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.  However, the poor thermostability and low catalytic efficiency are serious constraints on industrial application.  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.  Moreover, Co(2+) enhanced the thermostability significantly, including the t1/2 and Tm values. 
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