InterPro: IPR016066 Alpha-D-phosphohexomutase, conserved site

The alpha-D-phosphohexomutase superfamily is composed of four related enzymes, each of which catalyses a phosphoryl transfer on their sugar substrates: phosphoglucomutase (PGM), phosphoglucomutase/phosphomannomutase (PGM/PMM), phosphoglucosamine mutase (PNGM), and phosphoacetylglucosamine mutase (PAGM) [1]. PGM (EC:5.4.2.2) converts D-glucose 1-phosphate into D-glucose 6-phosphate, and participates in both the breakdown and synthesis of glucose [2]. PGM/PMM (EC:5.4.2.2; EC:5.4.2.8) are primarily bacterial enzymes that use either glucose or mannose as substrate, participating in the biosynthesis of a variety of carbohydrates such as lipopolysaccharides and alginate [3, 4]. Both PNGM (EC:5.4.2.3) and PAGM (EC:5.4.2.10) are involved in the biosynthesis of UDP-N-acetylglucosamine [5, 6].

Despite differences in substrate specificity, these enzymes share a similar catalytic mechanism, converting 1-phospho-sugars to 6-phospho-sugars via a biphosphorylated 1,6-phospho-sugar. The active enzyme is phosphorylated at a conserved serine residue and binds one magnesium ion; residues around the active site serine are well conserved among family members. The reaction mechanism involves phosphoryl transfer from the phosphoserine to the substrate to create a biophosphorylated sugar, followed by a phosphoryl transfer from the substrate back to the enzyme [7].

The structures of PGM and PGM/PMM have been determined, and were found to be very similar in topology. These enzymes are both composed of four domains and a large central active site cleft, where each domain contains residues essential for catalysis and/or substrate recognition. Domain I contains the catalytic phosphoserine, domain II contains a metal-binding loop to coordinate the magnesium ion, domain III contains the sugar-binding loop that recognises the two different binding orientations of the 1- and 6-phospho-sugars, and domain IV contains a phosphate-binding site required for orienting the incoming phospho-sugar substrate.

This entry represents the conserved site at the N-terminal region of alpha-D-phosphohexomutase enzymes.