Application of microbial glucose isomerases in a cascade to convert xylose to rare sugars

Isomerization cascade is a promising strategy to prepare valuable rare sugars from biomass xylose. However current conversions are not satisfactory due to factors of enzyme efficiency and reaction equilibrium. An isomerase/isomerase cascade has been newly developed employing an isomerase of xylose and a tagatose-3-epimerase (DTE) to prepare xylulose and ribulose from xylose, with a total xylose conversion of 43.2 %. In this study, two microbial glucose isomerases (GI) were tested as alternatives to isomerize xylose for their high activity towards xylose and thermal stability, aiming to explore the adaption effect of enzyme in the isomerization type of cascade. GIs from Streptomyces rubiginosus (SrGI) and Piromyces sp. (PsGI) both showed higher xylose conversions than the previously used isomerase of xylose, ribose-5-phosphate isomerase. The optimal reaction conditions of GIs for xylose conversion were compared in the single transformation of xylose and the cascade reactions catalyzed by GI/DTE, and it revealed the importance of high reaction temperature to substrate conversion and enzyme deactivation. The activity profile and thermal stability of SrGI were better than PsGI, making it a potential biocatalyst in biomass sugar isomerization which was favorable at high temperatures. The conversion of xylose by the PsGI/DTE cascade was improved to 71.3 % by efforts of enzyme adaption and reaction condition adjusting. Additionally, the standstill of isomerization reaction of monosaccharide can be attributed to thermal dynamic equilibrium at low temperatures and enzyme deactivation at high temperatures based on the results of this study.