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.
Keywords: aldehydes; biocatalysis; carboxylate reductase; carboxylic acids; flavours and fragrances.