The investigation of water oxidation in photosynthesis has remained a central topic in biochemical research for the last few decades due to the importance of this catalytic process for technological ...applications. Significant progress has been made following the 2011 report of a high-resolution X-ray crystallographic structure resolving the site of catalysis, a protein-bound Mn
4
CaO
x
complex, which passes through ≥5 intermediate states in the water-splitting cycle. Spectroscopic techniques complemented by quantum chemical calculations aided in understanding the electronic structure of the cofactor in all (detectable) states of the enzymatic process. Together with isotope labeling, these techniques also revealed the binding of the two substrate water molecules to the cluster. These results are described in the context of recent progress using X-ray crystallography with free-electron lasers on these intermediates. The data are instrumental for developing a model for the biological water oxidation cycle.
The understanding of light‐induced biological water oxidation in oxygenic photosynthesis is of great importance both for biology and (bio)technological applications. The chemically difficult ...multistep reaction takes place at a unique protein‐bound tetra‐manganese/calcium cluster in photosystem II whose structure has been elucidated by X‐ray crystallography (Umena et al. Nature 2011, 473, 55). The cluster moves through several intermediate states in the catalytic cycle. A detailed understanding of these intermediates requires information about the spatial and electronic structure of the Mn4Ca complex; the latter is only available from spectroscopic techniques. Here, the important role of Electron Paramagnetic Resonance (EPR) and related double resonance techniques (ENDOR, EDNMR), complemented by quantum chemical calculations, is described. This has led to the elucidation of the cluster's redox and protonation states, the valence and spin states of the manganese ions and the interactions between them, and contributed substantially to the understanding of the role of the protein surrounding, as well as the binding and processing of the substrate water molecules, the O‐O bond formation and dioxygen release. Based on these data, models for the water oxidation cycle are developed.
Light‐induced water oxidation and dioxygen release in photosynthesis is catalyzed by a paramagnetic μ‐oxo‐bridged Mn4Ca cofactor. It passes through five metastable states (S0 – S4) whose structure is described, focusing on the essential electronic structure obtained from spectroscopy, especially EPR techniques, supported by quantum chemistry. A catalytic cycle is presented, which describes structural isomers of key S‐state intermediates facilitating substrate binding and cofactor activation.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The photosynthetic protein complex photosystem II oxidizes water to molecular oxygen at an embedded tetramanganese-calcium cluster. Resolving the geometric and electronic structure of this cluster in ...its highest metastable catalytic state (designated S₃) is a prerequisite for understanding the mechanism of O-O bond formation. Here, multifrequency, multidimensional magnetic resonance spectroscopy reveals that all four manganese ions of the catalyst are structurally and electronically similar immediately before the final oxygen evolution step; they all exhibit a 4+ formal oxidation state and octahedral local geometry. Only one structural model derived from quantum chemical modeling is consistent with all magnetic resonance data; its formation requires the binding of an additional water molecule. O-O bond formation would then proceed by the coupling of two proximal manganese-bound oxygens in the transition state of the cofactor.
A central question in biological water splitting concerns the oxidation states of the manganese ions that comprise the oxygen-evolving complex of photosystem II. Understanding the nature and order of ...oxidation events that occur during the catalytic cycle of five S
states (
= 0-4) is of fundamental importance both for the natural system and for artificial water oxidation catalysts. Despite the widespread adoption of the so-called "high-valent scheme"-where, for example, the Mn oxidation states in the S
state are assigned as III, IV, IV, IV-the competing "low-valent scheme" that differs by a total of two metal unpaired electrons (
III, III, III, IV in the S
state) is favored by several recent studies for the biological catalyst. The question of the correct oxidation state assignment is addressed here by a detailed computational comparison of the two schemes using a common structural platform and theoretical approach. Models based on crystallographic constraints were constructed for all conceivable oxidation state assignments in the four (semi)stable S states of the oxygen evolving complex, sampling various protonation levels and patterns to ensure comprehensive coverage. The models are evaluated with respect to their geometric, energetic, electronic, and spectroscopic properties against available experimental EXAFS, XFEL-XRD, EPR, ENDOR and Mn K pre-edge XANES data. New 2.5 K
Mn ENDOR data of the S
state are also reported. Our results conclusively show that the entire S state phenomenology can only be accommodated within the high-valent scheme by adopting a single motif and protonation pattern that progresses smoothly from S
(III, III, III, IV) to S
(IV, IV, IV, IV), satisfying all experimental constraints and reproducing all observables. By contrast, it was impossible to construct a consistent cycle based on the low-valent scheme for all S states. Instead, the low-valent models developed here may provide new insight into the over-reduced S states and the states involved in the assembly of the catalytically active water oxidizing cluster.
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IJS, KILJ, NUK, UL, UM, UPUK
Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the ...similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II.
► Charge separation in Photosystem II is reviewed from a historical perspective. ► The electron acceptor reactions of Type II reaction centres is described and compared. ► Differences among Type II reaction centres are associated with photoprotection. ► We discuss some evolutionary aspects of Photosystem II photochemistry.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Among the four photo-driven transitions of the water-oxidizing tetramanganese-calcium cofactor of biological photosynthesis, the second-last step of the catalytic cycle, that is the S
to S
state ...transition, is the crucial step that poises the catalyst for the final O-O bond formation. This transition, whose intermediates are not yet fully understood, is a multi-step process that involves the redox-active tyrosine residue and includes oxidation and deprotonation of the catalytic cluster, as well as the binding of a water molecule. Spectroscopic data has the potential to shed light on the sequence of events that comprise this catalytic step, which still lacks a structural interpretation. In this work the S
-S
state transition is studied and a key intermediate species is characterized: it contains a Mn
O
Ca cubane subunit linked to a five-coordinate Mn(iv) ion that adopts an approximately trigonal bipyramidal ligand field. It is shown using high-level density functional and multireference wave function calculations that this species accounts for the near-infrared absorption and electron paramagnetic resonance observations on metastable S
-S
intermediates. The results confirm that deprotonation and Mn oxidation of the cofactor must precede the coordination of a water molecule, and lead to identification of a novel low-energy water binding mode that has important implications for the identity of the substrates in the mechanism of biological water oxidation.
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The ideal prostate cancer diagnostic pathway would maximise detection of clinically-significant prostate cancer (csPCa) while avoiding unnecessary biopsies and other investigations. The introduction ...of pre-biopsy MRI has done much to aid this goal. However, referrals into the image-based diagnostic pathway still depends on prostate-specific antigen (PSA) testing performed in primary care and interpreted using referral guidelines. In the UK, the National Institute for Health and Care Excellence (NICE) only provides guidance on PSA thresholds for men aged 50-69 years (PSA ≥3.0 ng/mL) 1. For other age groups, PSA thresholds are set by regional cancer networks without any unified consensus. Here we explored if different regional guidelines impacted csPCa detection in modern image-based pathways.
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BFBNIB, DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UILJ, UKNU, UL, UM, UPUK
Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, ...steering the mechanistic pathway, which involves at least five redox state intermediates S n (n = 0–4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S2 state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn4O5Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (MnA) of the cluster is deprotonated in the S2 state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal–metal distances. Deprotonation of Ca2+-bound water molecules is strongly disfavored in the S2 state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to MnIII. The present results impose limits for the total charge and the proton configuration of the OEC in the S2 state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.
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IJS, KILJ, NUK, PNG, UL, UM
Photosynthetic complex I enables cyclic electron flow around photosystem I, a regulatory mechanism for photosynthetic energy conversion. We report a 3.3-angstrom-resolution cryo-electron microscopy ...structure of photosynthetic complex I from the cyanobacterium
The model reveals structural adaptations that facilitate binding and electron transfer from the photosynthetic electron carrier ferredoxin. By mimicking cyclic electron flow with isolated components in vitro, we demonstrate that ferredoxin directly mediates electron transfer between photosystem I and complex I, instead of using intermediates such as NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). A large rate constant for association of ferredoxin to complex I indicates efficient recognition, with the protein subunit NdhS being the key component in this process.
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