Biosynthesis of D-xylulose 5-phosphate from D-xylose and polyphosphate through a minimized two-enzyme cascade.

Sugar phosphates cannot be produced easily by microbial fermentation because negatively-charged compounds cannot be secreted across intact cell membrane. D-xylulose 5-phosphate (Xu5P)， a very expensive sugar phosphate， was synthesized from D-xylose and polyphosphate catalyzed by enzyme cascades in one pot. The synthetic enzymatic pathway comprised of xylose isomerase and xylulokinase was designed to produce Xu5P， along with a third enzyme， polyphosphate kinase， responsible for in site ATP regeneration. Due to the promiscuous activity of the ATP-based xylulokinase from a hyperthermophilic bacterium Thermotoga maritima on polyphosphate， the number of enzymes in the pathway was minimized to two without polyphosphate kinase. The reactions catalyzed by the two-enzyme and three-enzyme pathways were compared for Xu5P production， and the reaction conditions were optimized by examining effects of reaction temperature， enzyme ratio and substrate concentration. The optimized two-enzyme system produced 32 mM Xu5P from 50 mM xylose and polyphosphate after 36 h at 45°C. Biosynthesis of less costly Xu5P from D-xylose and polyphosphate could be highly feasible via this minimized two-enzyme pathway.