Rational hinge engineering of carboxylic acid reductase from Mycobacterium smegmatis enhances its catalytic efficiency in biocatalysis

Background: Carboxylic acid reductases (CARs) represent useful tools for the production of aldehydes from ubiquitous organic carboxylic acids. However, the low catalytic efficiency of these enzymes hampers their application. Methods: Herein, a CAR originating from Mycobacterium smegmatis was redesigned through rational hinge engineering to enhance the catalytic efficiency. Results: Based on the unique domain architecture of CARs and their superfamily, a mutagenesis library of the hinge region was designed. The best mutant R505I/N506K showed a 6.57-fold improved catalytic efficiency. Molecular dynamics simulations showed the increased catalytic efficiency was due to the strong binding of the acyl-AMP complex with it. Meanwhile, the ε-nitrogen atom of Lys610 frequently interacted with the ribose-ring oxygen atom of the complex, the distance (d1) between them represents a great indicator for that. The d1 value was used as a nimble indicator to evaluate unexplored mutants of that region for enhanced activity by in silico mutational experiments. Overall, eight mutants were identified to show higher enhanced activity compared with wild-type enzyme and R505F/N506G showed the highest catalytic efficiency. Conclusion: Altogether, the two-step strategy used here provided useful references for the engineering of CARs and other similar multiple-domain enzymes. Graphical and Lay Summary Carboxylic acid reductases (CARs) provide useful tools for the production of aldehydes from organic carboxylic acids but the low catalytic efficiency hinders their application. In this study, the catalytic efficiency of CARs originating from Mycobacterium smegmatis was rationally enhanced through the two-step hinge region engineering. And the method used provides useful guidance for engineering other CARs and related enzymes.