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Carboxylate reductases (CARs) are valuable catalysts for the selective one‐step reduction of carboxylic acids to their corresponding aldehydes. In recent years, numerous new CARs have been made available, studied and applied in the context of biocatalytic syntheses. The preparation of aldehydes as end products for the flavor and fragrance sector and the integration of CARs in cascade reactions with aldehydes as the key intermediates represent the two major fields of applications. This review gives a comprehensive overview of the current toolbox of recombinant CARs and their numerous carboxylate substrates. Non‐natural functions of CARs are highlighted and recent insight into the structure‐function relationship of CARs are summarized. [ABSTRACT FROM AUTHOR]

Carboxylate reductases (CARs, E.C. 1.2.1.30) generate aldehydes from their corresponding carboxylic acid with high selectivity. Little is known about the structure of CARs and their catalytically important amino acid residues. The identification of key residues for carboxylate reduction provides a starting point to gain deeper understanding of enzymatic carboxylate reduction. A multiple sequence alignment of CARs with confirmed activity recently identified in our lab and fromthe literature revealed a fingerprint of conserved amino acids. We studied the function of conserved residues by multiple sequence alignments and mutational replacements of these residues. In this study, single-site alanine variants of Neurospora crassa CAR were investigated to determine the contribution of conserved residues to the function, expressability or stability of the enzyme. The effect of amino acid replacements was investigated by analyzing enzymatic activity of the variants in vivo and in vitro. Supported by molecular modeling, we interpreted that five of these residues are essential for catalytic activity, or substrate and co-substrate binding. We identified amino acid residues having significant impact on CAR activity. Replacement of His 237, Glu 433, Ser 595, Tyr 844, and Lys 848 by Ala abolish CAR activity, indicating their key role in acid reduction. These results may assist in the functional annotation of CAR coding genes in genomic databases. While some other conserved residues decreased activity or had no significant impact, four residues increased the specific activity of NcCAR variants when replaced by alanine. Finally, we showed that NcCAR wild-type and mutants efficiently reduce aliphatic acids. [ABSTRACT FROM AUTHOR]

• Polyphosphate (polyP) sources differ significantly in their capability to provide reactive phosphate groups for the polyphosphate kinase-based ATP regeneration • The concentration of ATP and NADP+ in the in vitro reduction of carboxylic acids can be reduced to 200 μM and 100 μM, respectively • Lower NADP+ concentrations allow reducing the formation of alcohol from the aldehyde products through side activities in the enzyme preparations Addressing the challenges associated with the development of in vitro biocatalytic carboxylate reductions for potential applications, important aspects of the co-factor regeneration systems and strategies for minimizing over-reduction were investigated. The ATP recycling can be performed with similarly high efficiency exploiting the polyphosphate source by combining Meiothermus ruber polyphosphate kinase and adenylate kinase or with Sinorhizobium meliloti polyphosphate kinase instead of the latter. Carboxylate reductions with the enzyme candidates used in this work allow operating at co-factor concentrations of adenosine 5′-triphosphate and β-nicotinamide adenine dinucleotide 2′-phosphate of 100 μM and, thereby, reducing the amounts of alcohols formed by side activities in the enzyme preparations. This study confirmed the expected benefits of carboxylic acid reductases in chemoselectively reducing the carboxylates to the corresponding aldehydes while leaving reductively-sensitive nitro, ester and cyano groups intact. [ABSTRACT FROM AUTHOR]