Essential in nucleic acid metabolism of T.foetus because the parasite is unable to synthesize purine nucleotides de novo and relies on the HGXPRTase activities for its purine requirements by salvaging purine bases from the host. Works with guanine, hypoxanthine and xanthine. diphosphate + IMP = 5-phospho-alpha-D-ribose 1-diphosphate + hypoxanthine diphosphate + GMP = 5-phospho-alpha-D-ribose 1-diphosphate + guanine diphosphate + XMP = 5-phospho-alpha-D-ribose 1-diphosphate + xanthine 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; GMP biosynthesis via salvage pathway; GMP from guanine: step 1/1. Purine metabolism; IMP biosynthesis via salvage pathway; IMP from hypoxanthine: step 1/1. Purine metabolism; XMP biosynthesis via salvage pathway; XMP from xanthine: step 1/1. Homodimer. Belongs to the purine/pyrimidine phosphoribosyltransferase family.