• Phosphoglucomutase (EC 5.4.2.2) (PGM). PGM is an enzyme responsible for the conversion of D-glucose 1-phosphate into D-glucose 6-phosphate. PGM participates in both the breakdown and synthesis of glucose [1].
• Phosphomannomutase (EC 5.4.2.8) (PMM). PMM is an enzyme responsible for the conversion of D-mannose 1-phosphate into D-mannose 6-phosphate. PMM is required for different biosynthetic pathways in bacteria. For example, in enterobacteria such as Escherichia coli there are two different genes coding for this enzyme: rfbK which is involved in the synthesis of the O antigen of lipopolysaccharide and cpsG which is required for the synthesis of the M antigen capsular polysaccharide [2]. In Pseudomonas aeruginosa PMM (gene algC) is involved in the biosynthesis of the alginate layer [3] and in Xanthomonas campestris (gene xanA) it is involved in the biosynthesis of xanthan [4]. In Rhizobium strain ngr234 (gene noeK) it is involved in the biosynthesis of the nod factor.
• Phosphoacetylglucosamine mutase (EC 5.4.2.3) which converts N-acetyl-D- glucosamine 1-phosphate into the 6-phosphate isomer.

The catalytic mechanism of both PGM and PMM involves the formation of a phosphoserine intermediate [1]. The sequence around the serine residue is well conserved and can be used as a signature pattern.

In addition to PGM and PMM there are at least three uncharacterized proteins that belong to this family [5,6]:

• Urease operon protein ureC from Helicobacter pylori.
• Escherichia coli protein mrsA.
• Paramecium tetraurelia parafusin, a phosphoglycoprotein involved in exocytosis.
• A Methanococcus vannielii hypothetical protein in the 3'region of the gene for ribosomal protein S10.