Purine nucleoside phosphorylase I, inosine/guanosine-specific <p>This entry represents a family of bacterial and metazoan purine phosphorylases acting primarily on inosine and guanosine and not acting on adenosine. PNP-I refers to the nomenclature from <taxon tax_id="1422">Bacillus stearothermophilus</taxon> (Geobacillus stearothermophilus) [<cite idref="PUB00013854"/>] where PHP-II refers to the nucleotidase acting on adenosine as the primary substrate. The bacterial enzymes (PUNA) are typified by the Bacillus PupG protein [<cite idref="PUB00013820"/>] which is involved in the metabolism of nucleosides as a carbon source. Several metazoan enzymes (PNPH) are well characterised including the human [<cite idref="PUB00002584"/>] and bovine [<cite idref="PUB00013855"/>] enzymes which have been crystallised.</p>