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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) are emerging as valuable catalysts for the selective one-step reduction of carboxylic acids to their corresponding aldehydes. The substrate scope of CARs is exceptionally broad and offers potential for their application in diverse synthetic processes. Two major fields of application are 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. The latest applications of CARs are dominated by in vivo cascades and chemo-enzymatic reaction sequences. The challenge to fully exploit product selectivity is discussed. Recent developments in the characterization of CARs are summarized, with a focus on aspects related to the domain architecture and protein sequences of CAR enzymes. [ABSTRACT FROM AUTHOR]

Carboxylic acid reductases (CARs) are well-known for their eminent selective one-step synthesis of carboxylic acids to aldehydes. To date, however, few CARs have been identified and characterized, especially from fungal sources. In this study, the CAR from the white rot fungus Pycnoporus cinnabarinus (Pc CAR2) was expressed in Escherichia coli. Pc CAR2′s biochemical properties were explored in vitro after purification, revealing a melting temperature of 53 °C, while the reaction temperature optimum was at 35 °C. In the tested buffers, the enzyme showed a pH optimum of 6.0 and notably, a similar activity up to pH 7.5. Pc CAR2 was immobilized to explore its potential as a recyclable biocatalyst. Pc CAR2 showed no critical loss of activity after six cycles, with an average conversion to benzaldehyde of more than 85% per cycle. Immobilization yield and efficiency were 82% and 76%, respectively, on Ni-sepharose. Overall, our findings contribute to the characterization of a thermotolerant fungal CAR, and established a more sustainable use of the valuable biocatalyst. [Display omitted] • The carboxylic acid reductase (CAR) from Pycnoporus cinnabarinus Pc CAR2 is a pH and thermotolerant enzyme. • Targeted immobilization of Pc CAR2 maintains essential conformational flexibility of the enzyme and enables recycling. • All known CARs from subtype IV are from Basidiomycetes. [ABSTRACT FROM AUTHOR]

The chemoselective synthesis of aldehydes is a challenging task. Nature provides carboxylic acid reductases (CARs) as elegant tools for the direct reduction of carboxylic acids to their respective aldehydes. The discovery of new CARs and strains that efficiently produce these enzymes necessitates a robust high‐throughput assay with selectivity for aldehydes. We recently reported a simple assay that allows the substrate independent and chemoselective quantification of aldehydes (irrespective of their chemical structure). The assay utilized amino benzamidoxime (ABAO), which forms UV‐active and fluorescent dihydroquinazolines. In this study, we adapted the ABAO‐assay for the identification and comparison of Pichia pastoris clones with the ability to produce aldehydes from carboxylic acids. Specifically, CAR and PPTase from Mycobacterium marinum (MmCAR and MmPPTase) were co‐expressed using different bidirectional promoters (BDPs). A library of 598 clones was screened for piperonal production with the ABAO assay and the results were validated by HPLC quantification. 1 OD unit of the best Pichia pastoris clone 2.A7, regulating MmCAR and MmPPTase expression by two strong constitutive promoters, fully converted 5 mM of piperonylic acid within 2 h. [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]

The selective and direct reduction of carboxylic acids into the corresponding aldehydes by chemical methods is still a challenging task in synthesis. Several reductive and oxidative chemical methods are known to produce aldehydes, but most of them require expensive reagents, special reaction conditions, are two‐step procedures and often lack chemoselectivity. Nature provides an elegant tool, so called carboxylic acid reductases (CARs) for the direct reduction of carboxylic acids to aldehydes. Discovery as well as engineering of novel CAR enzymes necessitates a robust, product selective high‐throughput assay (HTA). We report a simple and fast HTA that allows the substrate‐independent and chemoselective quantification of aldehydes (irrespective of their chemical structure) and is sensitive to the nM range. The HTA was validated by NMR and GC analyses and in microbial cells by reexamination of the substrate scope of CAR from Nocardia iowensis (CARNi). The results were fully consistent with reported data. [ABSTRACT FROM AUTHOR]

The enzymatic reduction of carboxylic acids is in its infancy with only a handful of biocatalysts available to this end. We have increased the spectrum of carboxylate-reducing enzymes (CARs) with the sequence of a fungal CAR from Neurospora crassa OR74A ( NcCAR). NcCAR was efficiently expressed in E. coli using an autoinduction protocol at low temperature. It was purified and characterized in vitro, revealing a broad substrate acceptance, a pH optimum at pH 5.5-6.0, a Tm of 45 °C and inhibition by the co-product pyrophosphate which can be alleviated by the addition of pyrophosphatase. The synthetic utility of NcCAR was demonstrated in a whole-cell biotransformation using the Escherichia coli K-12 MG1655 RARE strain in order to suppress overreduction to undesired alcohol. The fragrance compound piperonal was prepared from piperonylic acid (30 mM) on gram scale in 92 % isolated yield in >98% purity. This corresponds to a productivity of 1.5 g/L/h. [ABSTRACT FROM AUTHOR]