Purine salvage pathway enzyme that catalyzes the transfer of the ribosyl-5-phosphate group from 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) to the N9 position of the 6-oxopurines hypoxanthine and guanine to form the corresponding ribonucleotides IMP (inosine 5'-monophosphate) and GMP (guanosine 5'-monophosphate), with the release of PPi. 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. Purine metabolism; GMP biosynthesis via salvage pathway; GMP from guanine: step 1/1. Belongs to the purine/pyrimidine phosphoribosyltransferase family. Extended N-terminus.