = what the step spends and makes (toggle ⚡), dashed = allosteric signal, shaded bands = compartment. Drag to pan.

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Methionine cycle & homocysteine's two fates

CYSTEINE

Made from Cystathionine by 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 →