In chlorophyll biosynthesis, the light-activated enzyme protochlorophyllide oxidoreductase catalyzes trans addition of hydrogen across the C-17-C-18 double bond of the chlorophyll precursor protochlorophyllide (Pchlide). This unique lightdriven reaction plays a key role in the assembly of the photosynthetic apparatus, but despite its biological importance, the mechanism of light-activated catalysis is unknown. In this study, we show that Pchlide reduction occurs by dynamically coupled nuclear quantum tunneling of a hydride anion followed by a proton on the microsecond time scale in the Pchlide excited and ground states, respectively. We demonstrate the need for fast dynamic searches to form degenerate "tunneling-ready" configurations within the lifetime of the Pchlide excited state from which hydride transfer occurs. Moreover, we have found a breakpoint at ؊27°C in the temperature dependence of the hydride transfer rate, which suggests that motions/vibrations that are important for promoting light-activated hydride tunneling are quenched below ؊27°C. We observed no such breakpoint for the proton-tunneling reaction, indicating a reliance on different promoting modes for this reaction in the enzyme-substrate complex. Our studies indicate that the overall photoreduction of Pchlide is endothermic and that rapid dynamic searches are required to form distinct tunneling-ready configurations within the lifetime of the photoexcited state. Consequently, we have established the first important link between photochemical and nuclear quantum tunneling reactions, linked to protein dynamics, in a biologically significant system.Hydrogen transfer reactions are fundamental chemical processes that are essential for almost all biological reactions. H-transfer by tunneling is an important feature of these reactions in enzymes (1-3), but mechanistic understanding of how protein motions (from the millisecond to sub-picosecond time domain) facilitate the H-tunneling reactions remains elusive (4 -6). A major limitation has been the inability to synchronously trigger catalysis on ultrafast time scales for the majority of enzymes that require mixing strategies to initiate the reaction. However, by using the light-activated enzyme, protochlorophyllide oxidoreductase (POR 4 ; EC 1.3.1.33) (7), we have triggered two enzymatic H-transfer reactions using a single pulse of light, and we show these reactions occur sequentially by quantum tunneling in a pre-formed enzyme-substrate complex. This has provided a unique opportunity to analyze these reactions at physiological and cryogenic temperatures, on very fast time scales, that are experimentally inaccessible with other enzyme systems.POR catalyzes the trans addition of hydrogen across the C-17-C-18 double bond of the chlorophyll precursor protochlorophyllide (Pchlide) to produce chlorophyllide (Chlide) (7), a unique light-driven reaction in the synthesis of the most abundant pigment on earth, which plays a key role in the assembly of the photosynthetic apparatus (8, 9). In addition to POR, non...

The enzyme protochlorophyllide oxidoreductase (POR) catalyses a lightdependent step in chlorophyll biosynthesis that is essential to photosynthesis and ultimately all life on Earth. 1-3 POR, which is one of three known light-dependent enzymes, 4,5 catalyzes reduction of the photosensitizer and substrate protochlorophyllide to form the pigment chlorophyllide. Despite its biological importance, a structural basis for POR photocatalysis has remained elusive. Here, we report crystal structures of cyanobacterial PORs from Thermosynechococcus elongatus and Synechocystis sp. in their free forms, and in complex with nicotinamide coenzyme. Our structural models and simulations of the ternary protochlorophyllide-NADPH-POR complex have identified multiple interactions in the POR active site that are important for protochlorophyllide binding, photosensitization and photochemical conversion to chlorophyllide. We demonstrate the importance of active-site architecture and protochlorophyllide structure in experiments using POR variants and protochlorophyllide analogues. These studies reveal how the POR active site facilitates light-driven reduction of protochlorophyllide by localized hydride transfer from NADPH and long-range proton transfer along structurally defined proton-transfer pathways. As the light-driven step in the chlorophyll biosynthetic pathway (Fig. 1), the POR reaction acts as the trigger for the germination of seedlings =in plants and provokes a marked change in the morphological development of the plant. 2,3 Given this crucial biological role, POR has been the focus of numerous mechanistic and biophysical investigations. A combination of time-resolved (at the femtosecond-to-second scale) and cryogenic spectroscopy methods have provided some understanding of the mechanism of POR photocatalysis in a range of photosynthetic organisms, including cyanobacteria and plants. Picosecond excited-state dynamics in the protochlorophyllide (Pchlide) molecule are thought to result in excited state interactions between the substrate and active-site residues that are necessary to trigger the subsequent reaction chemistry. 6-12 This involves sequential transfer of a hydride equivalent from NADPH and a proton transfer from either an active site residue or solvent. Proton transfer is reliant on solvent dynamics and an implied network of extended protein motions that occur on the microsecond timescale. 13-17 Hydride transfer from NADPH is not concerted, but occurs in a stepwise manner that involves

We show that pentaerythritol tetranitrate reductase (PETNR), a member of the 'ene' reductase old yellow enzyme family, catalyses the asymmetric reduction of a variety of industrially relevant activated alpha,beta-unsaturated alkenes including enones, enals, maleimides and nitroalkenes. We have rationalised the broad substrate specificity and stereochemical outcome of these reductions by reference to molecular models of enzyme-substrate complexes based on the crystal complex of the PETNR with 2-cyclohexenone 4a. The optical purity of products is variable (49-99% ee), depending on the substrate type and nature of substituents. Generally, high enantioselectivity was observed for reaction products with stereogenic centres at Cbeta (>99% ee). However, for the substrates existing in two isomeric forms (e.g., citral 11a or nitroalkenes 18-19a), an enantiodivergent course of the reduction of E/Z-forms may lead to lower enantiopurities of the products. We also demonstrate that the poor optical purity obtained for products with stereogenic centres at Calpha is due to non-enzymatic racemisation. In reactions with ketoisophorone 3a we show that product racemisation is prevented through reaction optimisation, specifically by shortening reaction time and through control of solution pH. We suggest this as a general strategy for improved recovery of optically pure products with other biocatalytic conversions where there is potential for product racemisation.

Fatty acid photodecarboxylase (FAP) is a promising target for the production of biofuels and fine chemicals. It contains a flavin adenine dinucleotide cofactor and catalyzes the blue-light-dependent decarboxylation of fatty acids to generate the corresponding alkane. However, little is known about the catalytic mechanism of FAP, or how light is used to drive enzymatic decarboxylation. Here, we have used a combination of time-resolved and cryogenic trapping UV–visible absorption spectroscopy to characterize a red-shifted flavin intermediate observed in the catalytic cycle of FAP. We show that this intermediate can form below the “glass transition” temperature of proteins, whereas the subsequent decay of the species proceeds only at higher temperatures, implying a role for protein motions in the decay of the intermediate. Solvent isotope effect measurements, combined with analyses of selected site-directed variants of FAP, suggest that the formation of the red-shifted flavin species is directly coupled with hydrogen atom transfer from a nearby active site cysteine residue, yielding the final alkane product. Our study suggests that this cysteine residue forms a thiolate-flavin charge-transfer species, which is assigned as the red-shifted flavin intermediate. Taken together, our data provide insights into light-dependent decarboxylase mechanisms catalyzed by FAP and highlight important considerations in the (re)design of flavin-based photoenzymes.

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Protein dynamics are crucial for realizing the catalytic power of enzymes, but how enzymes have evolved to achieve catalysis is unknown. The light-activated enzyme protochlorophyllide oxidoreductase (POR) catalyzes sequential hydride and proton transfers in the photoexcited and ground states, respectively, and is an excellent system for relating the effects of motions to catalysis. Here, we have used the temperature dependence of isotope effects and solvent viscosity measurements to analyze the dynamics coupled to the hydride and proton transfer steps in three cyanobacterial PORs and a related plant enzyme. We have related the dynamic profiles of each enzyme to their evolutionary origin. Motions coupled to light-driven hydride transfer are conserved across all POR enzymes, but those linked to thermally activated proton transfer are variable. Cyanobacterial PORs require complex and solvent-coupled dynamic networks to optimize the proton donor-acceptor distance, but evolutionary pressures appear to have minimized such networks in plant PORs. POR from Gloeobacter violaceus has features of both the cyanobacterial and plant enzymes, suggesting that the dynamic properties have been optimized during the evolution of POR. We infer that the differing trajectories in optimizing a catalytic structure are related to the stringency of the chemistry catalyzed and define a functional adaptation in which active site chemistry is protected from the dynamic effects of distal mutations that might otherwise impact negatively on enzyme catalysis.Currently, one of the most challenging questions in biology is how enzymes have evolved to optimize the dynamic processes that enable their extraordinary rate enhancements (1-8). The role of protein motions and mechanisms of coupling to active site chemistry and solvent dynamics have been debated extensively (1, 5-10), but how the intrinsic motions of enzyme molecules have been affected by millions of years of evolutionary pressure (and the influence these motions have on catalysis) remains an open question. An evolutionary perspective of protein dynamics can be acquired only by considering functional differences between enzymes from species that span the evolutionary time scale. Hence, we have now studied this problem in the light-activated enzyme protochlorophyllide oxidoreductase (POR 3 ; EC 1.3.1.33), which is an excellent system for relating the effects of motions to catalysis in the context of proton and hydride transfer chemistry (11,12).POR catalyzes the light-dependent trans-addition of hydrogen across the C17-C18 double bond of the D-ring of protochlorophyllide (Pchlide) to produce chlorophyllide, an essential step in the synthesis of chlorophyll, the most abundant pigment on Earth (Fig. 1) (11, 13). The reaction involves a highly endergonic (ground state to excited state) light-driven hydride transfer from the pro-S face of the nicotinamide ring of NADPH to C17 of the Pchlide molecule (12, 14), followed by an exergonic (ground state) proton transfer from a conserved Tyr residue to...

We have conducted a site-specific saturation mutagenesis study of H181 and H184 of flavoprotein pentaerythritol tetranitrate reductase (PETN reductase) to probe the role of these residues in substrate binding and catalysis with a variety of α,β-unsaturated alkenes. Single mutations at these residues were sufficient to dramatically increase the enantiopurity of products formed by reduction of 2-phenyl-1-nitropropene. In addition, many mutants exhibited a switch in reactivity to predominantly catalyse nitro reduction, as opposed to CC reduction. These mutants showed an enhancement in a minor side reaction and formed 2-phenylpropanal oxime from 2-phenyl-1-nitropropene. The multiple binding conformations of hydroxy substituted nitro-olefins in PETN reductase were examined by using both structural and catalytic techniques. These compounds were found to bind in both active and inhibitory complexes; this highlights the plasticity of the active site and the ability of the H181/H184 couple to coordinate with multiple functional groups. These properties demonstrate the potential to use PETN reductase as a scaffold in the development of industrially useful biocatalysts.

Dysregulation of the kynurenine pathway (KP) leads to imbalances in neuroactive metabolites associated with the pathogenesis of several neurodegenerative disorders, including Huntington’s disease (HD). Inhibition of the enzyme kynurenine 3-monooxygenase (KMO) in the KP normalises these metabolic imbalances and ameliorates neurodegeneration and related phenotypes in several neurodegenerative disease models. KMO is thus a promising candidate drug target for these disorders, but known inhibitors are not brain permeable. Here, 19 new KMO inhibitors have been identified. One of these ( 1 ) is neuroprotective in a Drosophila HD model but is minimally brain penetrant in mice. The prodrug variant ( 1b ) crosses the blood–brain barrier, releases 1 in the brain, thereby lowering levels of 3-hydroxykynurenine, a toxic KP metabolite linked to neurodegeneration. Prodrug 1b will advance development of targeted therapies against multiple neurodegenerative and neuroinflammatory diseases in which KP likely plays a role, including HD, Alzheimer’s disease, and Parkinson’s disease.