guanine phosphoribosyltransferase activity KEGG_REACTION:R01229 guanosine phosphoribosyltransferase activity guanylate pyrophosphorylase activity Transphosphoribosidase activity GPRT 6-mercaptopurine phosphoribosyltransferase activity guanosine 5'-phosphate pyrophosphorylase activity guanine-hypoxanthine phosphoribosyltransferase activity Catalysis of the reaction: GMP + diphosphate = guanine + 5-phospho-alpha-D-ribose 1-diphosphate. hypoxanthine-guanine phosphoribosyltransferase activity GMP pyrophosphorylase activity RHEA:25424 molecular_function GO:0052657 guanylic pyrophosphorylase activity 6-hydroxypurine phosphoribosyltransferase activity purine-6-thiol phosphoribosyltransferase activity MetaCyc:GUANPRIBOSYLTRAN-RXN purine phosphoribosyltransferase activity