In the present study, cutinase-CBMCenA fusion protein was genetically modified in the carbohydrate-binding module (CBM) binding sites, by site-directed mutagenesis, to enhance its activity toward polyethylene terephthalate (PET) fiber. The effects of tryptophan at particular positions of CBMCenA on the binding and hydrolysis of polyester substrate were investigated by replacing each of Trp14, Trp50 and Trp68 with leucine or tyrosine, respectively. All the mutants were expressed in Escherichia coli and purified to homogeneity. Enzyme characterization showed that the mutants displayed similar thermostability and pH stabilities in response to the native enzyme. Furthermore, W68L and W68Y, among all the mutants, exhibited significant improvement in binding and catalytic efficiency (1.4-1.5 fold) toward PET fiber when compared to that of the native enzyme. The enhanced binding and hydrolytic activity might be a result of creating new hydrogen bond or hydrophobic interaction between the enzyme and PET fiber.
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