The phosphomannomutase/phosphoglucomutase (PMM/PGM) bifunctional enzyme catalyzes the reversible conversion of 1-phospho to 6-phospho-sugars (e.g. between mannose-1-phosphate and mannose-6-phosphate or glucose-1-phosphate and glucose-6-phosphate) via a bisphosphorylated sugar intermediate. The reaction involves two phosphoryl transfers, with an intervening 180 degree reorientation of the reaction intermediate during catalysis. Reorientation of the intermediate occurs without dissociation from the active site of the enzyme and is thus, a simple example of processivity, as defined by multiple rounds of catalysis without release of substrate. Glucose-6-phosphate and glucose-1-phosphate are known to be utilized for energy metabolism and cell surface construction, respectively. PMM/PGM belongs to the alpha-D-phosphohexomutase superfamily which includes several related enzymes that catalyze a reversible intramolecular phosphoryl transfer on their sugar substrates. Other members of this superfamily include phosphoglucosamine mutase (PNGM), phosphoacetylglucosamine mutase (PAGM), the bacterial phosphomannomutase ManB, the bacterial phosphoglucosamine mutase GlmM, and the phosphoglucomutases (PGM1 and PGM2). Each of these enzymes has four domains with a centrally located active site formed by four loops, one from each domain. All four domains are included in this alignment model.