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
METHIONINE (essential)
Made from HOMOCYSTEINE — the fork by METHIONINE SYNTHASE — remethylation, Fate 1
METHIONINE SYNTHASE — remethylation, Fate 1 — Methionine cycle & homocysteine's two fates
Cofactors: N⁵-methyl-THF → THF, Methylcobalamin (B₁₂)
Deficiency: B12 deficiency (pernicious anemia, gastrectomy, ileal disease, strict vegan diet) or folate deficiency — both raise homocysteine; only B12 also raises methylmalonic acid
Closes the methionine cycle. Note what this single reaction ties together: it needs B12, it consumes the folate-carried methyl group, and it is the escape route that lowers homocysteine. Lose B12, lose folate, or lose B6 on the other arm, and homocysteine rises — the deck's point that plasma homocysteine runs inversely to all three vitamins.
Condition cards:Hyperhomocysteinemia (acquired; B6/B12/folate deficiency) →
Becomes S-adenosylmethionine (SAM) via Methionine adenosyltransferase (MAT)
Methionine adenosyltransferase (MAT) · irreversible — Methionine cycle & homocysteine's two fates
Cofactors: ATP → PPi + Pi
Unusual in that ALL THREE phosphates of ATP are lost. The product is a high-energy sulfonium compound — the methyl group on SAM's sulfur is activated, which is what makes it a good donor.