Four PBGs combine through deamination to form hydroxymethylbilane (HMB) Authored: Jassal, B, D'Eustachio, P, 2007-01-24 10:19:49 Cytosolic porphobilinogen deaminase catalyzes the polymerization of four molecules of porphobilinogen (PBG) to generate hydroxymethylbilane (HMB), an unstable tetrapyrrole. This reaction is the first step in the formation of the tetrapyrrole macrocycle. Two isoforms of porphobilinogen deaminase are generated by alternative splicing, one expresssed in erythroid tissues and one ubiquitously expressed in the body. Deficiencies of both forms of PBG deaminase are associated with acute intermittent porphyria. EC Number: 2.5.1.61 Edited: Jassal, B, D'Eustachio, P, 2007-01-24 10:19:49 Pubmed12773194 Pubmed7354069 Reactome Database ID Release 43189406 Reactome, http://www.reactome.org ReactomeREACT_9446 Reviewed: Sassa, S, 2007-01-24 10:18:36 has a Stoichiometric coefficient of 4