Catalyzes the reversible isomerization of D-mannose to D-fructose. Can also isomerize D-lyxose, with lower efficiency. In longer reaction with a higher concentration of enzyme, it can isomerize 4-OH D-mannose derivatives (D-talose and 4-O-monosaccharyl-D-mannose). Cannot use D-glucose. D-mannose = D-fructose D-lyxose = D-xylulose kcat is 329 sec(-1) with D-mannose as substrate. kcat is 64.4 sec(-1) with D-lyxose as substrate. kcat is 0.319 sec(-1) with D-talose as substrate. Optimum pH is 7.3 with D-mannose as substrate. Optimum temperature is 30 degrees Celsius with D-mannose as substrate. Monomer. The alpha7-alpha8 and alpha11-alpha12 loops of the catalytic domain may participate in the formation of an open substrate-binding site to provide sufficient space to bind 4-OH D-mannose derivatives. Belongs to the N-acylglucosamine 2-epimerase family.