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A new approach, time-resolved serial femtosecond crystallography, is used to view the intermediate states of a photosystem complex following illumination, shedding light on proton transfer and O=O bond formation. Bond formation in photosystem II Technical developments, such as X-ray free electron lasers (XFEL), allow for a more detailed view of the structure of the photosystem complexes, making it possible to get a glimpse of the mechanisms of proton transfer and bond formation. Jian-Ren Shen and colleagues use a new approach, time-resolved serial femtosecond crystallography, with X-ray free electron lasers to view the intermediate states formed after two-flash illumination. Upon illumination, the authors see that the disappearance of one water molecule relocates another water molecule towards an oxygen atom, in a manner that may reflect proton transfer. They also gain evidence for the inclusion of a new oxygen atom that would be positioned to form an O=O bond that has been hypothesized but never previously detected. These insights increase our understanding of the mechanism of water oxidation in photosystem II. Photosystem II (PSII) is a huge membrane-protein complex consisting of 20 different subunits with a total molecular mass of 350 kDa for a monomer. It catalyses light-driven water oxidation at its catalytic centre, the oxygen-evolving complex (OEC) 1 , 2 , 3 . The structure of PSII has been analysed at 1.9 Å resolution by synchrotron radiation X-rays, which revealed that the OEC is a Mn 4 CaO 5 cluster organized in an asymmetric, ‘distorted-chair’ form 4 . This structure was further analysed with femtosecond X-ray free electron lasers (XFEL), providing the ‘radiation damage-free’ 5 structure. The mechanism of O=O bond formation, however, remains obscure owing to the lack of intermediate-state structures. Here we describe the structural changes in PSII induced by two-flash illumination at room temperature at a resolution of 2.35 Å using time-resolved serial femtosecond crystallography with an XFEL provided by the SPring-8 ångström compact free-electron laser. An isomorphous difference Fourier map between the two-flash and dark-adapted states revealed two areas of apparent changes: around the Q B /non-haem iron and the Mn 4 CaO 5 cluster. The changes around the Q B /non-haem iron region reflected the electron and proton transfers induced by the two-flash illumination. In the region around the OEC, a water molecule located 3.5 Å from the Mn 4 CaO 5 cluster disappeared from the map upon two-flash illumination. This reduced the distance between another water molecule and the oxygen atom O4, suggesting that proton transfer also occurred. Importantly, the two-flash-minus-dark isomorphous difference Fourier map showed an apparent positive peak around O5, a unique μ 4 -oxo-bridge located in the quasi-centre of Mn1 and Mn4 (refs 4 , 5 ). This suggests the insertion of a new oxygen atom (O6) close to O5, providing an O=O distance of 1.5 Å between these two oxygen atoms. This provides a mechanism for the O=O bond formation consistent with that proposed previously 6 , 7 .

Enzymes are increasingly explored for use in asymmetric synthesis 1 – 3 , but their applications are generally limited by the reactions available to naturally occurring enzymes. Recently, interest in photocatalysis 4 has spurred the discovery of novel reactivity from known enzymes 5 . However, so far photoinduced enzymatic catalysis 6 has not been used for the cross-coupling of two molecules. For example, the intermolecular coupling of alkenes with α-halo carbonyl compounds through a visible-light-induced radical hydroalkylation, which could provide access to important γ-chiral carbonyl compounds, has not yet been achieved by enzymes. The major challenges are the inherent poor photoreactivity of enzymes and the difficulty in achieving stereochemical control of the remote prochiral radical intermediate 7 . Here we report a visible-light-induced intermolecular radical hydroalkylation of terminal alkenes that does not occur naturally, catalysed by an ‘ene’ reductase using readily available α-halo carbonyl compounds as reactants. This method provides an efficient approach to the synthesis of various carbonyl compounds bearing a γ-stereocentre with excellent yields and enantioselectivities (up to 99 per cent yield with 99 per cent enantiomeric excess), which otherwise are difficult to access using chemocatalysis. Mechanistic studies suggest that the formation of the complex of the substrates (α-halo carbonyl compounds) and the ‘ene’ reductase triggers the enantioselective photoinduced radical reaction. Our work further expands the reactivity repertoire of biocatalytic, synthetically useful asymmetric transformations by the merger of photocatalysis and enzyme catalysis. A transformation in which an ‘ene’ reductase catalyses the visible-light-induced intermolecular radical hydroalkylation of alkenes gives carbonyl compounds with a remote stereocentre in high yield and enantioselectivity.

Photoexcitation is a common strategy for initiating radical reactions in chemical synthesis. We found that photoexcitation of flavin-dependent “ene”-reductases changes their catalytic function, enabling these enzymes to promote an asymmetric radical cyclization. This reactivity enables the construction of five-, six-, seven-, and eight-membered lactams with stereochemical preference conferred by the enzyme active site. After formation of a prochiral radical, the enzyme guides the delivery of a hydrogen atom from flavin—a challenging feat for small-molecule chemical reagents. The initial electron transfer occurs through direct excitation of an electron donor-acceptor complex that forms between the substrate and the reduced flavin cofactor within the enzyme active site. Photoexcitation of promiscuous flavoenzymes has thus furnished a previously unknown biocatalytic reaction.

hotoexcitation of a catalytic enzyme enzyme’s co-factor is shown to change the reactivity of the enzyme, enabling it to carry out a non-natural enantioselective dehalogenation of lactone molecules. New activity for nicotinamide-dependent enzymes Enzymatic catalysis offers high selectivity and efficiency for specific chemical reactions in complex environments, but these reactions are limited to those found in nature. Todd Hyster and colleagues report a new method for altering the catalytic behaviour of an enzyme, using the photoexcited state of its cofactor. Nicotinamide-dependent ketoreductases are transformed from a hydride source into a radical initiator and chiral hydrogen atom source. Using the new reactivity of the ketoreductase, the authors carry out enantioselective dehalogenation of lactones by irradiation with visible light, a challenging transformation by traditional small-molecule catalysis. 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 methods 1 . However, enzymes are limited to reactions that are found in nature and, as such, facilitate fewer types of transformation than do other forms of catalysis 2 . Thus, a longstanding challenge in the field of biologically mediated catalysis has been to develop enzymes with new catalytic functions 3 . 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 catalysts 4 , 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 regeneration 8 , 9 .

The ability to program new modes of catalysis into proteins would allow the development of enzyme families with functions beyond those found in nature. To this end, genetic code expansion methodology holds particular promise, as it allows the site-selective introduction of new functional elements into proteins as noncanonical amino acid side chains 1 – 4 . Here we exploit an expanded genetic code to develop a photoenzyme that operates by means of triplet energy transfer (EnT) catalysis, a versatile mode of reactivity in organic synthesis that is not accessible to biocatalysis at present 5 – 12 . Installation of a genetically encoded photosensitizer into the beta-propeller scaffold of DA_20_00 (ref.  13 ) converts a de novo Diels–Alderase into a photoenzyme for [2+2] cycloadditions (EnT1.0). Subsequent development and implementation of a platform for photoenzyme evolution afforded an efficient and enantioselective enzyme (EnT1.3, up to 99% enantiomeric excess (e.e.)) that can promote intramolecular and bimolecular cycloadditions, including transformations that have proved challenging to achieve selectively with small-molecule catalysts. EnT1.3 performs >300 turnovers and, in contrast to small-molecule photocatalysts, can operate effectively under aerobic conditions and at ambient temperatures. An X-ray crystal structure of an EnT1.3-product complex shows how multiple functional components work in synergy to promote efficient and selective photocatalysis. This study opens up a wealth of new excited-state chemistry in protein active sites and establishes the framework for developing a new generation of enantioselective photocatalysts. A genetically encoded triplet photosensitizer is used to develop an efficient photoenzyme that can promote enantioselective intramolecular and bimolecular [2+2] cycloadditions by means of triplet energy transfer.

Photobiocatalysis—where light is used to expand the reactivity of an enzyme—has recently emerged as a powerful strategy to develop chemistries that are new to nature. These systems have shown potential in asymmetric radical reactions that have long eluded small-molecule catalysts 1 . So far, unnatural photobiocatalytic reactions are limited to overall reductive and redox-neutral processes 2 – 9 . Here we report photobiocatalytic asymmetric sp 3 – sp 3 oxidative cross-coupling between organoboron reagents and amino acids. This reaction requires the cooperative use of engineered pyridoxal biocatalysts, photoredox catalysts and an oxidizing agent. We repurpose a family of pyridoxal-5′-phosphate-dependent enzymes, threonine aldolases 10 – 12 , for the α-C–H functionalization of glycine and α-branched amino acid substrates by a radical mechanism, giving rise to a range of α-tri- and tetrasubstituted non-canonical amino acids 13 – 15 possessing up to two contiguous stereocentres. Directed evolution of pyridoxal radical enzymes allowed primary and secondary radical precursors, including benzyl, allyl and alkylboron reagents, to be coupled in an enantio- and diastereocontrolled fashion. Cooperative photoredox–pyridoxal biocatalysis provides a platform for sp 3 – sp 3 oxidative coupling 16 , permitting the stereoselective, intermolecular free-radical transformations that are unknown to chemistry or biology. We report on the oxidative cross-coupling of organoboron reagents and amino acids via pyridoxal biocatalysis to produce non-canonical amino acids, uncovering stereoselective, intermolecular free-radical transformations.

Multicomponent reactions—those where three or more substrates combine into a product—have been highly useful in rapidly building chemical building blocks of increased complexity 1 , but achieving this enzymatically has remained rare 2 , 3 , 4 – 5 . This limitation primarily arises because an enzyme’s active site is not typically set up to address multiple substrates, especially in cases involving multiple radical intermediates 6 . Recently, chemical catalytic radical sorting has emerged as an enabling strategy for a variety of useful reactions 7 , 8 . However, making such processes enantioselective is highly challenging owing to the inherent difficulty in the stereochemical control of radicals 9 . Here we repurpose a thiamine-dependent enzyme 10 , 11 through directed evolution and combine it with photoredox catalysis to achieve a photobiocatalytic enantioselective three-component radical cross-coupling. This approach combines three readily available starting materials—aldehydes, α-bromo-carbonyls and alkenes—to give access to enantioenriched ketone products. Mechanistic investigations provide insights into how this dual photocatalyst–enzyme system precisely directs the three distinct radicals involved in the transformation, unlocking enzyme reactivity. Our approach has achieved exceptional stereoselectivity, with 24 out of 33 examples achieving ≥97% enantiomeric excess. Enantioselective three-component radical cross-coupling is achieved using a thiamine-dependent enzyme and photoredox catalysis, giving access to ketone products with exceptional stereoselectivity.

The maintenance of functional chloroplasts in photosynthetic eukaryotes requires real-time coordination of the nuclear and plastid genomes. Tetrapyrroles play a significant role in plastid-to-nucleus retrograde signaling in plants to ensure that nuclear gene expression is attuned to the needs of the chloroplast. Well-known sites of synthesis of chlorophyll for photosynthesis, plant chloroplasts also export heme and heme-derived linear tetrapyrroles (bilins), two critical metabolites respectively required for essential cellular activities and for light sensing by phytochromes. Here we establish that Chlamydomonas reinhardtii, one of many chlorophyte species that lack phytochromes, can synthesize bilins in both plastid and cytosol compartments. Genetic analyses show that both pathways contribute to iron acquisition from extracellular heme, whereas the plastid-localized pathway is essential for light-dependent greening and phototrophic growth. Our discovery of a bilin-dependent nuclear gene network implicates a widespread use of bilins as retrograde signals in oxygenic photosynthetic species. Our studies also suggest that bilins trigger critical metabolic pathways to detoxify molecular oxygen produced by photosynthesis, thereby permitting survival and phototrophic growth during the light period.

Yeast lacks dedicated photoreceptors; however, blue light still causes pronounced oscillations of the transcription factor Msn2 into and out of the nucleus. Here we show that this poorly understood phenomenon is initiated by a peroxisomal oxidase, which converts light into a hydrogen peroxide (H 2 O 2 ) signal that is sensed by the peroxiredoxin Tsa1 and transduced to thioredoxin, to counteract PKA-dependent Msn2 phosphorylation. Upon H 2 O 2 , the nuclear retention of PKA catalytic subunits, which contributes to delayed Msn2 nuclear concentration, is antagonized in a Tsa1-dependent manner. Conversely, peroxiredoxin hyperoxidation interrupts the H 2 O 2 signal and drives Msn2 oscillations by superimposing on PKA feedback regulation. Our data identify a mechanism by which light could be sensed in all cells lacking dedicated photoreceptors. In particular, the use of H 2 O 2 as a second messenger in signalling is common to Msn2 oscillations and to light-induced entrainment of circadian rhythms and suggests conserved roles for peroxiredoxins in endogenous rhythms. While yeasts lack dedicated photoreceptors, they nonetheless possess metabolic rhythms responsive to light. Here the authors find that light signalling in budding yeast involves the production of H 2 O 2 , which in turn regulates protein kinase A through a peroxiredoxin-thioredoxin redox relay.

BLUF photoreceptor: light-activated scissors BLUF is a photoreceptor protein domain that uses an FAD chromophore to sense blue light. Although X-ray crystal structures of single-domain BLUF proteins have been determined, there have not been any reports of a structure of a BLUF protein that also contained a functional 'output' domain. For this reason, the mechanism(s) of light activation for this class of photoreceptors has remained enigmatic. Here, Thomas Barends and colleagues report the first biochemical, structural, and mechanistic characterization of a full-length, active photoreceptor. The protein is from the bacterium Klebsiella pneumoniae , and it contains the BLUF sensor domain and a phosphodiesterase output domain that hydrolyses cyclic dimeric GMP. The structures of this protein co-complexed with its substrate and metal ions provide a detailed understanding of how light absorbed by the BLUF domain leads to activation of the phosphodiesterase output domain. Although structures of single-domain BLUF proteins—a photoreceptor protein domain that senses blue light—have been determined, there have been no reports of the structure of a BLUF protein containing a functional output domain; for this reason, the mechanism of light activation has remained enigmatic. The first biochemical, structural and mechanistic characterization of a full-length, active photoreceptor containing a BLUF sensor domain and a phosphodiesterase EAL output domain is now reported. The ability to respond to light is crucial for most organisms. BLUF is a recently identified photoreceptor protein domain that senses blue light using a FAD chromophore 1 . BLUF domains are present in various proteins from the Bacteria, Euglenozoa and Fungi. Although structures of single-domain BLUF proteins have been determined 2 , 3 , 4 , none are available for a BLUF protein containing a functional output domain; the mechanism of light activation in this new class of photoreceptors has thus remained poorly understood. Here we report the biochemical, structural and mechanistic characterization of a full-length, active photoreceptor, BlrP1 (also known as KPN_01598), from Klebsiella pneumoniae 5 . BlrP1 consists of a BLUF sensor domain and a phosphodiesterase EAL output domain which hydrolyses cyclic dimeric GMP (c-di-GMP). This ubiquitous second messenger controls motility, biofilm formation, virulence and antibiotic resistance in the Bacteria 6 , 7 , 8 , 9 . Crystal structures of BlrP1 complexed with its substrate and metal ions involved in catalysis or in enzyme inhibition provide a detailed understanding of the mechanism of the EAL-domain c-di-GMP phosphodiesterases. These structures also sketch out a path of light activation of the phosphodiesterase output activity. Photon absorption by the BLUF domain of one subunit of the antiparallel BlrP1 homodimer activates the EAL domain of the second subunit through allosteric communication transmitted through conserved domain–domain interfaces.