<p>Phosphomannose isomerase (PMI) [<cite idref="PUB00001448"/>, <cite idref="PUB00000378"/>] is the enzyme that catalyzes the interconversion of mannose-6-phosphate and fructose-6-phosphate. In eukaryotes, it is involved in the synthesis of GDP-mannose which is a constituent of N- and O-linked glycans as well as GPI anchors. In prokaryotes, it is involved in a variety of pathways including capsular polysaccharide biosynthesis and D-mannose metabolism. </p><p>Three classes of PMI have been defined on the basis of sequence similarities [<cite idref="PUB00001448"/>]. The first class comprises all known eukaryotic PMI as well as the enzyme encoded by the manA gene in enterobacteria such as <taxon tax_id="562">Escherichia coli</taxon>. Class I PMI's are proteins of about 42 to 50 kDa which bind a zinc ion essential for their activity. </p><p>Two conserved regions define class I PMI. The first one is located in the N-terminal section of the proteins, the second in the C-terminal half. Both patterns contain a residue involved in the binding of the zinc ion [<cite idref="PUB00003929"/>].</p>