Sugar isomerase that catalyzes the reversible isomerization of D-lyxose to D-xylulose, and D-mannose to D-fructose (PubMed:20615418). Shows similar activity toward D-lyxose and D-mannose with a turnover and catalytic efficiency for D-lyxose as a substrate only 1.1- and 1.3-fold higher than those for D-mannose, respectively (PubMed:20615418). Shows weaker activity with L-gulose, D-talose, L-ribose and L-allose (PubMed:20615418). Overexpression enables cell growth on the rare pentose D-lyxose as the sole carbon source (PubMed:20615418). D-lyxose = D-xylulose D-mannose = D-fructose kcat is 13.7 sec(-1) with D-lyxose as substrate. kcat is 12.7 sec(-1) with D-mannose as substrate. kcat is 8.78 sec(-1) with L-gulose as substrate. Optimum pH is 7.5. Optimum temperature is 50 degrees Celsius. Homodimer; disulfide-linked (PubMed:20615418). Dimerization is facilitated through a disulfide bond between the two monomers of the dimeric enzyme (PubMed:20615418). Belongs to the D-lyxose ketol-isomerase family.