Amino acid residues involved in the coordination and assembly of the manganese cluster of photosystem II. Proton-coupled electron transport of the redox-active tyrosines and its relationship to water oxidation

Control of charge transport in electronically active systems towards integrated biomolecular circuits (IbC)

The miniaturization of traditional silicon-based electronics will soon reach its limitation as quantum tunneling and heat become serious problems at the several-nanometer scale. Crafting integrated circuits via self-assembly of electronically active molecules using a "bottom-up" paradigm provides a potential solution to these technological challenges. In particular, integrated biomolecular circuits (IbC) offer promising advantages to achieve this goal, as nature offers countless examples of functionalities entailed by self-assembly and examples of controlling charge transport at the molecular level within the self-assembled structures. To this end, the review summarizes the progress in understanding how charge transport is regulated in biosystems and the key redox-active amino acids that enable the charge transport. In addition, charge transport mechanisms at different length scales are also reviewed, offering key insights for controlling charge transport in IbC in the future.

Modeling the Characteristic Residues of Chlorophyll f Synthase (ChlF) from Halomicronema hongdechloris to Determine Its Reaction Mechanism

Photosystem II (PSII) is a quinone-utilizing photosynthetic system that converts light energy into chemical energy and catalyzes water splitting. PsbA (D1) and PsbD (D2) are the core subunits of the reaction center that provide most of the ligands to redox-active cofactors and exhibit photooxidoreductase activities that convert quinone and water into quinol and dioxygen. The performed analysis explored the putative uncoupled electron transfer pathways surrounding P680+ induced by far-red light (FRL) based on photosystem II (PSII) complexes containing substituted D1 subunits in Halomicronema hongdechloris. Chlorophyll f-synthase (ChlF) is a D1 protein paralog. Modeling PSII-ChlF complexes determined several key protein motifs of ChlF. The PSII complexes included a dysfunctional Mn4CaO5 cluster where ChlF replaced the D1 protein. We propose the mechanism of chlorophyll f synthesis from chlorophyll a via free radical chemistry in an oxygenated environment created by over-excited pheophytin a and an inactive water splitting reaction owing to an uncoupled Mn4CaO5 cluster in PSII-ChlF complexes. The role of ChlF in the formation of an inactive PSII reaction center is under debate, and putative mechanisms of chlorophyll f biosynthesis are discussed.

Mutation of a metal ligand stabilizes the high-spin form of the S2 state in the O2-producing Mn4CaO5 cluster of photosystem II

The residue D1-D170 bridges Mn4 with the Ca ion in the O₂-evolving Mn₄CaO₅ cluster of Photosystem II. Recently, the D1-D170E mutation was shown to substantially alter the S_n+1-minus-S_n FTIR difference spectra [Debus RJ (2021) Biochemistry 60:3841-3855]. The mutation was proposed to alter the equilibrium between different Jahn-Teller conformers of the S₁ state such that (i) a different S₁ state conformer is stabilized in D1-D170E than in wild-type and (ii) the S₁ to S₂ transition in D1-D170E produces a high-spin form of the S₂ state rather than the low-spin form that is produced in wild-type. In this study, we employed EPR spectroscopy to test if a high-spin form of the S₂ state is formed preferentially in D1-D170E PSII. Our data show that illumination of dark-adapted D1-D170E PSII core complexes does indeed produce a high-spin form of the S₂ state rather than the low-spin multiline form that is produced in wild-type. This observation provides further experimental support for a change in the equilibrium between S state conformers in both the S₁ and S₂ states in a site-directed mutant that retains substantial O₂ evolving activity.

From manganese oxidation to water oxidation: assembly and evolution of the water-splitting complex in photosystem II

The manganese cluster of photosystem II has been the focus of intense research aiming to understand the mechanism of H₂O-oxidation. Great effort has also been applied to investigating its oxidative photoassembly process, termed photoactivation that involves the light-driven incorporation of metal ions into the active Mn₄CaO₅ cluster. The knowledge gained on these topics has fundamental scientific significance, but may also provide the blueprints for the development of biomimetic devices capable of splitting water for solar energy applications. Accordingly, synthetic chemical approaches inspired by the native Mn cluster are actively being explored, for which the native catalyst is a useful benchmark. For both the natural and artificial catalysts, the assembly process of incorporating Mn ions into catalytically active Mn oxide complexes is an oxidative process. In both cases this process appears to share certain chemical features, such as producing an optimal fraction of open coordination sites on the metals to facilitate the binding of substrate water, as well as the involvement of alkali metals (e.g., Ca²⁺) to facilitate assembly and activate water-splitting catalysis. This review discusses the structure and formation of the metal cluster of the PSII H₂O-oxidizing complex in the context of what is known about the formation and chemical properties of different Mn oxides. Additionally, the evolutionary origin of the Mn₄CaO₅ is considered in light of hypotheses that soluble Mn²⁺ was an ancient source of reductant for some early photosynthetic reaction centers ('photomanganotrophy'), and recent evidence that PSII can form Mn oxides with structural resemblance to the geologically abundant birnessite class of minerals. A new functional role for Ca²⁺ to facilitate sustained Mn²⁺ oxidation during photomanganotrophy is proposed, which may explain proposed physiological intermediates during the likely evolutionary transition from anoxygenic to oxygenic photosynthesis.

Alteration of the O2-Producing Mn4Ca Cluster in Photosystem II by the Mutation of a Metal Ligand

The O₂-evolving Mn₄Ca cluster in photosystem II (PSII) is arranged as a distorted Mn₃Ca cube that is linked to a fourth Mn ion (denoted as Mn4) by two oxo bridges. The Mn4 and Ca ions are bridged by residue D1-D170. This is also the only residue known to participate in the high-affinity Mn(II) site that participates in the light-driven assembly of the Mn₄Ca cluster. In this study, we use Fourier transform infrared difference spectroscopy to characterize the impact of the D1-D170E mutation. On the basis of analyses of carboxylate and carbonyl stretching modes and the O-H stretching modes of hydrogen-bonded water molecules, we show that this mutation alters the extensive network of hydrogen bonds that surrounds the Mn₄Ca cluster in the same manner as that of many other mutations. It also alters the equilibrium between conformers of the Mn₄Ca cluster in the dark-stable S₁ state so that a high-spin form of the S₂ state is produced during the S₁-to-S₂ transition instead of the low-spin form that gives rise to the S₂ state multiline electron paramagnetic resonance signal. The mutation may also change the coordination mode of the carboxylate group at position 170 to unidentate ligation of Mn4. This is the first mutation of a metal ligand in PSII that substantially impacts the spectroscopic signatures of the Mn₄Ca cluster without substantially eliminating O₂ evolution. The results have significant implications for our understanding of the roles of alternate active/inactive conformers of the Mn₄Ca cluster in the mechanism of O₂ formation.

Resolution of chemical shift anisotropy in 19F ENDOR spectroscopy at 263 GHz/9.4 T

Pulsed ¹⁹F ENDOR spectroscopy provides a selective method for measuring angstrom to nanometer distances in structural biology. Here, the performance of ¹⁹F ENDOR at fields of 3.4 T and 9.4 T is compared using model compounds containing one to three ¹⁹F atoms. CF₃ groups are included in two compounds, for which the possible occurrence of uniaxial rotation might affect the distance distribution. At 9.4 T, pronounced asymmetric features are observed in many of the presented ¹⁹F ENDOR spectra. Data analysis by spectral simulations shows that these features arise from the chemical shift anisotropy (CSA) of the ¹⁹F nuclei. This asymmetry is also observed at 3.4 T, albeit to a much smaller extent, confirming the physical origin of the effect. The CSA parameters are well consistent with DFT predicted values and can be extracted from simulation of the experimental data in favourable cases, thereby providing additional information about the geometrical and electronic structure of the spin system. The feasibility of resolving the CSA at 9.4 T provides important information for the interpretation of line broadening in ENDOR spectra also at lower fields, which is relevant for developing methods to extract distance distributions from ¹⁹F ENDOR spectra.

Mimicking the Catalytic Center for the Water-Splitting Reaction in Photosystem II

The oxygen-evolving center (OEC) in photosystem II (PSII) of plants, algae and cyanobacteria is a unique natural catalyst that splits water into electrons, protons and dioxygen. The crystallographic studies of PSII have revealed that the OEC is an asymmetric Mn4CaO5-cluster. The understanding of the structure-function relationship of this natural Mn4CaO5-cluster is impeded mainly due to the complexity of the protein environment and lack of a rational chemical model as a reference. Although it has been a great challenge for chemists to synthesize the OEC in the laboratory, significant advances have been achieved recently. Different artificial complexes have been reported, especially a series of artificial Mn4CaO4-clusters that closely mimic both the geometric and electronic structures of the OEC in PSII, which provides a structurally well-defined chemical model to investigate the structure-function relationship of the natural Mn4CaO5-cluster. The deep investigations on this artificial Mn4CaO4-cluster could provide new insights into the mechanism of the water-splitting reaction in natural photosynthesis and may help the development of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

Tuning Radical Relay Residues by Proton Management Rescues Protein Electron Hopping

Transient tyrosine and tryptophan radicals play key roles in the electron transfer (ET) reactions of photosystem (PS) II, ribonucleotide reductase (RNR), photolyase, and many other proteins. However, Tyr and Trp are not functionally interchangeable, and the factors controlling their reactivity are often unclear. Cytochrome c peroxidase (CcP) employs a Trp191^•+ radical to oxidize reduced cytochrome c (Cc). Although a Tyr191 replacement also forms a stable radical, it does not support rapid ET from Cc. Here we probe the redox properties of CcP Y191 by non-natural amino acid substitution, altering the ET driving force and manipulating the protic environment of Y191. Higher potential fluorotyrosine residues increase ET rates marginally, but only addition of a hydrogen bond donor to Tyr191^• (via Leu232His or Glu) substantially alters activity by increasing the ET rate by nearly 30-fold. ESR and ESEEM spectroscopies, crystallography, and pH-dependent ET kinetics provide strong evidence for hydrogen bond formation to Y191^• by His232/Glu232. Rate measurements and rapid freeze quench ESR spectroscopy further reveal differences in radical propagation and Cc oxidation that support an increased Y191^• formal potential of ∼200 mV in the presence of E232. Hence, Y191 inactivity results from a potential drop owing to Y191^•+ deprotonation. Incorporation of a well-positioned base to accept and donate back a hydrogen bond upshifts the Tyr^• potential into a range where it can effectively oxidize Cc. These findings have implications for the Y_Z/Y_D radicals of PS II, hole-hopping in RNR and cryptochrome, and engineering proteins for long-range ET reactions.

'Photosystem II: the water splitting enzyme of photosynthesis and the origin of oxygen in our atmosphere'

About 3 billion years ago an enzyme emerged which would dramatically change the chemical composition of our planet and set in motion an unprecedented explosion in biological activity. This enzyme used solar energy to power the thermodynamically and chemically demanding reaction of water splitting. In so doing it provided biology with an unlimited supply of reducing equivalents needed to convert carbon dioxide into the organic molecules of life while at the same time produced oxygen to transform our planetary atmosphere from an anaerobic to an aerobic state. The enzyme which facilitates this reaction and therefore underpins virtually all life on our planet is known as Photosystem II (PSII). It is a pigment-binding, multisubunit protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Today we have detailed understanding of the structure and functioning of this key and unique enzyme. The journey to this level of knowledge can be traced back to the discovery of oxygen itself in the 18th-century. Since then there has been a sequence of mile stone discoveries which makes a fascinating story, stretching over 200 years. But it is the last few years that have provided the level of detail necessary to reveal the chemistry of water oxidation and O-O bond formation. In particular, the crystal structure of the isolated PSII enzyme has been reported with ever increasing improvement in resolution. Thus the organisational and structural details of its many subunits and cofactors are now well understood. The water splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino-acid side chains, of which seven provide direct ligands to the metals. The metal cluster is organised as a cubane structure composed of three Mn ions and a Ca2+ linked by oxo-bonds with the fourth Mn ion attached to the cubane. This structure has now been synthesised in a non-protein environment suggesting that it is a totally inorganic precursor for the evolution of the photosynthetic oxygen-evolving complex. In summary, the overall structure of the catalytic site has given a framework on which to build a mechanistic scheme for photosynthetic dioxygen generation and at the same time provide a blue-print and incentive to develop catalysts for artificial photo-electrochemical systems to split water and generate renewable solar fuels.

Photoactivation: The Light-Driven Assembly of the Water Oxidation Complex of Photosystem II

Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II. The assembly of the Mn4O5Ca requires light and involves a sequential process called photoactivation. This process harnesses the charge-separation of the photochemical reaction center and the coordination environment provided by the amino acid side chains of the protein to oxidize and organize the incoming manganese ions to form the oxo-bridged metal cluster capable of H2O-oxidation. Although most aspects of this assembly process remain poorly understood, recent advances in the elucidation of the crystal structure of the fully assembled cyanobacterial PSII complex help in the interpretation of the rich history of experiments designed to understand this process. Moreover, recent insights on the structure and stability of the constituent ions of the Mn4CaO5 cluster may guide future experiments. Here we consider the literature and suggest possible models of assembly including one involving single Mn(2+) oxidation site for all Mn but requiring ion relocation.

Bioinspired Immobilization of Glycerol Dehydrogenase by Metal Ion-Chelated Polyethyleneimines as Artificial Polypeptides

In this study, a novel, simple and generally applicable strategy for multimeric oxidoreductase immobilization with multi-levels interactions was developed and involved activity and stability enhancements. Linear polyethyleneimines (PEIs) are flexible cationic polymers with molecular weights that span a wide range and are suitable biomimic polypeptides for biocompatible frameworks for enzyme immobilization. Metal ion-chelated linear PEIs were applied as a heterofunctional framework for glycerol dehydrogenase (GDH) immobilization by hydrogen bonds, electrostatic forces and coordination bonds interactions. Nanoparticles with diameters from 250-650 nm were prepared that exhibited a 1.4-fold enhancement catalytic efficiency. Importantly, the half-life of the immobilized GDH was enhanced by 5.6-folds in aqueous phase at 85 °C. A mechanistic illustration of the formation of multi-level interactions in the PEI-metal-GDH complex was proposed based on morphological and functional studies of the immobilized enzyme. This generally applicable strategy offers a potential technique for multimeric enzyme immobilization with the advantages of low cost, easy operation, high activity reservation and high stability.

Water oxidation energy diagrams for photosystem II for different protonation states, and the effect of removing calcium

The main parts of the water oxidation mechanism in photosystem II have now been established both from theory and experiments. Still, there are minor questions remaining. One of them concerns the charge and the protonation state of the oxygen evolving complex (OEC). Previously, theory and experiments have agreed that the two water derived ligands on the outer manganese should be one hydroxide and one water. In the present study it is investigated whether both of them could be water. This question is addressed by a detailed study of energy diagrams, but in this context it is more conclusive to compare the redox potential of the OEC to the one of TyrZ. Both procedures lead to the conclusion that one of the ligands is a hydroxide. Another question concerns the protonation of the second shell His337, where the results are more ambiguous. The final part of the present study describes results when calcium is removed from the OEC. Even though protons enter to compensate the charge of the missing Ca(2+), the redox potential and the pKa value of the OEC change dramatically and prevent the progress after S2.

Ammonium tolerance in the cyanobacterium Synechocystis sp. strain PCC 6803 and the role of the psbA multigene family

Ammonium is one of the major nutrients for plants, and a ubiquitous intermediate in plant metabolism, but it is also known to be toxic to many organisms, in particular to plants and oxygenic photosynthetic microorganisms. Although previous studies revealed a link between ammonium toxicity and photodamage in cyanobacteria under in vivo conditions, ammonium-induced photodamage of photosystem II (PSII) has not yet been investigated with isolated thylakoid membranes. We show here that ammonium directly accelerated photodamage of PSII in Synechocystis sp. strain PCC6803, rather than affecting the repair of photodamaged PSII. Using isolated thylakoid membranes, it could be demonstrated that ammonium-induced photodamage of PSII primarily occurred at the oxygen evolution complex, which has a known binding site for ammonium. Wild-type Synechocystis PCC6803 cells can tolerate relatively high concentrations of ammonium because of efficient PSII repair. Ammonium tolerance requires all three psbA genes since mutants of any of the three single psbA genes are more sensitive to ammonium than wild-type cells. Even the poorly expressed psbA1 gene, whose expression was studied in some detail, plays a detectable role in ammonium tolerance.

Synthesis and evaluation of heterocyclic analogues of bromoxynil

One attractive strategy to discover more active and/or crop-selective herbicides is to make structural changes to currently registered compounds. This strategy is especially appealing for those compounds with limited herbicide resistance and whose chemistry is accompanied with transgenic tools to enable herbicide tolerance in crop plants. Bromoxynil is a photosystem II (PSII) inhibitor registered for control of broadleaf weeds in several agronomic and specialty crops. Recently at the University of Tennessee-Knoxville several analogues of bromoxynil were synthesized including a previously synthesized pyridine (2,6-dibromo-5-hydroxypyridine-2-carbonitrile sodium salt), a novel pyrimidine (4,6-dibromo-5-hydroxypyrimidine-2-carbonitrile sodium salt), and a novel pyridine N-oxide (2,6-dibromo-1-oxidopyridin-1-ium-4-carbonitrile). These new analogues of bromoxynil were also evaluated for their herbicidal activity on soybean (Glycine max), cotton (Gossypium hirsutum), redroot pigweed (Amaranthus retroflexus), velvetleaf (Abutilon theophrasti), large crabgrass (Digitaria sanguinalis), and pitted morningglory ( Ipomoea lacunose ) when applied at 0.28 kg ha(-1). A second study was conducted on a glyphosate-resistant weed (Amaranthus palmeri) with the compounds being applied at 0.56 kg ha(-1). Although all compounds were believed to inhibit PSII by binding in the quinone binding pocket of D1, the pyridine and pyridine-N-oxide analogues were clearly more potent than bromoxynil on Amaranthus retroflexus. However, application of the pyrimidine herbicide resulted in the least injury to all species tested. These variations in efficacy were investigated using molecular docking simulations, which indicate that the pyridine analogue may form a stronger hydrogen bond in the pocket of the D1 protein than the original bromoxynil. A pyridine analogue was able to control the glyphosate-resistant Amaranthus palmeri with >80% efficacy. The pyridine analogues of bromoxynil showed potential to have a different weed control spectrum compared to bromoxynil. A pyridine analogue of bromoxynil synthesized in this research controlled several weed species greater than bromoxynil itself, potentially due to enhanced binding within the PSII binding pocket. Future research should compare this analogue to bromoxynil using optimized formulations at higher application rates.

Solvent effect on electron and proton transfer in the excited state of a hydrogen bonded phenol–imidazole complex

Density functional theory calculations have been carried out for the ground state (S₀) and the first excited state (S₁) of the H-bonded phenol and imidazole complex as a model system for the active site of photosystem II.

Water oxidation mechanism in photosystem II, including oxidations, proton release pathways, O-O bond formation and O2 release

The present status of DFT studies on water oxidation in photosystem II is described. It is argued that a full understanding of all steps is close. In each S-transition, the manganese that is oxidized and the proton released are strongly implicated, and structures of all intermediates have been determined. For the S2-state, recent important experimental findings support key elements of the structure and the mechanism. In this mechanism, the O-O bond is formed between an oxyl radical in the center of the cluster and an Mn-bridging μ-oxo ligand, which was suggested already in 2006. The DFT structure of the oxygen evolving complex, suggested in 2008, is very similar to the recent high-resolution X-ray structure. Some new aspects of the interaction between P680 and the OEC are suggested. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Photosynthetic energy storage efficiency in Chlamydomonas reinhardtii, based on microsecond photoacoustics

Using a novel, pulsed micro-second time-resolved photoacoustic (PA) instrument, we measured thermal dissipation and energy storage (ES) in the intact cells of wild type (WT) Chlamydomonas reinhardtii, and mutants lacking either PSI or PSII reaction centers (RCs). On this time scale, the kinetic contributions of the thermal expansion component due to heat dissipation of absorbed energy and the negative volume change due to electrostriction induced by charge separation in each of the photosystems could be readily distinguished. Kinetic analysis revealed that PSI and PSII RCs exhibit strikingly different PA signals where PSI is characterized by a strong electrostriction signal and a weak thermal expansion component while PSII has a small electrostriction component and large thermal expansion. The calculated ES efficiencies at ~10 μs were estimated to be 80 ± 5 and 50 ± 13% for PSII-deficient mutants and PSI-deficient mutants, respectively, and 67 ± 2% for WT. The overall ES efficiency was positively correlated with the ratio of PSI to PSI + PSII. Our results suggest that the shallow excitonic trap in PSII limits the efficiency of ES as a result of an evolutionary frozen metabolic framework of two photosystems in all oxygenic photoautotrophs.

Ligation of D1-His332 and D1-Asp170 to the manganese cluster of photosystem II from Synechocystis assessed by multifrequency pulse EPR spectroscopy

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is used to ascertain the nature of the bonding interactions of various active site amino acids with the Mn ions that compose the oxygen-evolving cluster (OEC) in photosystem II (PSII) from the cyanobacterium Synechocystis sp. PCC 6803 poised in the S(2) state. Spectra of natural isotopic abundance PSII ((14)N-PSII), uniformly (15)N-labeled PSII ((15)N-PSII), and (15)N-PSII containing (14)N-histidine ((14)N-His/(15)N-PSII) are compared. These complementary data sets allow for a precise determination of the spin Hamiltonian parameters of the postulated histidine nitrogen interaction with the Mn ions of the OEC. These results are compared to those from a similar study on PSII isolated from spinach. Upon mutation of His332 of the D1 polypeptide to a glutamate residue, all isotopically sensitive spectral features vanish. Additional K(a)- and Q-band ESEEM experiments on the D1-D170H site-directed mutant give no indication of new (14)N-based interactions.

Molecular mechanisms of photodamage in the Photosystem II complex

Light induced damage of the photosynthetic apparatus is an important and highly complex phenomenon, which affects primarily the Photosystem II complex. Here the author summarizes the current state of understanding of the molecular mechanisms, which are involved in the light induced inactivation of Photosystem II electron transport together with the relevant mechanisms of photoprotection. Short wavelength ultraviolet radiation impairs primarily the Mn₄Ca catalytic site of the water oxidizing complex with additional effects on the quinone electron acceptors and tyrosine donors of PSII. The main mechanism of photodamage by visible light appears to be mediated by acceptor side modifications, which develop under conditions of excess excitation in which the capacity of light-independent photosynthetic processes limits the utilization of electrons produced in the initial photoreactions. This situation of excess excitation facilitates the reduction of intersystem electron carriers and Photosystem II acceptors, and thereby induces the formation of reactive oxygen species, especially singlet oxygen whose production is sensitized by triplet chlorophyll formation in the reaction center of Photosystem II. The highly reactive singlet oxygen and other reactive oxygen species, such as H₂O₂ and O₂⁻, which can also be formed in Photosystem II initiate damage of electron transport components and protein structure. In parallel with the excess excitation dependent mechanism of photodamage inactivation of the Mn₄Ca cluster by visible light may also occur, which impairs electron transfer through the Photosystem II complex and initiates further functional and structural damage of the reaction center via formation of highly oxidizing radicals, such as P 680(+) and Tyr-Z(+). However, the available data do not support the hypothesis that the Mn-dependent mechanism would be the exclusive or dominating pathway of photodamage in the visible spectral range. This article is part of a Special Issue entitled: Photosystem II.

Recent theoretical studies of water oxidation in photosystem II

In the present mini-review, computational work over the past decade on water oxidation in photosystem II (PSII) is summarized. The size of the chemical model used for the oxygen evolving complex (OEC) has during this time increased from the initial 20 atoms to the present day 220 atoms. The electronic structure methods used have during the same period only undergone minor improvements. It is concluded that the results have now reached a high level of convergence and the predictions for both the structure of the OEC and the O-O bond formation mechanism are most probably of higher accuracy than presently available from experiments.

Protonation, tautomerization, and rotameric structure of histidine: a comprehensive study by magic-angle-spinning solid-state NMR

Histidine structure and chemistry lie at the heart of many enzyme active sites, ion channels, and metalloproteins. While solid-state NMR spectroscopy has been used to study histidine chemical shifts, the full pH dependence of the complete panel of (15)N, (13)C, and (1)H chemical shifts and the sensitivity of these chemical shifts to tautomeric structure have not been reported. Here we use magic-angle-spinning solid-state NMR spectroscopy to determine the (15)N, (13)C, and (1)H chemical shifts of histidine from pH 4.5 to 11. Two-dimensional homonuclear and heteronuclear correlation spectra indicate that these chemical shifts depend sensitively on the protonation state and tautomeric structure. The chemical shifts of the rare π tautomer were observed for the first time, at the most basic pH used. Intra- and intermolecular hydrogen bonding between the imidazole nitrogens and the histidine backbone or water was detected, and N-H bond length measurements indicated the strength of the hydrogen bond. We also demonstrate the accurate measurement of the histidine side-chain torsion angles χ(1) and χ(2) through backbone-side chain (13)C-(15)N distances; the resulting torsion angles were within 4° of the crystal structure values. These results provide a comprehensive set of benchmark values for NMR parameters of histidine over a wide pH range and should facilitate the study of functionally important histidines in proteins.

Zwitterion modulation of O(2)-evolving activity of cyanobacterial photosystem II

Photosystem II (PSII) is the only enzyme in nature that can catalyze the challenging catalytic photooxidation of H(2)O into four protons, four electrons, and O(2). Slowing down turnover of the O(2)-evolving complex (OEC) is a plausible approach to gain mechanistic information on the reaction. However, modulating the kinetics of the reaction without perturbing the active site is a challenge. In this study, it is shown that the steady-state activity of cyanobacterial PSII is inhibited by small zwitterions, such as glycine betaine and β-alanine. We show that the binding of zwitterions is nondenaturing, is highly reversible, and results in the decrease of the rate of catalytic turnover by ∼50% in the presence of excess zwitterion. Control measurements of photoinduced electron transfer in O(2)-inactive PSII show that the inhibition by zwitterions is the result of a specific decrease in the rate of catalytic turnover of the OEC. Recovery of activity upon addition of an exogenous proton carrier (HCO(3)(-)) provides evidence that proton-transfer pathways, thought to be essential for the relay of protons from the OEC to the lumen, are affected. Interestingly, no inhibition is observed for spinach PSII, suggesting that zwitterions act specifically by binding to the extrinsic proteins on the lumenal side of PSII, which differ significantly between plants and cyanobacteria, to slow proton transfer on the electron donor side of PSII.

Evidence from FTIR difference spectroscopy of an extensive network of hydrogen bonds near the oxygen-evolving Mn(4)Ca cluster of photosystem II involving D1-Glu65, D2-Glu312, and D1-Glu329

Analyses of the refined X-ray crystallographic structures of photosystem II (PSII) at 2.9-3.5 A have revealed the presence of possible channels for the removal of protons from the catalytic Mn(4)Ca cluster during the water-splitting reaction. As an initial attempt to verify these channels experimentally, the presence of a network of hydrogen bonds near the Mn(4)Ca cluster was probed with FTIR difference spectroscopy in a spectral region sensitive to the protonation states of carboxylate residues and, in particular, with a negative band at 1747 cm(-1) that is often observed in the S(2)-minus-S(1) FTIR difference spectrum of PSII from the cyanobacterium Synechocystis sp. PCC 6803. On the basis of its 4 cm(-1) downshift in D(2)O, this band was assigned to the carbonyl stretching vibration (C horizontal lineO) of a protonated carboxylate group whose pK(a) decreases during the S(1) to S(2) transition. The positive charge that forms on the Mn(4)Ca cluster during the S(1) to S(2) transition presumably causes structural perturbations that are transmitted to this carboxylate group via electrostatic interactions and/or an extended network of hydrogen bonds. In an attempt to identify the carboxylate group that gives rise to this band, the FTIR difference spectra of PSII core complexes from the mutants D1-Asp61Ala, D1-Glu65Ala, D1-Glu329Gln, and D2-Glu312Ala were examined. In the X-ray crystallographic models, these are the closest carboxylate residues to the Mn(4)Ca cluster that do not ligate Mn or Ca and all are highly conserved. The 1747 cm(-1) band is present in the S(2)-minus-S(1) FTIR difference spectrum of D1-Asp61Ala but absent from the corresponding spectra of D1-Glu65Ala, D2-Glu312Ala, and D1-Glu329Gln. The band is also sharply diminished in magnitude in the wild type when samples are maintained at a relative humidity of </=85%. It is proposed that D1-Glu65, D2-Glu312, and D1-Glu329 participate in a common network of hydrogen bonds that includes water molecules and the carboxylate group that gives rise to the 1747 cm(-1) band. It is further proposed that the mutation of any of these three residues, or partial dehydration caused by maintaining samples at a relative humidity of <or=85%, disrupts the network sufficiently that the structural perturbations associated with the S(1) to S(2) transition are no longer transmitted to the carboxylate group that gives rise to the 1747 cm(-1) band. Because D1-Glu329 is located approximately 20 A from D1-Glu65 and D2-Glu312, the postulated network of hydrogen bonds must extend for at least 20 A across the lumenal face of the Mn(4)Ca cluster. The D1-Asp61Ala, D1-Glu65Ala, and D2-Glu312Ala mutations also appear to substantially decrease the fraction of PSII reaction centers that undergo the S(3) to S(0) transition in response to a saturating flash. This behavior is consistent with D1-Asp61, D1-Glu65, and D2-Glu312 participating in a dominant proton egress channel that links the Mn(4)Ca cluster with the thylakoid lumen.

Substitution of chloride by bromide modifies the low-temperature tyrosine Z oxidation in active photosystem II

Chloride is an essential cofactor for photosynthetic water oxidation. However, its location and functional roles in active photosystem II are still a matter of debate. We have investigated this issue by studying the effects of Cl- replacement by Br- in active PSII. In Br- substituted samples, Cl- is effectively replaced by Br- in the presence of 1.2 M NaBr under room light with protection of anaerobic atmosphere followed by dialysis. The following results have been obtained. i) The oxygen-evolving activities of the Br--PSII samples are significantly lower than that of the Cl--PSII samples; ii) The same S2 multiline EPR signals are observed in both Br- and Cl--PSII samples; iii) The amplitudes of the visible light induced S1TyrZ* and S2TyrZ* EPR signals are significantly decreased after Br- substitution; the S1TyrZ* EPR signal is up-shifted about 8G, whereas the S2TyrZ* signal is down-shifted about 12 G after Br- substitution. These results imply that the redox properties of TyrZ and spin interactions between TyrZ* and Mn-cluster could be significantly modified due to Br- substitution. It is suggested that Cl-/Br- probably coordinates to the Ca2+ ion of the Mn-cluster in active photosystem II.

Structures and energetics for O2 formation in photosystem II

Water oxidation, forming O(2) from water and sunlight, is a fundamental process for life on earth. In nature, the enzyme photosystem II (PSII) catalyzes this reaction. The oxygen evolving complex (OEC), the complex within PSII that catalyzes the actual formation of the O-O bond, contains four manganese atoms and one calcium atom connected by oxo bonds. Seven amino acid side chains in the structure, mostly carboxylates, are ligated to the metal atoms. In the study of many enzyme mechanisms, theoretical modeling using density functional theory has served as an indispensable tool. This Account summarizes theoretical research to elucidate the mechanism for water oxidation in photosynthesis, including the most recent findings. The development of successively larger models, ranging from 50 atoms in the active site up to the present model size of 170 atoms, has revealed the mechanism of O(2) formation with increasing detail. The X-ray crystal structures of PSII have provided a framework for optimizing the theoretical models. By constraint of the backbone atoms to be at the same positions as those in the X-ray structures, the theoretical structures are in good agreement with both the measured electron density and extended X-ray absorption fine structure (EXAFS) interpretations. By following the structural and energetic changes in those structures through the different steps in the catalytic process, we have modeled the oxidation of the catalytic complex, the binding of the two substrate water molecules, and the subsequent deprotonations of those substrate molecules. In these models, the OEC forms a basin into which the water molecules naturally fit. These findings demonstrate that the binding of the second water molecule causes a reconstruction, results that are consistent with earlier EXAFS measurements. Most importantly, this Account describes a low-barrier mechanism for formation of the O-O bond, involving an oxygen radical that reacts with a mu-oxo ligand of the OEC. Further research revealed that the oxygen radical is bound in the Mn(3)Ca cube rather than to the outside manganese. This Account provides detailed diagrams of the energetics of the different S-transitions both without and with a membrane gradient. An interesting detail of these reactions concerns the role of the tyrosine (Tyr(Z)), which appears as an intermediate radical in the oxidation of the OEC. By simple electrostatic arguments, these results show that the initial oxidation of Tyr(Z) is downhill for the first two transitions but uphill for the final ones. In these later transitions, the oxidation of the OEC is coupled to deprotonations of water.

Effect of anions on the binding and oxidation of divalent manganese and iron in modified bacterial reaction centers

The influence of different anions on the binding and oxidation of manganous and ferrous cations was studied in four mutants of bacterial reaction centers that can bind and oxidize these metal ions. Light-minus-dark difference optical and electron paramagnetic resonance spectroscopies were applied to monitor electron transfer from bound divalent metal ions to the photo-oxidized bacteriochlorophyll dimer in the presence of five different anions. At pH 7, bicarbonate was found to be the most effective for both manganese and iron binding, with dissociation constants around 1 muM in three of the mutants. The pH dependence of the dissociation constants for manganese revealed that only bicarbonate and acetate were able to facilitate the binding and oxidation of the metal ion between pH 6 and 8 where the tight binding in their absence could not otherwise be established. The data are consistent with two molecules of bicarbonate or one molecule of acetate binding to the metal binding site. For ferrous ion, the binding and oxidation was facilitated not only by bicarbonate and acetate, but also by citrate. Electron paramagnetic resonance spectra suggest differences in the arrangement of the iron ligands in the presence of the various anions.

Probing tyrosine Z oxidation in Photosystem II core complex isolated from spinach by EPR at liquid helium temperatures

Tyrosine Z (Tyr(Z)) oxidation observed at liquid helium temperatures provides new insights into the structure and function of Tyr(Z) in active Photosystem II (PSII). However, it has not been reported in PSII core complex from higher plants. Here, we report Tyr(Z) oxidation in the S(1) and S(2) states in PSII core complex from spinach for the first time. Moreover, we identified a 500 G-wide symmetric EPR signal (peak position g = 2.18, trough position g = 1.85) together with the g = 2.03 signal induced by visible light at 10 K in the S(1) state in the PSII core complex. These two signals decay with a similar rate in the dark and both disappear in the presence of 6% methanol. We tentatively assign this new feature to the hyperfine structure of the S(1)Tyr(Z)(*) EPR signal. Furthermore, EPR signals of the S(2) state of the Mn-cluster, the oxidation of the non-heme iron, and the S(1)Tyr(Z)(*) in PSII core complexes and PSII-enriched membranes from spinach are compared, which clearly indicate that both the donor and acceptor sides of the reaction center are undisturbed after the removal of LHCII. These results suggest that the new spinach PSII core complex is suitable for the electron transfer study of PSII at cryogenic temperatures.

Comparison of bacterial reaction centers and photosystem II

In photosynthetic organisms, the utilization of solar energy to drive electron and proton transfer reactions across membranes is performed by pigment-protein complexes including bacterial reaction centers (BRCs) and photosystem II. The well-characterized BRC has served as a structural and functional model for the evolutionarily-related photosystem II for many years. Even though these complexes transfer electrons and protons across cell membranes in analogous manners, they utilize different secondary electron donors. Photosystem II has the unique ability to abstract electrons from water, while BRCs use molecules with much lower potentials as electron donors. This article compares the two complexes and reviews the factors that give rise to the functional differences. Also discussed are the modifications that have been performed on BRCs so that they perform reactions, such as amino acid and metal oxidation, which occur in photosystem II.

Isotopic labelling of photosystem II in Thermosynechococcus elongatus

This report describes a protocol to incorporate isotopically labelled aromatic amino acids into the proteins of the thermophilic cyanobacterium Thermosynechoccus elongatus. By using the EPR signal of the two redox active tyrosines of Photosystem II, Tyr(D)(*) and Tyr(Z)(*), as spectroscopic probes it is shown that labelled tyrosines can be incorporated with a high yield in this cyanobacterium. The production of a fully (13)C- or (2)H-labelled enzyme is also described.

The D1 and D2 proteins of dinoflagellates: unusually accumulated mutations which influence on PSII photoreaction

Plastid encoded genes of the dinoflagellates are rapidly evolving and most divergent. The importance of unusually accumulated mutations on structure of PSII core protein and photosynthetic function was examined in the dinoflagellates, Symbiodinium sp. and Alexandrium tamarense. Full-length cDNA sequences of psbA (D1 protein) and psbD (D2 protein) were obtained and compared with the other oxygen-evolving photoautotrophs. Twenty-three amino acid positions (7%) for the D1 protein and 34 positions (10%) for the D2 were mutated in the dinoflagellates, although amino acid residues at these positions were conserved in cyanobacteria, the other algae, and plant. Many mutations were likely to distribute in the N-terminus and the D-E interhelical loop of the D1 protein and helix B of D2 protein, while the remaining regions were well conserved. The different structural properties in these mutated regions were supported by hydropathy profiles. The chlorophyll fluorescence kinetics of the dinoflagellates was compared with Synechocystis sp. PCC6803 in relation to the altered protein structure.

Photosynthetic generation of oxygen

The oxygen in the atmosphere is derived from light-driven oxidation of water at a catalytic centre contained within a multi-subunit enzyme known as photosystem II (PSII). PSII is located in the photosynthetic membranes of plants, algae and cyanobacteria and its oxygen-evolving centre (OEC) consists of four manganese ions and a calcium ion surrounded by a highly conserved protein environment. Recently, the structure of PSII was elucidated by X-ray crystallography thus revealing details of the molecular architecture of the OEC. This structural information, coupled with an extensive knowledge base derived from a wide range of biophysical, biochemical and molecular biological studies, has provided a framework for understanding the chemistry of photosynthetic oxygen generation as well as opening up debate about its evolutionary origin.

A bioinspired construct that mimics the proton coupled electron transfer between P680*+ and the Tyr(Z)-His190 pair of photosystem II

A bioinspired hybrid system, composed of colloidal TiO2 nanoparticles surface modified with a photochemically active mimic of the PSII chlorophyll-Tyr-His complex, undergoes photoinduced stepwise electron transfer coupled to proton motion at the phenolic site. Low temperature electron paramagnetic resonance studies reveal that injected electrons are localized on TiO2 nanoparticles following photoexcitation. At 80 K, 95% of the resulting holes are localized on the phenol moiety and 5% are localized on the porphyrin. At 4.2 K, 52% of the holes remain trapped on the porphyrin. The anisotropic coupling tensors of the phenoxyl radical are resolved in the photoinduced D-band EPR spectra and are in good agreement with previously reported g-tensors of tyrosine radicals in photosystem II. The observed temperature dependence of the charge shift is attributed to restricted nuclear motion at low temperature and is reminiscent of the observation of a trapped high-energy state in the natural system. Electrochemical studies show that the phenoxyl/phenol couple of the model system is chemically reversible and thermodynamically capable of water oxidation.

Computational insights into the O2-evolving complex of photosystem II

Mechanistic investigations of the water-splitting reaction of the oxygen-evolving complex (OEC) of photosystem II (PSII) are fundamentally informed by structural studies. Many physical techniques have provided important insights into the OEC structure and function, including X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy as well as mass spectrometry (MS), electron paramagnetic resonance (EPR) spectroscopy, and Fourier transform infrared spectroscopy applied in conjunction with mutagenesis studies. However, experimental studies have yet to yield consensus as to the exact configuration of the catalytic metal cluster and its ligation scheme. Computational modeling studies, including density functional (DFT) theory combined with quantum mechanics/molecular mechanics (QM/MM) hybrid methods for explicitly including the influence of the surrounding protein, have proposed chemically satisfactory models of the fully ligated OEC within PSII that are maximally consistent with experimental results. The inorganic core of these models is similar to the crystallographic model upon which they were based, but comprises important modifications due to structural refinement, hydration, and proteinaceous ligation which improve agreement with a wide range of experimental data. The computational models are useful for rationalizing spectroscopic and crystallographic results and for building a complete structure-based mechanism of water-splitting in PSII as described by the intermediate oxidation states of the OEC. This review summarizes these recent advances in QM/MM modeling of PSII within the context of recent experimental studies.

The EPR spectrum of tyrosine Z* and its decay kinetics in O2-evolving photosystem II preparations

The O2-evolving complex of photosystem II, Mn 4Ca, cycles through five oxidation states, S0,..., S4, during its catalytic function, which involves the gradual abstraction of four electrons and four protons from two bound water molecules. The direct oxidant of the complex is the tyrosine neutral radical, YZ(*), which is transiently produced by the highly oxidizing power of the photoexcited chlorophyll species P680. EPR characterization of YZ(*) has been limited, until recently, to inhibited (non-oxygen-evolving) preparations. A number of relatively recent papers have demonstrated the trapping of YZ(*) in O2-evolving preparations at liquid helium temperatures as an intermediate of the S0 to S1, S1 to S2, and S2 to S3 transitions. The respective EPR spectra are broadened and split at g approximately 2 by the magnetic interaction with the Mn cluster, but this interaction collapses at temperatures higher than about 100K [Zahariou et al. (2007) Biochemistry 46, 14335 -14341]. We have conducted a study of the Tyr Z(*) transient in the temperature range 77-240 K by employing rapid or slow EPR scans. The results reveal for the first time high-resolution X-band spectra of Tyr Z(*) in the functional system and at temperatures close to the onset of the S-state transitions. We have simulated the S 2Y Z(*) spectrum using the simulation algorithm of Svistunenko and Cooper [(2004) Biophys. J. 87, 582 -595]. The small g(x) = 2.00689 value inferred from the analysis suggests either a H-bonding of Tyr Z (*) (presumably with His190) that is stronger than what has been assumed from studies of Tyr D(*) or Tyr Z(*) in Mn-depleted preparations or a more electropositive environment around Tyr Z(*). The study has also yielded for the first time direct information on the temperature variation of the YZ(*)/QA(-) recombination reaction in the various S states. The reaction follows biphasic kinetics with the slow phase dominating at low temperatures and the fast phase dominating at high temperatures. It is tentatively proposed that the slow phase represents the action of the YZ(*)/YZ(-) redox couple while the fast phase represents that of the YZ(*)/YZH couple; it is inferred that Tyr Z at elevated temperatures is protonated at rest. It is also proposed that YZ(*)/YZH is the couple that oxidizes the Mn cluster during the S1-S2 and S2-S3 transitions. A simple mechanism ensuring a rapid (concerted) protonation of Tyr Z upon oxidation of the Mn cluster is discussed, and also, a structure-based molecular model suggesting the participation of His190 into two hydrogen bonds is proposed.

Multifrequency pulsed electron paramagnetic resonance study of the S2 state of the photosystem II manganese cluster

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is employed to measure the strength of the hyperfine coupling of magnetic nuclei to the paramagnetic (S = 1/2) S2 form of photosystem II (PSII). Previous X-band-frequency ESEEM studies indicated that one or more histidine nitrogens are electronically coupled to the tetranuclear manganese cluster in the S2 state of PSII. However, the spectral resolution was relatively poor at the approximately 9 GHz excitation frequency, precluding any in-depth analysis of the corresponding bonding interaction between the detected histidine and the manganese cluster. Here we report ESEEM experiments using higher X-, P-, and Ka-band microwave frequencies to target PSII membranes isolated from spinach. The X- to P-band ESEEM spectra suffer from the same poor resolution as that observed in previous experiments, while the Ka-band spectra show remarkably well-resolved features that allow for the direct determination of the nuclear quadrupolar couplings for a single I = 1(14)N nucleus. The Ka-band results demonstrate that at an applied field of 1.1 T we are much closer to the exact cancellation limit (alpha iso = 2nu(14)N) that optimizes ESEEM spectra. These results reveal hyperfine (alpha iso = 7.3 +/- 0.20 MHz and alpha dip = 0.50 +/- 0.10 MHz) and nuclear quadrupolar (e(2)qQ = 1.98 +/- 0.05 MHz and eta = 0.84 +/- 0.06) couplings for a single (14)N nucleus magnetically coupled to the manganese cluster in the S 2 state of PSII. These values are compared to the histidine imidazole nitrogen hyperfine and nuclear quadrupolar couplings found in superoxidized manganese catalase as well as (14)N couplings in relevant manganese model complexes.