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Porphobilinogen deaminase
Porphobilinogen deaminase ((EC 2.5.1.61 ), Older sources categorize it under ), also known as hydroxymethylbilane synthase or uroporphyrinogen I synthase is an enzyme that in humans is encoded by the HMBS gene. Porphobilinogen deaminase is involved in the third step of the heme biosynthetic pathway. It catalyzes the head to tail condensation of four porphobilinogen molecules into the linear hydroxymethylbilane while releasing four ammonia molecules.
== Structure and function ==
Functionally, porphobilinogen deaminase catalyzes the loss of ammonia from the porphobilinogen monomer (deamination) and its subsequent polymerization to a linear tetrapyrrole, which is released as hydroxymethylbilane:
The structure of 40-42 kDa porphobilinogen deaminase, which is highly conserved amongst organisms, consists of three domains. Domains 1 and 2 are structurally very similar: each consisting of five beta-sheets and three alpha helices in humans. Domain 3 is positioned between the other two and has a flattened beta-sheet geometry. A dipyrrole, a cofactor of this enzyme consisting of two condensed porphobilinogen molecules, is covalently attached to domain 3 and extends into the active site, the cleft between domains 1 and 2. Several positively charged arginine residues, positioned to face the active site from domains 1 and 2, have been shown to stabilize the carboxylate functionalities on the incoming porphobilinogen as well as the growing pyrrole chain. These structural features presumably favor the formation of the final hydroxymethylbilane product. Porphobilinogen deaminase usually exists in dimer units in the cytoplasm of the cell.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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