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
THF — usable folate
Made from N⁵-methyl-THF (trapped form) by METHIONINE SYNTHASE regenerates THF — the ONLY route
METHIONINE SYNTHASE regenerates THF — the ONLY route · irreversible — Methionine cycle & homocysteine's two fates
Cofactors: Methylcobalamin (B₁₂)
Deficiency: THE FOLATE TRAP — without B12 this reaction stalls, folate piles up as N⁵-methyl-THF and cannot be used for purine or dTMP synthesis, so B12 deficiency produces the SAME megaloblastic anemia as folate deficiency even with normal folate intake. B12 deficiency adds neurologic signs (peripheral neuropathy, subacute combined degeneration) that folate deficiency does not
The same enzyme, viewed from the folate side: handing the methyl group to homocysteine is what converts N⁵-methyl-THF back into usable THF, and it is the only reaction that does so. THF is what purine and dTMP synthesis need.
Condition cards:Folate (folic acid) deficiency →