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An uncharacterized subgroup of the PBGD family; the type 2 periplasmic binding protein fold Hydroxymethylbilane synthase (HMBS), also known as porphobilinogen deaminase (PBGD), is an intermediate enzyme in the biosynthetic pathway of tetrapyrrolic ring systems, such as heme, chlorophyll, and vitamin B12. HMBS catalyzes the conversion of porphobilinogen (PBG) into hydroxymethylbilane (HMB). This subfamily includes the three domains of HMBS. The enzyme is believed to bind substrate through a hinge-bending motion of domains 1 and 2. The C-terminal domain 3 contains an invariant cysteine that forms the covalent attachment site for the DPM (dipyrromethane) cofactor. HMBS is found in all organisms except viruses. The domains 1 and 2 have the same overall topology as found in the type 2 periplasmic-binding proteins (PBP2), many of which are involved in chemotaxis and uptake of nutrients and other small molecules from the extracellular space as a primary receptor.
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Jiyao W et al.(2023). "The conserved domain database in 2023.",