CLINICAL CASES
403
Figure 44-2. Heme biosynthetic pathway involving (1) ALAS, (2) ALAD,
(3) PBGD, (4) UROS, (5) UROD, (6) CPO, (7) PPO, and (8) Ferrochelatase.
HMB = hydroxymethylbilane; CM = carboxymethyl; CE = carboxyethyl; M =
methyl; V = vinyl.
steps, uroporphyrinogen decarboxylase (UROD) recognizes either isomer
URO I or III and removes specific carboxyl groups leaving coproporphyrino-
gen I or III, respectively. Coproporphyrinogen oxidase (CPO) acts exclu-
sively on the type III isomer of coproporphyrinogen; after its substrate enters
the mitochondrion CPO catalyzes the conversion of COPRO III to protopor-
phyrinogen III. This intermediate is oxidized by protoporphyrinogen oxi-
dase (PPO) to form protoporphyrin IX. In the final step of heme synthesis, the
enzyme ferrochelatase inserts a ferrous iron into water-insoluble protopor-
phyrin IX. The excess protoporphyrin IX not converted to heme is removed via
biliary excretion into the intestine.
previous page 417 Case Files   Biochemistry read online next page 419 Case Files   Biochemistry read online Home Toggle text on/off