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DOI: 10.1055/sos-SD-215-00109

Faber, K.; Hall, M.Science of Synthesis: Biocatalysis in Organic Synthesis, (20152236.

The high cost of the nicotinamide (NADH or NADPH) required as hydride source does not allow the application of ene-reductases on a large scale with molar equivalents of cofactor. An efficient alternative is to recycle the cofactor by employing the so-called coupled-enzyme approach, where an additional enzyme is present in the reaction medium, together with an inexpensive sacrificial cosubstrate. In case of ene-reductases, the cosubstrate is oxidized by the second enzyme at the expense of NAD(P)+, thereby allowing regeneration of the expensive NAD(P)H (Scheme 15). Care should be taken that the coproduct thus formed does not impede the efficiency of the overall redox process (via inhibition of ene-reductase, for instance) and the second enzyme should not display any activity (such as carbonyl reduction) on the substrate and product. Several approaches are available. The common systems used for this purpose include glucose dehydrogenase (GDH)/glucose, glucose 6-phosphate dehydrogenase (G6PDH)/glucose 6-phosphate, formate dehydrogenase (FDH)/formate, alcohol dehydrogenase (ADH)/propan-2-ol, and phosphite dehydrogenase (PDH)/phosphite. These helper proteins can be used either in soluble form or as whole cells or cell extracts (see Table 9, Section Alternatively, they may be coexpressed with ene-reductases in a single host to create a so-called “designer-bug” (see Schemes 1719, Section In the former case, all components are added at once into the reaction mixture and in the latter, whole cells are employed as a single catalyst.

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