A highly active carboxylic acid reductase from Mycobacterium abscessus for biocatalytic reduction of vanillic acid to vanillin

Vanillin is a value added compound used in wide range of areas including food, beverage, fragrance and pharmaceutical. It can be produced by reduction of vanillic acid, a by-product in lignin degradation processes or a competing component in the metabolic pathways in biological vanillin production. The reduction process is energetically disadvantageous due to the high activation energy in the reaction coordinates, but can be overcome by an enzymatic strategy using carboxylic acid reductases (CARs). In this study, fourteen CARs heterologously expressed in Escherichia coli were tested for vanillic acid reduction, of which a CAR from Mycobacterium abscessus B1MLD7 gene, as a result, was selected for the highest activity. The selected CAR exhibited 8.6-fold higher in vitro activity than that of the CAR from Nocardia iowensis Q6RKB1, previously reported to have the highest activity in vanillic acid reduction. The effect of substrate concentration and reducing power supply on the biocatalytic reaction has been investigated to enhance the yield while minimizing the by-product formation, resulting in 2.86 g L⁻¹ of vanillin production in the whole cell biocatalytic reaction. The sequential and structural analyses of the enzyme with molecular modeling also have been carried out to discuss the catalytic characteristics of the enzyme.