This entry represents the methylthioadenosine phosphorylase (MTAP). MTAP catalyses the reversible phosphorylation of S-methyl-5'-thioadenosine (MTA) to adenine and 5-methylthioribose-1-phosphate. It is involved in the breakdown of MTA, a major by-product of polyamine biosynthesis. It is responsible for the first step in the methionine salvage pathway after MTA has been generated from S-adenosylmethionine. It has broad substrate specificity with 6-aminopurine nucleosides as preferred substrates [, ].
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 () 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 [].Of the two remaining clades, one is xanthosine phosphorylase (XAPA,); it is limited to certain gammaproteobacteria and constitutes a special purine phosphorylase found in a specialised operon for xanthosine catabolism []. 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, ) from metazoa []and bacteria []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.
