Seems to be involved in the degradation of glucomannan. D-mannose 6-phosphate = D-fructose 6-phosphate Binds 1 zinc ion per subunit. Up-regulated by konjac glucomannan and by cellobiose and mannobiose, the possible degradation products of glucomannan. Repressed by glucose via the carbon catabolite repression system. Also repressed by GmuR. Belongs to the mannose-6-phosphate isomerase type 1 family.