Bioproduction of high-allulose-fructose syrup from D-glucose catalyzed by recombinant E. coli co-expressing glucose isomerase and D-allulose 3-epimerase

High-fructose syrup (HFS) is a sugar substitute widely used in food and beverages and can also be used as an inexpensive substrate for production of rare sugars, such as D-allulose (or D-psicose) etc. Biotransformation of HFS to produce D-allulose contained HFS, called High-allulose-fructose syrup (HAFS), can be used as a value added sweetener with healthy functions. The recombinant Escherichia coli expressing glucose isomerase (GI) from Caldicellulosiruptor bescii DSM 6725 was constructed and used for production of HFS from D-glucose. A dual enzyme expression system for expressing above GI and D-allulose 3-epimerase (DAE or D-psicose 3-epimerase, DPE) from Ruminococcus sp. 5_1_39BFAA was constructed for production of HAFS from D-glucose. The recombinant strains developed had strong substrate tolerance and high capacity in production of HFS and HAFS. The cultivation conditions of above strains were made, and the biotransformation conditions of HAFS catalyzed by the cells of two recombinant strains expressing GI and DAE, respectively, or by the cells of the single strain coexpressing GI and DAE were optimized. Under the optimum conditions, about 18.6, 110.4 and 163.8 g/L Dallulose can be obtained from 100, 600 and 900 g/L D-glucose respectively. The HAFS containing about 18.2% (163.8 g/L) D-allulose, 36.3% (326.7 g/L) D-fructose and 45.5% (409.5 g/L) D-glucose was obtained at the initial D-glucose concentration of 900 g/L catalyzed by the cells of the single strain co-expressing GI and DAE. The production process developed in this study was simple, highly efficient, and can provide reference for the production of HAFS.