The catalytic domain of ATP phosphoribosyltransferase contains the type 2 periplasmic substrate-binding fold
Encoded by the hisG gene, the ATP phosphoribosyltransferase (ATP-PRT, EC 2.4.2.17) is the first enzyme in histidine biosynthetic pathway that catalyzes the condensation of ATP and PRPP (5'-phosphoribosyl 1'-pyrophosphate), and is regulated by a feedback inhibition from the product histidine. ATP-PRT has two distinct forms: a hexameric long form, HisGL, containing two catalytic domains and a C-terminal regulatory domain; and a hetero-octomeric short form, HisGs, without the regulatory domain. HisGL is catalytically competent, but the hetero-octameric HisGs requires the second subunit HisZ, a paralog to the catalytic domain of functional histidyl-tRNA synthetases (HisRSs), for the enzyme activity. This catalytic domain belongs to the type 2 periplasmic binding fold protein superfamily (PBP2). The PBP2 proteins are typically comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap. The majority of PBP2 proteins function in the uptake of small soluble substrates in eubacteria and archaea.
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