Involved in the biosynthesis of the capsular polysaccharide colanic acid. 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. Belongs to the phosphohexose mutase family.