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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]

Fatty aldehyde production by chemical synthesis causes an immense burden to the environment. Within this study, we explored a sustainable, aldehyde‐selective and mild alternative approach by utilizing carboxylic acid reductases (CARs). CARs from Neurospora crassa (NcCAR), Thermothelomyces thermophila (TtCAR), Nocardia iowensis (NiCAR), Mycobacterium marinum (MmCAR) and Trametes versicolor (TvCAR) were overexpressed in E. coli K‐12 MG1655 RARE (DE3) and screened for medium‐ to long‐chain fatty acid (C6–C18) reduction. MmCAR showed the broadest tolerance towards all carbon‐chain lengths and was selected for further investigations of fatty aldehyde synthesis in whole cells. To yield relevant product concentrations, different limitations of CAR whole‐cell conversions were elucidated and compensated. We coupled an in vitro cofactor recycling system to a whole‐cell biocatalyst to support cofactor supply and achieved 12.36 g L−1 of octanal (STY 0.458 g L−1 h−1) with less than 1.5 % of 1‐octanol. [ABSTRACT FROM AUTHOR]

Fragrance and flavor industries could not imagine business without aldehydes. Processes for their commercial production raise environmental and ecological concerns. The chemical reduction of organic acids to aldehydes is challenging. To fulfill the demand of a mild and selective reduction of carboxylic acids to aldehydes, carboxylic acid reductases (CARs) are gaining importance. We identified two new subtype IV fungal CARs from Dichomitus squalens CAR (DsCAR) and Trametes versicolor CAR (Tv2CAR) in addition to literature known Trametes versicolor CAR (TvCAR). Expression levels were improved by the co‐expression of GroEL‐GroES with either the trigger factor or the DnaJ‐DnaK‐GrpE system. Investigation of the substrate scope of the three enzymes revealed overlapping substrate‐specificities. Tv2CAR and DsCAR showed a preferred pH range of 7.0 to 8.0 in bicine buffer. TvCAR showed highest activity at pH 6.5 to 7.5 in MES buffer and slightly reduced activity at pH 6.0 or 8.0. TvCAR appeared to tolerate a wider pH range without significant loss of activity. Type IV fungal CARs optimal temperature was in the range of 25–35 °C. TvCAR showed a melting temperature (Tm) of 55 °C indicating higher stability compared to type III and the other type IV fungal CARs (Tm 51–52 °C). Finally, TvCAR was used as the key enzyme for the bioreduction of 3,4‐dihydroxyphenylacetic acid to the antioxidant 3‐hydroxytyrosol (3‐HT) and gave 58 mM of 3‐HT after 24 h, which correlates to a productivity of 0.37 g L−1 h−1. [ABSTRACT FROM AUTHOR]