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Porphobilinogen synthase (PBGS), which is also called delta-aminolevulinic acid dehydratase (ALAD), catalyzes the condensation of two 5-aminolevulinic acid (ALA) molecules to form the pyrrole porphobilinogen (PBG), which is the second step in the biosynthesis of tetrapyrroles, such as heme, vitamin B12 and chlorophyll. This reaction involves the formation of a Schiff base link between the substrate and the enzyme. PBGSs are metalloenzymes, some of which have a second, allosteric metal binding site, beside the metal ion binding site in their active site. Although PBGS is a family of homologous enzymes, its metal ion utilization at catalytic site varies between zinc and magnesium and/or potassium. PBGS can be classified into two groups based on differences in their active site metal binding site. They either contain a cysteine-rich zinc binding site (consensus DXCXCX(Y/F)X3G(H/Q)CG) or an aspartate-rich magnesium binding site (consensus DXALDX(Y/F)X3G(H/Q)DG). The cysteine-rich zinc binding site appears more common. Most members represented by this model also have a second allosteric magnesium binding site (consensus RX~164DX~65EXXXD, missing in a eukaryotic subfamily with cysteine-rich zinc binding site).
Jiyao W et al.(2023). "The conserved domain database in 2023.",