Two molecules of ALA condense to form porphobilinogen (PBG) 5-Aminolevulinic acid dehydratase (ALAD), also known as porphobilinogen synthase (PBGS), catalyzes the asymmetric condensation of two molecules of ALA to form porphobilinogen. The substrate that becomes the acetyl side chain-containing half of PBG is called A-side ALA; the half that becomes the propionyl side chains and the pyrrole nitrogen is called P-ALA (Jaffe 2004). Porphobilinogen (PBG) is the first pyrrole formed, the precursor to all tetrapyrrole pigments such as heme and chlorophyll. There are at least eight bonds that are made or broken during this reaction. The active form of the ALAD enzyme is an octamer complexed with eight Zn++ ions, four that are strongly bound and four that are weakly bound. The four weakly bound ones are dispensible for enzyme activity in vitro (Bevan et al. 1980; Mitchell et al. 2001).<br>Deficiencies of ALAD enzyme in vivo are associated with 5-aminolevulinate dehydratase-deficient porphyria (e.g., Akagi et al. 2000). Authored: Jassal, B, D'Eustachio, P, 2007-01-24 10:19:49 EC Number: 4.2.1.24 Edited: Jassal, B, D'Eustachio, P, 2007-01-24 10:19:49 Pubmed10706561 Pubmed11032836 Pubmed11032841 Pubmed15381398 Pubmed7354072 Reactome Database ID Release 43189439 Reactome, http://www.reactome.org ReactomeREACT_9430 Reviewed: Sassa, S, 2007-01-24 10:18:36 has a Stoichiometric coefficient of 2