Co-expression of d-glucose isomerase and d-psicose 3-epimerase: Development of an efficient one-step production of d-psicose

• We constructed a co-expression system of d-glucose isomerase and d-psicose 3-epimerase.
• We developed a one-step strategy to produce d-psicose using cheap sugar.
• HFCS was high-valued by converting d-fructose to d-psicose, a zero calorie sweetener.
• The amount of d-psicose was relatively high in the new products of HFCS.

d-Psicose has been attracting attention in recent years because of its alimentary activities and is used as an ingredient in a range of foods and dietary supplements. To develop a one-step enzymatic process of d-psicose production, thermoactive d-glucose isomerase and the d-psicose 3-epimerase obtained from Bacillus sp. and Ruminococcus sp., respectively, were successfully co-expressed in Escherichia coli BL21 strain. The substrate of one-step enzymatic process was d-glucose. The co-expression system exhibited maximum activity at 65 °C and pH 7.0. Mg2+ could enhance the output of d-psicose by 2.32 fold to 1.6 g/L from 10 g/L of d-glucose. When using high-fructose corn syrup (HFCS) as substrate, 135 g/L d-psicose was produced under optimum conditions. The mass ratio of d-glucose, d-fructose, and d-psicose was almost 3.0:2.7:1.0, when the reaction reached equilibrium after an 8 h incubation time. This co-expression system approaching to produce d-psicose has potential application in food and beverage products, especially softdrinks.