Characterization of a recombinant D-mannose-producing D-lyxose isomerase from Caldanaerobius polysaccharolyticus

Recently, functional sugars, such as d-mannose, have attracted considerable attention due to their excellent physiological benefits for human health and wide applications in food and pharmaceutical industries. Therefore, d-mannose production using a sugar isomerase such as d-lyxose isomerase (d-LIase) has emerged as a research hotspot owing to its advantages over plant extraction and chemical synthesis methods. In this study, a putative d-LIase gene from Caldanaerobius polysaccharolyticus was cloned and expressed in Escherichia coli. Then, a biochemical characterization of the recombinant d-LIase was carried out and its potential use in d-mannose production also assessed. Results showed that d-LIase exhibited its maximum activity under these optimal conditions: temperature of 65 °C, a pH of 6.5, and the Mn²⁺ metal ion. The d-LIase was active at pH 6.0-8.0; it was also quite thermostable up to 60 °C and approximately 85 % of its maximum activity was retained after incubating for 4 h. Further, our Nano-DSC analysis determined that its melting temperature (T_m) was 70.74 °C. Using 100, 300, and 500 g L^-1 of d-fructose as substrate, 25.6, 74.4, and 115 g L^-1 of d-mannose were produced respectively, corresponding to a conversion rate of 25.6 %, 24.8 %, and 23.0 % under optimal conditions. Taken together, our results provide evidence for a promising candidate d-LIase for producing d-mannose directly from d-fructose.