Methylthioadenosine phosphorylase <p>This entry represents the methylthioadenosine phosphorylase found in metazoa, cyanobacteria and a limited number of archaea such as Sulfolobus, Aeropyrum, Pyrobaculum, Pyrococcus, and Thermoplasma. This enzyme is responsible for the first step in the methionine salvage pathway after the transfer of the amino acid moiety from S-adenosylmethionine. The enzyme from human is well-characterised and its crystal structure has been described [<cite idref="PUB00000256"/>, <cite idref="PUB00013813"/>]. A misleading characterisation is found for a <taxon tax_id="2287">Sulfolobus solfataricus</taxon> enzyme [<cite idref="PUB00002872"/>] which is called a MTAP. In fact, as uncovered by the genome sequence of S. solfataricus, there are at least two nucleotide phosphorylases and the one found in the MTAP clade is not the one annotated as such. The sequence in this clade has not been isolated but is likely to be the authentic SsMTAP as it displays all of the conserved active site residues found in the human enzyme [<cite idref="PUB00013813"/>]. This explains the finding that the characterised enzyme has greater efficiency towards the purines inosine, guanosine and adenosine over MTA. In fact, this misnaming of this enzyme has been carried forward to several publications including a crystal structure [<cite idref="PUB00013791"/>].</p><p>Related sequences include:<ul> <li>1) several archaeal sequences which appear to contain several residues characteristic of phosphorylases which act on guanosine or inosine (according to the crystal structure of MTAP and alignments).</li><li>2) sequences from <taxon tax_id="1773">Mycobacterium tuberculosis</taxon> and <taxon tax_id="1902">Streptomyces coelicolor</taxon> which have better, although not perfect retention of the active site residues, but considering the general observation that bacteria utilise the MTA/SAH nucleotidase enzyme and a kinase to do this reaction.</li><li>3) a sequence from Drosophila which appears to be a recent divergence (long branch in neighbour-joining trees) and lacks some of the conserved active site residues.</li> </ul> </p>