InterPro: IPR001250 Mannose-6-phosphate isomerase, type I

Mannose-6-phosphate isomerase or phosphomannose isomerase (EC: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 [1]. Three classes of PMI have been defined [2].

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.

A crystal structure of PMI from Candida albicans shows that the enzyme has three distinct domains [3]. 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.