Enzymes are ideal for use in asymmetric catalysis by the chemical industry, because their chemical compositions can be tailored to a specific substrate and selectivity pattern while providing efficiencies and selectivities that surpass those of classical synthetic methods1. However, enzymes are limited to reactions that are found in nature and, as such, facilitate fewer types of transformation than do other forms of catalysis2. Thus, a longstanding challenge in the field of biologically mediated catalysis has been to develop enzymes with new catalytic functions3. Here we describe a method for achieving catalytic promiscuity that uses the photoexcited state of nicotinamide co-factors (molecules that assist enzyme-mediated catalysis). Under irradiation with visible light, the nicotinamide-dependent enzyme known as ketoreductase can be transformed from a carbonyl reductase into an initiator of radical species and a chiral source of hydrogen atoms. We demonstrate this new reactivity through a highly enantioselective radical dehalogenation of lactones—a challenging transformation for small-molecule catalysts4,5,6,7. Mechanistic experiments support the theory that a radical species acts as an intermediate in this reaction, with NADH and NADPH (the reduced forms of nicotinamide adenine nucleotide and nicotinamide adenine dinucleotide phosphate, respectively) serving as both a photoreductant and the source of hydrogen atoms. To our knowledge, this method represents the first example of photo-induced enzyme promiscuity, and highlights the potential for accessing new reactivity from existing enzymes simply by using the excited states of common biological co-factors. This represents a departure from existing light-driven biocatalytic techniques, which are typically explored in the context of co-factor regeneration8,9.
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Financial support was provided by Princeton University. D.G.O. also acknowledges financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC). We thank the MacMillan group for use of their chiral high-performance liquid-chromatography and cyclic-voltammetry equipment; G. Scholes for providing the time-resolved fluorescence instrument; H. Yayla of the Knowles group for assistance with cyclic-voltammetry experiments; B. Shields of the Doyle group and the Scholes Group for collection of the LED emission spectrum; and G. Huisman of Codexis for conversations regarding the nature of the mutants in the Codexis KRED kit.
This file contains Supplementary Methods, Supplementary Text and Data, Supplementary Figures 1-10 and additional references (see Contents for more details).
About this article
Scientific Reports (2017)