Improving the Catalytic Activity of Cyclohexanone Monooxygenase-based Whole-cell Biocatalysts under Substrate Toxic Conditions

Abstract

The catalytic activity of oxygenase-based whole-cell biocatalysts is heavily influenced by substrate and product toxicities due to cell membrane permeabilization and protein denaturation effects of the organic substrates and products. Therefore, stability of oxygenase-based whole-cell biocatalysts against solvent stress was investigated with recombinant Escherichia coil BL21 and Corynebacterium glutamicum ATCC13032 expressing the chnB gene of cyclohexanone monooxygenase of Acinetobacter calcoaceticus NCIMB 9871. The cyclohexanone oxygenation activity of the recombinant biocatalysts rapidly decreased as cyclohexanone concentration increased from 2.4 to 26 g/L. However, treatment of the recombinant cells with non-lethal doses of cyclohexanone or preadaptation to the toxic substrate led to the oxygenation activity being relatively maintained. For instance, the oxygenation activity of cyclohexanone-treated E. coli cells was ca. 13 U per g dry cells at the substrate concentration of 26 g/L, which was almost 5-fold higher than that of the cyclohexanone-nontreated cells. In addition, biocatalytic activity was better maintained when the genes encoding chaperones (i.e., GroEL-ES and DnaKJ-GrpE) were coexpressed with the chnB gene. The positive effects of chaperones on the catalytic activity of the recombinant E. coli-based biocatalyst appeared to be related with expression level of biotransformation enzymes rather than with solvent stress-response metabolism. Overall, molecular chaperones, of which expression can be induced by solvent treatment, were involved in catalytic stability of whole-cell biocatalysts during biotransformations involving toxic compounds as the reactants.