Involved in the synthesis of the GDP-mannose and dolichol-phosphate-mannose required for a number of critical mannosyl transfer reactions. D-mannose 6-phosphate = D-fructose 6-phosphate Binds 1 zinc ion per subunit. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; alpha-D-mannose 1-phosphate from D-fructose 6-phosphate: step 1/2. Monomer. Belongs to the mannose-6-phosphate isomerase type 1 family.