Phosphomannose isomerase (PMI) types I and II were found to possess a conserved protein motif. This motif coincides with the catalytic site of the Candida albicans type I PMI, indicating a common catalytic process for both PMI types. The type II PMI are bifunctional enzymes possessing PMI and guanosine diphospho-D-mannose pyrophosphorylase (GMP) activity in separate catalytic domains, which in some species may function as separate proteins.