Exploration of glucose isomerases in enzymatic cascades converting glucose and xylose synergistically

Lignocellulosic monosaccharides are valuable substrates for the production of high value-added chemicals. An enzymatic cascade was previously developed to convert two main biomass monosaccharides, D-xylose and d -glucose synergistically and produce rare sugars of D-xylulose and D-arabitol. However, the conversion of D-xylose was less than 80%, resulting in low reaction efficiency and tedious separation issues. In this study, the reaction was reevaluated from the viewpoint of enzyme adaption, and the isomerization of D-xylose was conducted by glucose isomerases instead of ribose-5-phosphate isomerase B to overcome the limitation of conversion. Under fine-tuned reaction conditions, glucose isomerases from Streptomyces rubiginosus (SrGI) and Piromyces sp. (PsGI) were combined with the arabitol dehydrogenase from Gluconobacter sp. JX-05 (GsArDH) and glucose dehydrogenase from Bacillus megaterium IWG3 (BmGDH), to convert D-xylose completely at 100 mM. In comparison, SrGI performed better than PsGI in the cascade system and the reaction was finished in 2 h. However, higher substrate concentrations led to uncompleted conversion of D-xylose due to substrate inhibition. This study demonstrated the feasibility of systematic refinement of lignocellulosic monosaccharides, achieved complete conversion of D-xylose, and provided insights relevant to the utilization potential of biomass lignocellulose.