Involved in GDP-mannose biosynthesis which serves as the activated sugar nucleotide precursor for mannose residues in cell surface polysaccharides. This enzyme participates in synthesis of the LPS group C2 O antigen. alpha-D-mannose 1-phosphate = D-mannose 6-phosphate Binds 1 Mg(2+) ion per subunit. 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; LPS O-antigen biosynthesis. Belongs to the phosphohexose mutase family.