Highly reversible phosphoryltransferase. The phosphomannomutase activity produces a precursor for alginate polymerization, the alginate layer causes a mucoid phenotype and provides a protective barrier against host immune defenses and antibiotics. Also involved in core lipopolysaccaride (LPS) biosynthesis due to its phosphoglucomutase activity. Essential for rhamnolipid production, an exoproduct correlated with pathogenicity (PubMed:10481091). Required for biofilm production. The reaction proceeds via 2 processive phosphoryl transferase reactions; first from enzyme-phospho-Ser-108 to the substrate (generating a bisphosphorylated substrate intermediate and a dephosphorylated enzyme), a 180 degree rotation of the intermediate (probably aided by movement of domain 4), and subsequent transfer of phosphate back to the enzyme (PubMed:11716469, PubMed:16880541, PubMed:16595672, PubMed:22242625). alpha-D-mannose 1-phosphate = D-mannose 6-phosphate alpha-D-glucose 1-phosphate = alpha-D-glucose 6-phosphate Binds 1 Mg(2+) ion per subunit (PubMed:23517223). Zn(2+) can substitute, but yields a catalytically inactive enzyme (PubMed:14725765, PubMed:16880541, PubMed:16595672). Requires glucose 1,6-bisphosphate (G1,6P) as an activator (PubMed:8050998, PubMed:11716469). Reaction making glucose 6-phosphate is subject to substrate inhibition, reactions making mannose 1-phosphate or glucose 1-phosphate are not. 1-deoxyglucose 6-phosphate competitively inhibits glucose 1-phosphate (PubMed:11716469). Inhibited by xylose 1-phosphate (PubMed:16880541). kcat is 3000 min(-1) for glucose 1-phosphate and 1350 min(-1) for mannose 1-phosphate. TM is 66 degrees Celsius for phosphorylated protein and 62 degrees Celsius for unphosphorylated protein. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; alpha-D-mannose 1-phosphate from D-fructose 6-phosphate: step 2/2. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Monomer. By D-mannose 6-phosphate. Consists of 4 domains; domains 1-3 have a similar toplological core while domain 4 folds over and closes the active site from a hinge region. Mutants in the hinge region (residues 262 and 368-369) generally increase KM for glucose 1-phosphate 2-fold while reducing kcat about 10-fold (PubMed:18690721). May be phosphorylated, protein expressed in E.coli. No longer expresses O-antigen LPS side chain or A-band LPS, sensitive to serum, resistant to virus E79. Has no phosphomannomutase nor phosphoglucomutase activities (PubMed:7515870, PubMed:8050998). Does not make rhamnolipid (PubMed:10481091). Most crystals have Zn(2+) rather than Mg(2+) and are catalytically inactive. Belongs to the phosphohexose mutase family.