InterPro: IPR011857 Bifunctional glucose-6-phosphate/mannose-6-phosphate isomerase

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].

These bifunctional isomerases form a distinct phylogenetic cluster within the larger phosphoglucose isomerase (PGI) superfamily [3]. They show relatively low sequence identity to other PGIs, but contain similar structural elements and show almost complete conservation of the catalytic residues in the active site, indicating they use a similar catalytic mechanism [4]. The family appears to have originated in the archaea, with the bacterial proteins being acquired through horizontal transfer.