Purine nucleoside phosphorylase which is highly specific for 6-oxopurine nucleosides. Cleaves guanosine or inosine to respective bases and sugar-1-phosphate molecules. Involved in purine salvage. a purine D-ribonucleoside + phosphate = a purine nucleobase + alpha-D-ribose 1-phosphate Purine metabolism; purine nucleoside salvage. Homohexamer. Dimer of a homotrimer. Although this enzyme belongs to the family of MTA phosphorylases based on sequence homology, it has been shown that conserved amino acid substitutions in the substrate binding pocket convert the substrate specificity of this enzyme from 6-aminopurines to 6-oxopurines. Belongs to the PNP/MTAP phosphorylase family. MTAP subfamily.