Development of a sugar isomerase cascade to convert D-xylose to rare sugars

Monosaccharides from biomass can be used as cost-effective raw materials to produce rare sugars which are valuable sweeteners. Enzymes play a pivotal role in such biomass biorefinery processes to develop novel synthetic routes, and improve the environmental friendliness of current routes. Here we propose a proof of concept biorefinery process to prepare useful rare sugars of D-xylulose and D-ribulose from D-xylose which was abundant in biomass. Ribose-5-phosphate isomerase B from Ochrobactrum sp. (OsRpiB) with a unique substrate scope was engineered to intensify the activity of transforming D-xylose to D-xylulose. A double mutant R94N/N137Q with 80% higher catalytic efficiency was identified, and kinetic and molecular dynamic studies were conducted to explore the mechanism underneath. The results revealed a correlation between enhancing stability and activity simultaneously. Then a one-pot enzyme cascade was constructed with OsRpiB and D-tagatose-3-epimerase from Pseudomonas cichorii and transformed D-xylulose further to D-ribulose. The conversions of D-xylose and D-xylulose in cascade reaction were 1.80-fold and 1.83-fold of that in separate reactions with the aid of process parameter adjustment. The presented work is the first reported enzyme cascade converting D-xylose to D-ribulose, and proves the promising role of monosaccharide isomerase platforms in the refinery of biomass.