Purine salvage pathway enzyme which catalyzes the transfer of the ribosyl-5-phosphate group from 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) to the N9 position of hypoxanthine to yield IMP (inosine 5'-monophosphate). To a lesser extent, can also act on guanine leading to GMP, but shows a highly less efficient activity with xanthine. diphosphate + IMP = 5-phospho-alpha-D-ribose 1-diphosphate + hypoxanthine diphosphate + GMP = 5-phospho-alpha-D-ribose 1-diphosphate + guanine Inhibited by the product IMP (PubMed:12070315). Highly inhibited by nucleoside phosphonates, which are product analogs (PubMed:23927482, Ref.6). kcat is 59.0 sec(-1) with hypoxanthine as substrate. kcat is 10.2 sec(-1) with guanine as substrate. kcat is 0.008 sec(-1) with xanthine as substrate (at pH 8.5 and 25 degrees Celsius). Purine metabolism; IMP biosynthesis via salvage pathway; IMP from hypoxanthine: step 1/1. Homotetramer. E.coli cells express two distinct 6-oxopurine PRTases, with very different specificities for hypoxanthine, guanine, and xanthine. Salvage enzymes allow a more energy efficient synthesis of purine nucleoside monophosphates compared with the de novo pathway. The kinetic analysis suggests that E.coli HPRT is mainly responsible for the synthesis of IMP and that XGPRT primarily salvages guanine and xanthine. Belongs to the purine/pyrimidine phosphoribosyltransferase family.