Catalyzes a salvage reaction resulting in the formation of IMP that is energically less costly than de novo synthesis. 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. Homodimer. Belongs to the purine/pyrimidine phosphoribosyltransferase family. Archaeal HPRT subfamily. Extended N-terminus.