<p>Mannose-6-phosphate isomerase or phosphomannose isomerase (<db_xref db="EC" dbkey="5.3.1.8"/>) (PMI) is the enzyme that catalyses the interconversion of mannose-6-phosphate and fructose-6-phosphate. In eukaryotes PMI is involved in the synthesis of GDP-mannose, a constituent of N- and O-linked glycans and GPI anchors and in prokaryotes it participates in a variety of pathways, including capsular polysaccharide biosynthesis and D-mannose metabolism. PMI's belong to the cupin superfamily whose functions range from isomerase and epimerase activities involved in the modification of cell wall carbohydrates in bacteria and plants, to non-enzymatic storage proteins in plant seeds, and transcription factors linked to congenital baldness in mammals [<cite idref="PUB00007419"/>]. Three classes of PMI have been defined [<cite idref="PUB00001448"/>].</p><p>Type I includes eukaryotic PMI and the enzyme encoded by the manA gene in enterobacteria. PMI has a bound zinc ion, which is essential for activity.</p><p>A crystal structure of PMI from <i>Candida albicans</i> shows that the enzyme has three distinct domains [<cite idref="PUB00003929"/>]. The active site lies in the central domain, contains a single essential zinc atom, and forms a deep, open cavity of suitable dimensions to contain M6P or F6P The central domain is flanked by a helical domain on one side and a jelly-roll like domain on the other.</p>