Converts guanine to guanosine monophosphate, and hypoxanthine to inosine monophosphate. Transfers the 5-phosphoribosyl group from 5-phosphoribosylpyrophosphate onto the purine. Plays a central role in the generation of purine nucleotides through the purine salvage pathway (By similarity). diphosphate + IMP = 5-phospho-alpha-D-ribose 1-diphosphate + hypoxanthine diphosphate + GMP = 5-phospho-alpha-D-ribose 1-diphosphate + guanine Binds 2 magnesium ions per subunit. The magnesium ions are essentially bound to the substrate and have few direct interactions with the protein. Purine metabolism; IMP biosynthesis via salvage pathway; IMP from hypoxanthine: step 1/1. Homotetramer. The cell lines from which this sequence was cloned are revertants from mutants with no detectable enzyme activity. The phenotypic reversions are the result of overproduction of variant enzymes because of gene amplification. This variant sequence is expected to be very similar to the wild-type. Belongs to the purine/pyrimidine phosphoribosyltransferase family.