Improving the Thermostability and Catalytic Efficiency of the d-Psicose 3-Epimerase from Clostridium bolteae ATCC BAA-613 Using Site-Directed Mutagenesis.

d-Psicose is a highly valuable rare sugar because of its excellent physiological properties and commercial potential. d-Psicose 3-epimerase (DPEase) is the key enzyme catalyzing the isomerization of d-fructose to d-psicose. However, the poor thermostability and low catalytic efficiency are serious constraints on industrial application. To address these issues, site-directed mutagenesis of Tyr68 and Gly109 of the Clostridium bolteae DPEase was performed. Compared with the wild-type enzyme, the Y68I variant displayed the highest substrate-binding affinity and catalytic efficiency, and the G109P variant showed the highest thermostability. Furthermore, the double-site Y68I/G109P variant was generated and exhibited excellent enzyme characteristics. The Km value decreased by 17.9%; the kcat/Km increased by 1.2-fold; the t1/2 increased from 156 to 260 min; and the melting temperature (Tm) increased by 2.4 °C. Moreover, Co(2+) enhanced the thermostability significantly, including the t1/2 and Tm values. All of these indicated that the Y68I/G109P variant would be appropriate for the industrial production of d-psicose.