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.
Heme degradation & bilirubin (jaundice)
Biliverdin (green)
Made from Senescent RBC heme (~120 days) by Heme oxygenase (microsomal) ★
Heme oxygenase (microsomal) ★ rate-limiting · irreversible — Heme degradation & bilirubin (jaundice)
Cofactors: O₂, NADPH
Happens INSIDE the macrophage, after a red cell roughly 120 days old is phagocytosed by the mononuclear phagocyte system — spleen, liver Kupffer cells and marrow. Heme → biliverdin + CO + Fe²⁺ — three products, all worth knowing: the CO is the ONLY endogenous source and is what an exhaled-CO breath test measures, and the iron is recycled to ferritin or straight back into new heme, which is why haemolysis does not by itself cause iron deficiency. ~80–85% of bilirubin comes from old RBCs, the rest from ineffective erythropoiesis, myoglobin and cytochromes.
Condition cards:Pre-hepatic (hemolytic) jaundice →Hereditary spherocytosis (as cause of hemolytic jaundice) →Autoimmune hemolytic anemia (as cause of hemolytic jaundice) →Hemolytic disease of the newborn (ABO/Rh incompatibility) →
Becomes Unconjugated bilirubin (yellow, lipophilic) via Biliverdin reductase
Biliverdin reductase — Heme degradation & bilirubin (jaundice)
Cofactors: NADPH
Green → yellow — the colour change of a healing bruise. Still UNCONJUGATED and still inside the macrophage — it has to be exported to plasma next, and it cannot travel alone.