Enzymatic One‐Step Reduction of Carboxylates to Aldehydes with Cell‐Free Regeneration of ATP and NADPH

The direct generation of aldehydes from carboxylic acids is often a challenging synthetic task but undoubtedly attractive in view of abundant supply of such feedstocks from nature. Though long known, biocatalytic carboxylate reductions are at an early stage of development, presumably because of their co‐factor requirement. To establish an alternative to whole‐cell‐based carboxylate reductions which are limited by side reactions, we developed an in vitro multi‐enzyme system that allows for quantitative reductions of various carboxylic acids with full recycling of all cofactors and prevention of undesired over‐reductions. Regeneration of adenosine 5′‐triphosphate is achieved through the simultaneous action of polyphosphate kinases from Meiothermus ruber and Sinorhizobium meliloti and β‐nicotinamide adenine dinucleotide 2′‐phosphate is reduced by a glucose dehydrogenase. Under these conditions and in the presence of the carboxylate reductases from Neurospora crassa or Nocardia iowensis, various aromatic, heterocyclic and aliphatic carboxylic acids were quantitatively reduced to the respective aldehydes.