Catalyzes a salvage reaction resulting in the formation of IMP that is energically less costly than de novo synthesis. Prefers hypoxanthine, has 66% activity with guanine while activity with adenine, xanthine, uracil, orotate, or cytosine is negligible. diphosphate + IMP = 5-phospho-alpha-D-ribose 1-diphosphate + hypoxanthine diphosphate + GMP = 5-phospho-alpha-D-ribose 1-diphosphate + guanine Optimum pH is 7.6-8.2. Optimum temperature is 70-80 degrees Celsius. Purine metabolism; IMP biosynthesis via salvage pathway; IMP from hypoxanthine: step 1/1. Homodimer. Belongs to the purine/pyrimidine phosphoribosyltransferase family. Archaeal HPRT subfamily.