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.
The authors declare no competing financial interests.
Nature thanks E. Meggers and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, b, Lactone (1) does not undergo spontaneous dehydrogenation to produce dehydrolactone (3) in solution, either under irradiation (a) or without irradiation (b). This further suggests that the reaction is not proceeding via KRED-catalysed alkene reduction.
Dehydrolactone (3, left) is unreactive under our reaction conditions; thus, product 2 is not formed by KRED-catalysed alkene reduction.
Product yield and enantiomeric excess remain relatively unchanged over a wide range of NADP loadings. This suggests that, once KRED is fully loaded with NADP, further increases in NADP concentration have little effect; that is, NADP needs to be bound to KRED in order to exert its effect in this reaction.
a, In the presence of KRED, there is a redshift in the absorbance spectrum of NADPH when substrate is added to the system. This shift is potentially indicative of a charge-transfer complex being formed between the substrate and NADPH. These spectra are overlaid with the emission spectrum of the blue LED source used in all experiments. b, In the absence of KRED, no such absorption shift is seen when substrate is added. Together, these data suggest that light (LED) irradiates a charge-transfer complex comprising substrate, NADPH and enzyme to initiate the catalytic cycle.
The lifetimes of fluorescence-excited states of NADPH, with and without enzyme and substrate, were determined using time-correlated single-photon counting (TCSPC) on a HORIBA Scientific DeltaFlex TCSPC system. Each sample was concentrated to an optical density of approximately 0.1 absorbance units and excited using a 305-nm laser; fluorescence emission decay was then probed at 460 nm. Data analysis was done using HORIBA Scientific DAS6 decay analysis software, whereby each data set was fit to an exponential curve to obtain the lifetimes.
Automated docking models were obtained using Autodock Vina28. Panels a and b indicate two different docking poses as predicted by Autodock. Chain C of RasADH (PDB = 4BMS) was used as a receptor and prepared using Autodock tools. Coordinates for all ligands were prepared using Gaussian and Autodock tools. The active site of RasADH was contained in a 10 × 10 × 10 grid with 1 Å spacing, centred around position 45 × 9.431 × 37.226, which approximately corresponds to the C4–H bond of NADPH. The exhaustiveness parameter was set to 10 and the rest of the docking parameters were set to default. Docking models for substrate 2 were accessed for their ability to rationalize the observed stereochemistry and provide a reasonable distance and geometry for hydrogen-atom transfer. The blue ribbon shows chain C of RasADH. Substrate 2 is shown in ball-and-stick format, with oxygens in red, lactone carbons in blue, and aromatic carbons in white. The left and right images are two different views of the same model.
The halolactone 1′ shows similar reactivity to its bromolactone counterpart (1) under the same reaction conditions.
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Emmanuel, M., Greenberg, N., Oblinsky, D. et al. Accessing non-natural reactivity by irradiating nicotinamide-dependent enzymes with light. Nature 540, 414–417 (2016). https://doi.org/10.1038/nature20569
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