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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

Pyrimidine synthesis (& thymidylate) · Cytosol, essentially all cells (CAD, UMP synthase); dihydroorotate dehydrogenase on the inner mitochondrial membrane. 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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Pyrimidine synthesis (& thymidylate)

UMP

Made from Orotate (free base) by UMP synthase (orotate phosphoribosyltransferase → OMP decarboxylase)

UMP synthase (orotate phosphoribosyltransferase → OMP decarboxylase) · irreversible — Pyrimidine synthesis (& thymidylate)

Cofactors: PRPP → PPi

Deficiency: HEREDITARY OROTIC ACIDURIA (AR) — orotic acid accumulates and crystallises in urine; pyrimidines run short so the fastest-dividing tissue fails first: megaloblastic anemia that does NOT respond to B12 or folate, plus failure to thrive. Ammonia is NORMAL, because nothing here touches the urea cycle — that is the discriminator against OTC deficiency. Treat with oral uridine, which bypasses the block

One bifunctional protein doing two consecutive jobs: orotate phosphoribosyltransferase attaches the free orotate ring to PRPP (giving OMP), then OMP decarboxylase removes the carboxyl to give UMP. Drawn as a single step because losing EITHER activity produces the same disease — splitting them adds a box without adding a decision.

Condition cards:Hereditary orotic aciduria (UMP synthase deficiency) →

Becomes UDP via UMP kinase

UMP kinase — Pyrimidine synthesis (& thymidylate)

Cofactors: ATP