Substrate specificity of a recombinant d-lyxose isomerase from Providencia stuartii for monosaccharides

The specific activity and catalytic efficiency (kcat/Km) of the recombinant putative protein from Providencia stuartii was the highest for d-lyxose among the aldose substrates, indicating that it is a d-lyxose isomerase. Gel filtration analysis suggested that the native enzyme is a dimer with a molecular mass of 44 kDa. The maximal activity for d-lyxose isomerization was observed at pH 7.5 and 45 °C in the presence of 1 mM Mn2+. The enzyme exhibited high isomerization activity for aldose substrates with the C2 and C3 hydroxyl groups in the left-hand configuration, such as d-lyxose, d-mannose, l-ribose, d-talose, and l-allose (listed in decreasing order of activity). The enzyme exhibited the highest activity for d-xylulose among all pentoses and hexoses. Thus, d-lyxose was produced at 288 g/l from 500 g/l d-xylulose by d-lyxose isomerase at pH 7.5 and 45 °C for 2 h, with a conversion yield of 58 % and a volumetric productivity of 144 g l− 1 h− 1. The observed kcat/Km (920 mM− 1 s− 1) of P. stuartiid-lyxose isomerase for d-xylulose is higher than any of the kcat/Km values previously reported for sugar and sugar phosphate isomerases with monosaccharide substrates. These results suggest that the enzyme will be useful as an industrial producer of d-lyxose.