GTP-mannose-1-phosphate guanyltransferase (MPG1 transferase), also known as GDP-mannose pyrophosphorylase, is a bifunctional enzyme with both phosphomannose isomerase (PMI) activity and GDP-mannose phosphorylase (GMP) activity. The protein contains an N-terminal NTP transferase domain, an L-beta-H domain, and a C-terminal PGM-like domain that belongs to the alpha-D-phosphohexomutase superfamily. This subfamily is limited to bacteria and archaea. The alpha-D-phosphohexomutases include several related enzymes that catalyze a reversible intramolecular phosphoryl transfer on their sugar substrates. Members of this group appear to lack conserved residues necessary for metal binding and catalytic activity. Other members of this superfamily include the phosphoglucomutases (PGM1 and PGM2), phosphoglucosamine mutase (PNGM), phosphoacetylglucosamine mutase (PAGM), the bacterial phosphomannomutase ManB, the bacterial phosphoglucosamine mutase GlmM, and the bifunctional phosphomannomutase/phosphoglucomutase (PMM/PGM). 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.