Technological developments enable the discovery of novel enzymes, the advancement of enzyme cascade designs and pathway engineering, moving biocatalysis into an era of technology integration, intelligent manufacturing and enzymatic total synthesis.

Carboxylic acid reductases (CARs) catalyze the direct adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) dependent reduction of carboxylic acids to their corresponding aldehydes. The identification and improvement of CARs by protein engineering is, however, severely limited by the lack of fast and generic methods to quantify aldehydes. Within this study, we applied a convenient high-throughput assay (HTA) based on amino benzamidoxime (ABAO) that allows the substrateindependent and chemoselective quantification of aldehydes. Random mutagenesis of the well-known CAR from Nocardia iowensis (CAR Ni ) to improve its activity for sterically demanding 2-substituted benzoic acid derivatives was conducted in a K M -dependent fashion, and the HTA applied in the presence of microbial cells. The study identified a hot spot in the active site of CAR Ni that increased the affinity to 2-methoxybenzoic acid 9-fold upon mutation from glutamine to proline (Q283P). The catalytic performance of CAR NiQ283P appeared to be significantly improved also for other substrates such as 2-substituted (2-Cl, 2-Br) as well as 3-and 4substituted benzoic acids (3-OMe, 4-OMe), and even aliphatic octanoic acid.