Electrostatics and proton transfer in photosynthetic water oxidation

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.

The terminal reaction cascade of water oxidation: proton and oxygen release

In cyanobacteria, algae and plants Photosystem II produces the oxygen we breathe. Driven and clocked by light quanta, the catalytic Mn(4)Ca-tyrosine centre accumulates four oxidising equivalents before it abstracts four electrons from water, liberating dioxygen and protons. Aiming at intermediates of the terminal four-electron cascade, we previously have suppressed this reaction by elevating the oxygen pressure, thereby stabilising one redox intermediate. Here, we established a similar suppression by increasing the proton concentration. Data were analysed in terms of only one (peroxy) redox intermediate between the fourfold oxidised Mn(4)Ca-tyrosine centre and oxygen release. The surprising result was that the release into the bulk of one proton per dioxygen is linked to the first and rate-limiting electron transfer in the cascade rather than to the second which produces free oxygen. The penultimate intermediate might thus be conceived as a fully deprotonated peroxy-moiety.

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.

In search of elusive high-valent manganese species that evaluate mechanisms of photosynthetic water oxidation

Significant progress in the understanding of biological water oxidation has occurred during the past 25 years. Today we have a somewhat clearer description of the structure of the Mn4Ca cluster and an idea of the appropriate oxidation states for the enzyme during catalysis. At issue is the mechanism of water oxidation. Depending on one's belief of the manganese ion oxidation levels at the catalytically active S4 configuration, one can invoke a variety of different processes that could lead to water oxidation. We have suggested that the most likely process is the nucleophilic attack of a water bound to calcium (or manganese) onto a highly electrophilic Mn(V)=O center. In this Article, we explore the difficulties of preparing Mn(V) in dimeric systems and the even more arduous task of definitively assigning oxidation states to such highly reactive species.

Coupled electron transfers in artificial photosynthesis

Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of coupled electron-transfer reactions, which have so far received only little attention. (i) Each absorbed photon leads to charge separation on a single-electron level only, while catalytic water splitting and hydrogen production are multi-electron processes; thus there is the need for controlling accumulative electron transfer on molecular components. (ii) Water splitting and proton reduction at the potential catalysts necessarily require the management of proton release and/or uptake. Far from being just a stoichiometric requirement, this controls the electron transfer processes by proton-coupled electron transfer (PCET). (iii) Redox-active links between the photosensitizers and the catalysts are required to rectify the accumulative electron-transfer reactions, and will often be the starting points of PCET.

The past and present of sodium energetics: may the sodium-motive force be with you

All living cells routinely expel Na(+) ions, maintaining lower concentration of Na(+) in the cytoplasm than in the surrounding milieu. In the vast majority of bacteria, as well as in mitochondria and chloroplasts, export of Na(+) occurs at the expense of the proton-motive force. Some bacteria, however, possess primary generators of the transmembrane electrochemical gradient of Na(+) (sodium-motive force). These primary Na(+) pumps have been traditionally seen as adaptations to high external pH or to high temperature. Subsequent studies revealed, however, the mechanisms for primary sodium pumping in a variety of non-extremophiles, such as marine bacteria and certain bacterial pathogens. Further, many alkaliphiles and hyperthermophiles were shown to rely on H(+), not Na(+), as the coupling ion. We review here the recent progress in understanding the role of sodium-motive force, including (i) the conclusion on evolutionary primacy of the sodium-motive force as energy intermediate, (ii) the mechanisms, evolutionary advantages and limitations of switching from Na(+) to H(+) as the coupling ion, and (iii) the possible reasons why certain pathogenic bacteria still rely on the sodium-motive force.

Evolutionary primacy of sodium bioenergetics

Background

The F- and V-type ATPases are rotary molecular machines that couple translocation of protons or sodium ions across the membrane to the synthesis or hydrolysis of ATP. Both the F-type (found in most bacteria and eukaryotic mitochondria and chloroplasts) and V-type (found in archaea, some bacteria, and eukaryotic vacuoles) ATPases can translocate either protons or sodium ions. The prevalent proton-dependent ATPases are generally viewed as the primary form of the enzyme whereas the sodium-translocating ATPases of some prokaryotes are usually construed as an exotic adaptation to survival in extreme environments.

Results

We combine structural and phylogenetic analyses to clarify the evolutionary relation between the proton- and sodium-translocating ATPases. A comparison of the structures of the membrane-embedded oligomeric proteolipid rings of sodium-dependent F- and V-ATPases reveals nearly identical sets of amino acids involved in sodium binding. We show that the sodium-dependent ATPases are scattered among proton-dependent ATPases in both the F- and the V-branches of the phylogenetic tree.

Conclusion

Barring convergent emergence of the same set of ligands in several lineages, these findings indicate that the use of sodium gradient for ATP synthesis is the ancestral modality of membrane bioenergetics. Thus, a primitive, sodium-impermeable but proton-permeable cell membrane that harboured a set of sodium-transporting enzymes appears to have been the evolutionary predecessor of the more structurally demanding proton-tight membranes. The use of proton as the coupling ion appears to be a later innovation that emerged on several independent occasions.

Reviewers

This article was reviewed by J. Peter Gogarten, Martijn A. Huynen, and Igor B. Zhulin. For the full reviews, please go to the Reviewers' comments section.

Water oxidation chemistry of photosystem II

Photosystem II (PSII) uses light energy to split water into protons, electrons and O2. In this reaction, nature has solved the difficult chemical problem of efficient four-electron oxidation of water to yield O2 without significant amounts of reactive intermediate species such as superoxide, hydrogen peroxide and hydroxyl radicals. In order to use nature's solution for the design of artificial catalysts that split water, it is important to understand the mechanism of the reaction. The recently published X-ray crystal structures of cyanobacterial PSII complexes provide information on the structure of the Mn and Ca ions, the redox-active tyrosine called YZ and the surrounding amino acids that comprise the O2-evolving complex (OEC). The emerging structure of the OEC provides constraints on the different hypothesized mechanisms for O2 evolution. The water oxidation mechanism of PSII is discussed in the light of biophysical and computational studies, inorganic chemistry and X-ray crystallographic information.

Computational studies of the O(2)-evolving complex of photosystem II and biomimetic oxomanganese complexes

In recent years, there has been considerable interest in studies of catalytic metal clusters in metalloproteins based on Density Functional Theory (DFT) quantum mechanics/molecular mechanics (QM/MM) hybrid methods. These methods explicitly include the perturbational influence of the surrounding protein environment on the structural/functional properties of the catalytic centers. In conjunction with recent breakthroughs in X-ray crystallography and advances in spectroscopic and biophysical studies, computational chemists are trying to understand the structural and mechanistic properties of the oxygen-evolving complex (OEC) embedded in photosystem II (PSII). Recent studies include the development of DFT-QM/MM computational models of the Mn(4)Ca cluster, responsible for photosynthetic water oxidation, and comparative quantum mechanical studies of biomimetic oxomanganese complexes. A number of computational models, varying in oxidation and protonation states and ligation of the catalytic center by amino acid residues, water, hydroxide and chloride have been characterized along the PSII catalytic cycle of water splitting. The resulting QM/MM models are consistent with available mechanistic data, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction data and extended X-ray absorption fine structure (EXAFS) measurements. Here, we review these computational efforts focused towards understanding the catalytic mechanism of water oxidation at the detailed molecular level.

The photosystem II-associated Cah3 in Chlamydomonas enhances the O2 evolution rate by proton removal

Water oxidation in photosystem II (PSII) is still insufficiently understood and is assumed to involve HCO(3)(-). A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O(2) evolution in the absence of HCO(3)(-). The O(2) evolution for saturating, continuous illumination (R(O2)) was slower than in the wild type, but was elevated by HCO(3)(-) and increased further by Cah3. The R(O2) limitation in the absence of Cah3/HCO(3)(-) was amplified by H(2)O/D(2)O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO(3)(-) in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO(3)(-) effect at the donor side of PSII. Time-resolved delayed fluorescence and O(2)-release measurements suggest specific effects on proton-release steps but not on electron transfer. We propose that Cah3 promotes proton removal from the Mn complex by locally providing HCO(3)(-), which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O(2) formation and thus, limit water oxidation under high light. Our results underlie the general importance of proton release at the donor side of PSII during water oxidation.

A comparison of catalytic site intermediates of cytochrome c oxidase and peroxidases

Compounds I and II of peroxidases such as horseradish peroxidase and cytochrome c peroxidase are relatively well understood catalytic intermediates in terms of their structures and redox states of iron, heme, and associated radical species. The intermediates involved in the oxygen reduction chemistry of the cytochrome c oxidase superfamily are more complicated because of the need for four reducing equivalents and because of the linkage of the oxygen chemistry with vectorial proton translocations. Nevertheless, two of these intermediates, the peroxy and ferryl forms, have characteristics that can in many ways be considered to be counterparts of peroxidase compounds I and II. We explore the primary factors that minimize the generation of unwanted reactive oxygen species products and ensure that the principal enzymological function becomes either that of a peroxidase or an oxidase. These comparisons can provide insights into the nature of biological oxygen reduction chemistry and guidance for the engineering of biomimetic synthetic materials.

Are there ecological implications for the proposed energetic restrictions on photosynthetic oxygen evolution at high oxygen concentrations?

It has recently been shown that, in subthylakoid particles prepared using detergent, there is inhibition of oxygen production reactions in photosynthesis by thermodynamic feedback from oxygen build-up, with 50% inhibition at 230 kPa partial pressure of oxygen. This article presents a comprehensive analysis of laboratory data on the effects of high oxygen partial pressures on photosynthesis, and on photo-lithotrophic and chemo-organotrophic growth, of oxygen-producing organisms. The article also contains an analysis of the extent to which high oxygen concentrations occur at the site of photosystem II (PSII) activity under natural conditions today and in the past. The conclusion is that the oxygen concentrations found in nature are very unlikely to reach that needed to cause 50% inhibition of the photosynthetic oxygen production reaction in subthylakoid particles, but that it is just possible that a small part of the inhibition of photosynthesis and of photo-lithotrophic growth by oxygen can be attributed to inhibition of oxygen production by PSII.

The Possible Role of Proton-Coupled Electron Transfer (PCET) in Water Oxidation by Photosystem II

All higher life forms use oxygen and respiration as their primary energy source. The oxygen comes from water by solar-energy conversion in photosynthetic membranes. In green plants, light absorption in photosystem II (PSII) drives electron-transfer activation of the oxygen-evolving complex (OEC). The mechanism of water oxidation by the OEC has long been a subject of great interest to biologists and chemists. With the availability of new molecular-level protein structures from X-ray crystallography and EXAFS, as well as the accumulated results from numerous experiments and theoretical studies, it is possible to suggest how water may be oxidized at the OEC. An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation. When these reactions are combined with long-range proton transfer (by sequential local proton transfers), it may be possible to view the OEC as an intricate structure that is "wired for protons".

Time-resolved X-ray spectroscopy leads to an extension of the classical S-state cycle model of photosynthetic oxygen evolution

In oxygenic photosynthesis, a complete water oxidation cycle requires absorption of four photons by the chlorophylls of photosystem II (PSII). The photons can be provided successively by applying short flashes of light. Already in 1970, Kok and coworkers [Photochem Photobiol 11:457-475, 1970] developed a basic model to explain the flash-number dependence of O2 formation. The third flash applied to dark-adapted PSII induces the S3-->S4-->S0 transition, which is coupled to dioxygen formation at a protein-bound Mn4Ca complex. The sequence of events leading to dioxygen formation and the role of Kok's enigmatic S4-state are only incompletely understood. Recently we have shown by time-resolved X-ray spectroscopy that in the S3-->S0 transition an interesting intermediate is formed, prior to the onset of O-O bond formation [Haumann et al. Science 310:1019-1021, 2005]. The experimental results of the time-resolved X-ray experiments are discussed. The identity of the reaction intermediate is considered and the question is addressed how the novel intermediate is related to the S4-state proposed in 1970 by Bessel Kok. This leads us to an extension of the classical S-state cycle towards a basic model which describes sequence and interplay of electron and proton abstraction events at the donor side of PSII [Dau and Haumann, Science 312:1471-1472, 2006].

Eight steps preceding O-O bond formation in oxygenic photosynthesis--a basic reaction cycle of the Photosystem II manganese complex

In oxygenic photosynthesis, water is split at a Mn(4)Ca complex bound to the proteins of photosystem II (PSII). Powered by four quanta of visible light, four electrons and four protons are removed from two water molecules before dioxygen is released. By this process, water becomes an inexhaustible source of the protons and electrons needed for primary biomass formation. On the basis of structural and spectroscopic data, we recently have introduced a basic reaction cycle of water oxidation which extends the classical S-state cycle [B. Kok, B. Forbush, M. McGloin, Cooperation of charges in photosynthetic O2 evolution- I. A linear four-step mechanism, Photochem. Photobiol. 11 (1970) 457-475] by taking into account also the role and sequence of deprotonation events [H. Dau, M. Haumann, Reaction cycle of photosynthetic water oxidation in plants and cyanobacteria, Science 312 (2006) 1471-1472]. We propose that the outwardly convoluted and irregular events of the classical S-state cycle are governed by a simple underlying principle: protons and electrons are removed strictly alternately from the Mn complex. Starting in I(0), eight successive steps of alternate proton and electron removal lead to I(8) and only then the O-O bond is formed. Thus not only four oxidizing equivalents, but also four bases are accumulated prior to the onset of dioxygen formation. After reviewing the kinetic properties of the individual S-state transition, we show that the proposed basic model explains a large body of experimental results straightforwardly. Furthermore we discuss how the I-cycle model addresses the redox-potential problem of PSII water oxidation and we propose that the accumulated bases facilitate dioxygen formation by acting as proton acceptors.

Distinct Mechanisms of Bridging-Oxo Exchange in Di-μ-O Dimanganese Complexes with and without Water-Binding Sites: Implications for Water Binding in the O2-Evolving Complex of Photosystem II

Isotopic exchange between oxygens of water and mu-O bridges in the di-mu-O dimanganese complexes, [(mes-terpy)2Mn2(III/IV)(mu-O)2(H2O)2](NO3)3 (1, mes-terpy = 4'-mesityl-2,2':6',2' '-terpyridine) and [(phen)4Mn2III/IV(mu-O)2](ClO4)3 (2, phen = 1,10-phenanthroline), has been investigated by a study of the kinetics of exchange. The data provide evidence for distinct mechanisms of exchange in 1 and 2 and suggest that these differences arise due to the presence and absence of terminal water-binding sites in 1 and 2, respectively. Exchange of oxygen atoms between water and mu-O bridges must involve the elementary steps of bridge protonation, deprotonation, opening, and closing. On the basis of the existing literature on these reactions in oxo-bridged metal complexes and our present data, we propose pathways of exchange in 1 and 2. The mechanism proposed for 1 involves an initial fast protonation of an oxo-bridge by water coordinated to Mn(IV), followed by a slow opening of the protonated bridge as proposed earlier for an analogous complex on the basis of DFT calculations. The mechanism proposed for 2 involves initial dissociation of phen, followed by coordination of water at the vacated sites, as observed for rearrangement of 2 to a trinuclear complex. The subsequent steps are proposed to be analogous to those for 1. Our results are discussed in the context of data on 18O-labeled water isotope exchange in photosystem II and provide support for the existence of fully protonated terminal waters bound to Mn in the O2-evolving complex of photosystem II.

A cluster of carboxylic groups in PsbO protein is involved in proton transfer from the water oxidizing complex of Photosystem II

The hypothesis presented here for proton transfer away from the water oxidation complex of Photosystem II (PSII) is supported by biochemical experiments on the isolated PsbO protein in solution, theoretical analyses of better understood proton transfer systems like bacteriorhodopsin and cytochrome oxidase, and the recently published 3D structure of PS II (Pdb entry 1S5L). We propose that a cluster of conserved glutamic and aspartic acid residues in the PsbO protein acts as a buffering network providing efficient acceptors of protons derived from substrate water molecules. The charge delocalization of the cluster ensures readiness to promptly accept the protons liberated from substrate water. Therefore protons generated at the catalytic centre of PSII need not be released into the thylakoid lumen as generally thought. The cluster is the beginning of a localized, fast proton transfer conduit on the lumenal side of the thylakoid membrane. Proton-dependent conformational changes of PsbO may play a role in the regulation of both supply of substrate water to the water oxidizing complex and the resultant proton transfer.

Comparative study of the water oxidizing reactions and the millisecond delayed chlorophyll fluorescence in photosystem II at different pH

Water splitting activity, the multiline EPR signal associated with S(2)-state of the CaMn(4)-cluster and the fast and slow phases of the induction curve of the millisecond delayed chlorophyll fluorescence from photosystem II (PSII) in the pH range of 4.5-8.5 were studied in the thylakoid membranes and purified PSII particles. It has been found that O(2) evolution and the multiline EPR signal were inhibited at acidic (pK approximately 5.3) and alkaline (pK approximately 8.1) pH values, and were maximal at pH 6.0-7.0. Our results indicate that the loss of O(2) evolution and the S(2)-state multiline EPR signal associated with the decrease of the millisecond delayed chlorophyll fluorescence only in alkaline region (pH 7.0-8.5). Possible correlations of the millisecond delayed chlorophyll fluorescence components with the donor side reactions in PSII are discussed.

Low-barrier hydrogen bond plays key role in active photosystem II--a new model for photosynthetic water oxidation

The function and mechanism of Tyr(Z) in active photosystem II (PSII) is one of the long-standing issues in the study of photosynthetic water oxidation. Based on recent investigations on active PSII and theoretical studies, a new model is proposed, in which D1-His190 acts as a bridge, to form a low-barrier hydrogen bond (LBHB) with Tyr(Z), and a coordination bond to Mn or Ca ion of the Mn-cluster. Accordingly, this new model differs from previous proposals concerning the mechanism of Tyr(Z) function in two aspects. First, the LBHB plays a key role to decrease the activation energy for Tyr(Z) oxidation and Tyr(Z)(.) reduction during photosynthetic water oxidation. Upon the oxidation of Tyr(Z), the hydrogen bond between Tyr(Z) and His190 changes from a LBHB to a weak hydrogen bond, and vice versa upon Tyr(Z)(.) reduction. In both stages, the electron transfer and proton transfer are coupled. Second, the positive charge formed after Tyr(Z) oxidation may play an important role for water oxidation. It can be delocalized on the Mn-cluster, thus helps to accelerate the proton release from substrate water on Mn-cluster. This model is well reconciled with observations of the S-state dependence of Tyr(Z) oxidation and Tyr(Z)(.) reduction, proton release, isotopic effect and recent EPR experiments. Moreover, the difference between Tyr(Z) and Tyr(D) in active PSII can also be readily rationalized. The His190 binding to the Mn-cluster predicted in this model is contradictious to the recent structure data, however, it has been aware that the crystal structure of the Mn-cluster and its environment are significantly modified by X-ray due to radiation damage and are different from that in active PSII. It is suggested that the His190 may be protonated during the radiation damage, which leads to the loss of its binding to Mn-cluster and the strong hydrogen bond with Tyr(Z). This type of change arising from radiation damage has been confirmed in other enzyme systems.

Interaction between tyrosineZ and substrate water in active photosystem II

In the field of photosynthetic water oxidation it has been under debate whether Tyrosine(Z) (Tyr(Z)) acts as a hydrogen or an electron acceptor from water. In the former concept, direct contact of Tyr(Z) with substrate water has been assumed. However, there is no direct evidence for the interaction between Tyr(Z) and substrate water in active Photosystem II (PSII), instead most experiments have been performed on inhibited PSII. Here, this problem is tackled in active PSII by combining low temperature EPR measurements and quantum chemistry calculations. EPR measurements observed that the maximum yield of Tyr(Z) oxidation at cryogenic temperature in the S(0) and S(1) states was around neutral pH and was essentially pH-independent. The yield of Tyr(Z) oxidation decreased at acidic and alkaline pH, with pKs at 4.7-4.9 and 7.7, respectively. The observed pH-dependent parts at low and high values of pH can be explained as due to sample inactivation, rather than active PSII. The reduction kinetics of Tyr(Z)(.) in the S(0) and S(1) states were pH independent at pH range from 4.5 to 8. Therefore, the change of the pH in bulk solution probably has no effect on the Tyr(Z) oxidation and Tyr(Z)(.) reduction at cryogenic temperature in the S(0) and S(1) states of the active PSII. Theoretical calculations indicate that Tyr(Z) becomes more difficult to oxidize when a H(2)O molecule interacts directly with it. It is suggested that Tyr(Z) is probably located in a hydrophobic environment with no direct interaction with the substrate H(2)O in active PSII. These results provide new insights on the function and mechanism of water oxidation in PSII.

Mimicking the electron donor side of Photosystem II in artificial photosynthesis

This review focuses on our recent efforts in synthetic ruthenium-tyrosine-manganese chemistry mimicking the donor side reactions of Photosystem II. Tyrosine and tryptophan residues were linked to ruthenium photosensitizers, which resulted in model complexes for proton-coupled electron transfer from amino acids. A new mechanistic model was proposed and used to design complexes in which the mechanism could be switched between concerted and step-wise proton-coupled electron transfer. Moreover, a manganese dimer linked to a ruthenium complex could be oxidized in three successive steps, from Mn (2) (II,II) to Mn (2) (III,IV) by the photo-oxidized ruthenium sensitizer. This was possible thanks to a charge compensating ligand exchange in the manganese complex. Detailed studies of the ligand exchange suggested that at high water concentrations, each oxidation step is coupled to a proton-release of water-derived ligands, analogous to the oxidation steps of the manganese cluster of Photosystem II.

Models for proton-coupled electron transfer in photosystem II

The coupling of proton and electron transfers is a key part of the chemistry of photosynthesis. The oxidative side of photosystem II (PS II) in particular seems to involve a number of proton-coupled electron transfer (PCET) steps in the S-state transitions. This mini-review presents an overview of recent studies of PCET model systems in the authors' laboratory. PCET is defined as a chemical reaction involving concerted transfer of one electron and one proton. These are thus distinguished from stepwise pathways involving initial electron transfer (ET) or initial proton transfer (PT). Hydrogen atom transfer (HAT) reactions are one class of PCET, in which H(+) and e (-) are transferred from one reagent to another: AH + B --> A + BH, roughly along the same path. Rate constants for many HAT reactions are found to be well predicted by the thermochemistry of hydrogen transfer and by Marcus Theory. This includes organic HAT reactions and reactions of iron-tris(alpha-diimine) and manganese-(mu-oxo) complexes. In PS II, HAT has been proposed as the mechanism by which the tyrosine Z radical (Y(Z)*) oxidizes the manganese cluster (the oxygen evolving complex, OEC). Another class of PCET reactions involves transfer of H(+) and e (-) in different directions, for instance when the proton and electron acceptors are different reagents, as in AH-B + C(+) --> A-HB(+) + C. The oxidation of Y(Z) by the chlorophyll P680 + has been suggested to occur by this mechanism. Models for this process - the oxidation of phenols with a pendent base - are described. The oxidation of the OEC by Y(Z)* could also occur by this second class of PCET reactions, involving an Mn-O-H fragment of the OEC. Initial attempts to model such a process using ruthenium-aquo complexes are described.

Mutagenesis of CP43-arginine-357 to serine reveals new evidence for (bi)carbonate functioning in the water oxidizing complex of Photosystem II

The chlorophyll-binding protein CP43 is an inner subunit of the Photosystem II (PSII) reaction center core complex of all oxygenic photoautotrophs. X-Ray structural evidence places the guanidinium cation of the conserved arginine 357 residue of CP43 within a few Angstroms to the Mn(4)Ca cluster of the water-oxidizing complex (WOC) and has been implicated as a possible carbonate binding site. To test the hypothesis, the serine mutant, CP43-R357S, from Synechocystis PCC 6803 was investigated by PSII variable fluorescence (F(v)/F(m)) and simultaneous flash O(2) yield measurements in cells and thylakoid membranes. The R357S mutant assembles PSII-WOC centers, but is unable to grow photoautotrophically. Reconstitution of O(2) evolution by photoactivation and the occurrence of period-four oscillations of F(v)/F(m) establishes that the R357S mutant contains an assembled Mn(4)Ca cluster, but turnover is impaired as seen by an 11-fold larger Kok double miss parameter and faster decay of upper S states. Using pulsed light to avoid photoinactivation, wild-type cells and thylakoid membranes exhibit a 2-4-fold loss in O(2) evolution rate upon partial bicarbonate depletion under multiple turnover conditions, while the R357S mutant is unaffected by bicarbonate. Arginine R357 appears to function in binding a (bi)carbonate ion essential to normal catalytic turnover of the WOC. The quantum yield of electron donation from the WOC into PSII increases with decreasing turnover rate in R357S mutant cells and involves an aborted two-flash pathway that is distinct from the classical four-flash pattern. We speculate that an altered photochemical mechanism for O(2) production occurs via formation of hydrogen peroxide, by analogy to other treatments that retard the kinetics of proton release into the lumen.

Moving a Phenol Hydroxyl Group from the Surface to the Interior of a Protein: Effects on the Phenol Potential and pKA

De novo protein design and electrochemistry were used to measure changes in the potential and pK(A) of a phenol when its OH group is moved from a solvent-exposed to a sequestered protein position. A "phenol rotation strategy" was adopted in which phenols, containing a SH in position 4, 3, or 2 relative to the OH group, were bound to a buried protein site. The alpha(3)C protein used here is a tryptophan to cysteine variant of the structurally defined alpha(3)W protein (Dai et al. (2002) J. Am. Chem. Soc. 124, 10952-10953). The protein characteristics of alpha(3)C and the three mercaptophenol-alpha(3)C (MP-alpha(3)C) proteins are shown to be close to those of alpha(3)W. Moreover, the phenol OH group is fully solvent exposed in 4MP-alpha(3)C and more sequestered in 3MP-alpha(3)C and 2MP-alpha(3)C. Here we compare the redox properties of the three mercaptophenols when bound to alpha(3)C and to cysteine free in water. The pK(A) and E(peak) values are essential identical when 4MP is ligated to alpha(3)C relative to when it is free in solution. In contrast, these values are increased in 3MP-alpha(3)C and 2MP-alpha(3)C relative to the solvated compounds. The E(peak) vs pH plots all display a approximately 59 mV/pH unit dependence. We conclude that interactions with the OH group dominate the phenol redox characteristics. In 3MP-alpha(3)C and 2MP-alpha(3)C, hydrogen bonds between the protein and the bound phenols appear to either stabilize the reduced phenol or destabilize the radical, relative to the aqueous buffer, raising the potential by 0.11 and 0.12 V, respectively.

Considerations on the mechanism of photosynthetic water oxidation - dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water

Two mechanistic problems of photosynthetic water oxidation at the Mn complex of Photosystem II (PS II) are considered. (I) In the four Mn-oxidizing transitions, any pure Mn oxidation is predicted to cause an increase in redox potential that renders subsequent oxidation steps impossible (redox-potential problem). Formation of unprotonated oxo-bridges may counteract the potential increase. (II) The O-O formation step without any high-pK bases acting as proton acceptors is energetically unfavorable (acceptor-base problem). The pK of oxides in a bridging position between Mn ions may increase drastically upon reduction of Mn in the water-oxidation step (>10 units), thus rendering them favorable proton acceptors. It is proposed that in PS II, in the course of the four oxidizing transitions at least two unprotonated oxo-bridges are formed. Thereby (i) a redox potential increase is prevented and (ii) proton acceptors are prepared for the O-O formation step. Water oxidation in the O-O bond formation step is facilitated by simultaneous Mn reduction and proton transfer to bridging oxides amounting to hydrogen atom or hydride transfer from substrate water to the Mn-oxo core of the Mn complex of PS II.

pH Dependence of charge transfer between tryptophan and tyrosine in dipeptides

Time-resolved absorption spectroscopy has been employed to study the directionality and rate of charge transfer in W-Y and Ac-W-Y dipeptides as a function of pH. Excitation with 266-nm nanosecond laser pulses produces both W (or [WH](+), depending on pH) and Y. Between pH 6 and 10, W to was found to oxidize Y with k(X)=9.0x10(4) s(-1) and 1.8x10(4) s(-1) for the W-Y and Ac-W-Y dipeptide systems, respectively. The intramolecular charge transfer rate increases as the pH is lowered over the range 6>pH>2. For 10<pH<12, the rate of radical transport for the W-Y dipeptide decreases and becomes convoluted with other radical decay processes, the timescales of which have been identified in studies of control dipeptides Ac-F-Y and W-F. Further increases in pH prompt the reverse reaction to occur, W-Y-->W-Y(-) (Y(-), tyrosinate anion), with a rate constant of k(X)=1.2x10(5) s(-1). The dependence of charge transfer directionality between W and Y on pH is important to the enzymatic function of several model and natural biological systems as discussed here for ribonucleotide reductase.

The complex architecture of oxygenic photosynthesis

Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on earth. The primary step in this process - the conversion of sunlight into chemical energy - is driven by four, multisubunit, membrane-protein complexes that are known as photosystem I, photosystem II, cytochrome b(6)f and F-ATPase. Structural insights into these complexes are now providing a framework for the exploration not only of energy and electron transfer, but also of the evolutionary forces that shaped the photosynthetic apparatus.

The mechanism for dioxygen formation in PSII studied by quantum chemical methods

The availability of an X-ray structure for PSII including the water-oxidizing cluster, where the metal atoms and the amino acids are assigned, has opened up new possibilities to study the mechanism for dioxygen formation. In the present paper the main results of an ongoing hybrid DFT study are presented. The model used follows the structure suggested by the X-ray analysis as closely as possible. After nearly one thousand optimizations of different structures, each one with about 70 atoms, the main features of a water oxidizing mechanism start to emerge. The key intermediate is an oxyl radical state in S(3), stabilized by a weak trans effect to a bridging oxo in the cube. To reach this radical state a structural rearrangement appears necessary, in which one additional bridging oxo is formed between the dangling manganese and a manganese in the cube. The calculated energetics is reasonable but still not fully consistent with a correct mechanism. It is suggested that some part of the structure is not correct, probably the presence of the bicarbonate.

Detection of an intermediate of photosynthetic water oxidation

The oxygen that we breathe is produced by photosystem II of cyanobacteria and plants. The catalytic centre, a Mn4Ca cluster, accumulates four oxidizing equivalents before oxygen is formed, seemingly in a single reaction step 2H2O<==>O2 + 4H+ + 4e-. The energy and cycling of this reaction derives solely from light. No intermediate oxidation product of water has been detected so far. Here, we shifted the equilibrium of the terminal reaction backward by increasing the oxygen pressure and monitoring (by absorption transients in the near-ultraviolet spectrum) the electron transfer from bound water into the catalytic centre. A tenfold increase of ambient oxygen pressure (2.3 bar) half-suppressed the full progression to oxygen. The remaining electron transfer at saturating pressure (30 bar) was compatible with the formation of a stabilized intermediate. The abstraction of four electrons from water was probably split into at least two electron transfers: mildly endergonic from the centre's highest oxidation state to an intermediate, and exergonic from the intermediate to oxygen. There is little leeway for photosynthetic organisms to push the atmospheric oxygen concentration much above the present level.

Evidence that the C-terminus of the D1 polypeptide of photosystem II is ligated to the manganese ion that undergoes oxidation during the S1 to S2 transition: an isotope-edited FTIR study

Isotope-edited FTIR difference spectroscopy was employed to determine if the C-terminal alpha-COO(-) group of the D1 polypeptide ligates the (Mn)(4) cluster in photosystem II (PSII) and, if so, if it ligates the Mn ion that undergoes an oxidation during the S(1) --> S(2) transition. Wild-type and mutant cells of the cyanobacterium Synechocystis sp. PCC 6803 were propagated photoautotrophically in the presence of L-[1-(13)C]alanine or unlabeled ((12)C) L-alanine. In wild-type cells, both the C-terminal alpha-COO(-) group of the D1 polypeptide at D1-Ala344 and all alanine-derived peptide carbonyl groups will be labeled. In D1-A344G and D1-A344S mutant cells, the C-terminal alpha-COO(-) group of the D1 polypeptide will not be labeled because this group is no longer provided by alanine. The resultant S(2)-minus-S(1) FTIR difference spectra of purified wild-type and mutant PSII particles showed that one symmetric carboxylate stretching mode that is altered during the S(1) --> S(2) transition is sensitive to L-[1-(13)C]alanine-labeling in wild-type PSII particles but not in D1-A344G and D1-A344S PSII particles. Because the only carboxylate group that can be labeled in the wild-type PSII particles but not in the mutant PSII particles is the C-terminal alpha-COO(-) group of the D1 polypeptide, we assign the L-[1-(13)C]alanine-sensitive symmetric carboxylate stretching mode to the alpha-COO(-) group of D1-Ala344. In unlabeled wild-type PSII particles, this mode appears at approximately 1356 cm(-1) in the S(1) state and at approximately 1339 or approximately 1320 cm(-1) in the S(2) state. These frequencies are consistent with unidentate ligation of the (Mn)(4) cluster by the alpha-COO(-) group of D1-Ala344 in both the S(1) and S(2) states. The apparent 17-36 cm(-1) downshift in frequency in response to the S(1) --> S(2) transition is consistent with the alpha-COO(-) group of D1-Ala344 ligating a Mn ion whose charge increases during the S(1) --> S(2) transition. Accordingly, we propose that the alpha-COO(-) group of D1-Ala344 ligates the Mn ion that undergoes an oxidation during the S(1) --> S(2) transition. Control experiments were conducted with Mn-depleted wild-type PSII particles. These experiments showed that tyrosine Y(D) may be structurally coupled to the carbonyl oxygen of an alanine-derived peptide carbonyl group.

Coupling of electron and proton transfer in oxidative water cleavage in photosynthesis

This minireview addresses questions on the mechanism of oxidative water cleavage with special emphasis on the coupling of electron (ET) and proton transfer (PT) of each individual redox step of the reaction sequence and on the mode of O-O bond formation. The following topics are discussed: (1) the multiphasic kinetics of Y(Z)(ox) formation by P680(+*) originate from three different types of rate limitations: (i) nonadiabatic electron transfer for the "fast" ns reaction, (ii) local "dielectric" relaxation for the "slow" ns reaction, and (iii) "large-scale" proton shift for the micros kinetics; (2) the ET/PT-coupling mode of the individual redox transitions within the water oxidizing complex (WOC) driven by Y(Z)(ox) is assumed to depend on the redox state S(i): the oxidation steps of S(0) and S(1) comprise separate ET and PT pathways while those of S(2) and S(3) take place via proton-coupled electron transfer (PCET) analogous to Jerry Babcock's hydrogen atom abstractor model [Biochim. Biophys. Acta, 1458 (2000) 199]; (3) S(3) is postulated to be a multistate redox level of the WOC with fast dynamic equilibria of both redox isomerism and proton tautomerism. The primary event in the essential O-O bond formation is the population of a state S(3)(P) characterized by an electronic configuration and nuclear geometry that corresponds with a complexed hydrogen peroxide; (4) the peroxidic type S(3)(P) is the entatic state for formation of complexed molecular oxygen through S(3) oxidation by Y(Z)(ox); and (5) the protein matrix itself is proposed to exert catalytic activity by functioning as "PCET director". The WOC is envisaged as a supermolecule that is especially tailored for oxidative water cleavage and acts as a molecular machine.

The function and characteristics of tyrosyl radical cofactors

Amino-acid radicals are involved in the catalytic cycles of a number of enzymes. The main focus of this mini-review is to discuss the function and properties of tyrosyl radical cofactors. We start by briefly summarizing the experimental studies that led to the detection and identification of the two redox-active tyrosines, denoted Y(Z) and Y(D), found in the water-oxidizing photosystem II (PSII) enzyme. More recent work that shows that the histidine-cross-linked tyrosine located in the active site of cytochrome c oxidase forms a radical during the catalytic oxygen-oxygen bond-cleavage process is also described. Advanced spectroscopic and structural studies have been performed to investigate the spin-density distribution, the protonation state and the hydrogen bonding of redox-active tyrosines. These studies have shown that the radical spin-density distribution is highly insensitive to the environment and that it is typical of a deprotonated species. In contrast, the hydrogen bonding and the nature of the proton acceptor or network of acceptors vary substantially in different systems. This is important for the function of the tyrosyl radical, as will be emphasized in a detailed discussion on the proposed function of Y(Z) as a proton coupled electron-transfer cofactor in photosynthetic water oxidation. Amino-acid radical enzymes are typically large complexes containing multiple subunits, chromophores and redox cofactors. The structural and mechanistic complexity of these systems has hampered the detailed characterization of their radical cofactors. In the final section of this mini-review, we will describe a project aimed at investigating how the protein controls the thermodynamic and kinetic redox properties of aromatic residues by using de novo protein design. Two model proteins of different size have been constructed. The smaller protein is a 67-residue three-helix bundle containing either a single buried tryptophan or tyrosine residue. The high-resolution NMR structure of the tryptophan-containing protein, denoted alpha(3)W, shows that the aromatic side chain is involved in a pi-cation interaction with a nearby lysine. The effects of this interaction on the tryptophan reduction potential were investigated by electrochemical and quantum mechanical methods. The calculations predict that the pi-cation interaction increases the potential, which is consistent with the electrochemical characterization of alpha(3)W. A larger 117-residue four-helix bundle, alpha(4)W, has more recently been constructed to complement the work on the three-helix-bundles and expand the family of model radical proteins.

Proton-coupled electron transfer: a unifying mechanism for biological charge transport, amino acid radical initiation and propagation, and bond making/breaking reactions of water and oxygen

Redox-driven proton pumps, radical initiation and propagation in biology, and small-molecule activation processes all involve the coupling of electron transfer to proton transport. A mechanistic framework in which to interpret these processes is being developed by examining proton-coupled electron transfer (PCET) in model and natural systems. Specifically, PCET investigations are underway on the following three fronts: (1) the elucidation of the PCET reaction mechanism by time-resolved laser spectroscopy of electron donors and acceptors juxtaposed by a proton transfer interface; (2) the role of amino acid radicals in biological catalysis with the radical initiation and transport processes of E. coli ribonucleotide reductase (RNR) as a focal point; and (3) the application of PCET towards small-molecule activation with emphasis on biologically relevant bond-breaking and bond-making processes involving oxygen and water. A review of recent developments in each of these areas is discussed.

Protons, proteins and ATP

The machinery of life has been disclosed in the second half of the 20th century to a degree not in the least envisioned previously by even the most daring players in this field. It has been extremely rewarding to start out from the fogs and to enjoy the brightness at the end of one's active career. Perhaps the most astounding lesson to learn is how conservative and modular is the construction of key devices. Oxidative and photophosphorylation are carried out by ATP synthase, which is unique in converting electrochemical, mechanical and chemical forms of energy within one nano-machine. This complex protein consists of more than 20 polypeptides of at least eight different kinds. Still, its activity survives in engineered chimerical constructs joining parts from organisms that underwent billions of years of separate evolution. The path of discovery of its structure and function is sketched here from a personal viewpoint. It has been a long way from before-structure-bioenergetics to the post-structural one (which now dominates the biology textbooks), but there is still a long way to go for a rigorous physical understanding. The author has been privileged to enjoy the friendship, cooperation and competition of excellent scientists from widely different backgrounds and expertise.