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 Purine metabolism; IMP biosynthesis via salvage pathway; IMP from hypoxanthine: step 1/1. Homotetramer. Belongs to the purine/pyrimidine phosphoribosyltransferase family.