<p>This entry describes a subset of the phosphorylase family. The entry excludes the methylthioadenosine phosphorylases (MTAP, <db_xref db="INTERPRO" dbkey="IPR010044"/>), which are believed to play a specific role in the recycling of methionine from methylthioadenosine. </p><p>This entry consists of three clades of purine phosphorylases based on a neighbour-joining tree using the MTAP family as an out group. The highest-branching clade (<db_xref db="INTERPRO" dbkey="IPR011269"/>) consists of a group of sequences from both Gram-positive and Gram-negative bacteria which have been shown to act as purine nucleotide phosphorylases but whose physiological substrate and role in vivo remain unknown [<cite idref="PUB00023847"/>].Of the two remaining clades, one is xanthosine phosphorylase (XAPA,<db_xref db="INTERPRO" dbkey="IPR010943"/>); it is limited to certain gammaproteobacteria and constitutes a special purine phosphorylase found in a specialised operon for xanthosine catabolism [<cite idref="PUB00028036"/>]. The enzyme also acts on the same purines (inosine and guanosine) as the other characterised members of this subfamily, but is only induced when xanthosine must be degraded. The remaining and largest clade consists of purine nucleotide phosphorylases (PNPH, <db_xref db="INTERPRO" dbkey="IPR011270"/>) from metazoa [<cite idref="PUB00002584"/>] and bacteria [<cite idref="PUB00013820"/>] which act primarily on guanosine and inosine, and do not act on adenosine. Sequences from Clostridium and Thermotoga fall between these last two clades and are uncharacterised with respect to substrate range.</p> Inosine guanosine/xanthosine phosphorylase