Catalyzes the reversible isomerization of D-mannose to D-fructose. Shows weaker activity on D-lyxose, but cannot use N-acetyl D-glucosamine. D-mannose = D-fructose D-lyxose = D-xylulose Significantly inhibited by divalent metal ions such as Cu(2+), Cd(2+) or Ca(2+). kcat is 788 sec(-1) with D-mannose as substrate. kcat is 63.3 sec(-1) with D-lyxose as substrate. Optimum pH is around 8.0. Stable between pH 4 and 11. Optimum temperature is 60 degrees Celsius. Homodimer. Belongs to the N-acylglucosamine 2-epimerase family.