The third enzyme (EC.22.214.171.124, porphobilinogen deaminase, hydroxymethylbilane synthase) in the biosynthetic pathway leading to haem. There are two isoforms (317 aa and 334 aa), one active in all tissues and the other restricted to erythrocytes. Mutations cause acute intermittent porphyria. Uroporphyrinogen I (UP1) is converted to UP III by UP III synthetase (hydroxymethylbilane hydrolyase (cyclizing), EC 126.96.36.199, 265 aa) that is defective in autosomal recessive erythropoietic porphyria.
Subjects: Medicine and Health.