Structure of Photosystems I and II

Photosynthesis is the major process that converts solar energy into chemical energy on Earth. Two and a half billion years ago, the ancestors of cyanobacteria were able to use water as electron source for the photosynthetic process, thereby evolving oxygen and changing the atmosphere of our planet Earth. Two large membrane protein complexes, Photosystems I and II, catalyze the primary step in this energy conversion, the light-induced charge separation across the photosynthetic membrane. This chapter describes and compares the structure of two Photosystems and discusses their function in respect to the mechanism of light harvesting, electron transfer and water splitting.

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.

Effect of charge distribution over a chlorophyll dimer on the redox potential of P680 in photosystem II as studied by density functional theory calculations

The effect of charge distribution over a chlorophyll dimer on the redox potential of P680 in photosystem II was studied by density functional theory calculations using the P680 coordinates in the X-ray structure. From the calculated ionization potentials of the dimer and the monomeric constituents, the decrease in the redox potential by charge delocalization over the dimer was estimated to be approximately 140 mV. Such charge delocalization was previously observed in the isolated D1-D2-Cyt b 559 complexes, whereas the charge was primarily localized on P D1 in the core complexes. The calculated potential decrease of approximately 140 mV can explain the inhibition of Y Z oxidation in the former complexes and in turn implies that the charge localization on P D1 upon formation of the core complex increases the P680 potential to the level necessary for water oxidation.

Crystal structure of the oxygen-evolving complex of photosystem II

The oxygen in our atmosphere is derived from and maintained by the water-splitting process of photosynthesis. The enzyme that facilitates this reaction and therefore underpins virtually all life on our planet is known as photosystem II (PSII). It is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae, and cyanobacteria. Powered by light, PSII catalyzes the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases molecular oxygen into the atmosphere and provides the reducing equivalents required for the conversion of carbon dioxide into the organic molecules of life. Recently, a fully refined structure of an isolated 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography, which gave organizational and structural details of the 19 subunits (16 intrinsic and 3 extrinsic) that make up each monomer and provided information about the position and protein environments of the many different cofactors it binds. 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 six or seven form direct ligands to the metals. The metal cluster was originally modeled as a cubane-like structure composed of three Mn ions and the Ca (2+) linked by oxo bonds and the fourth Mn attached to the cubane via one of its O atoms. New data from X-ray diffraction and X-ray spectroscopy suggest some alternative arrangements. Nevertheless, all of the models are sufficiently similar to provide a basis for discussing the chemistry by which PSII splits water and makes oxygen.

The structure of the Mn4Ca2+ cluster of photosystem II and its protein environment as revealed by X-ray crystallography

The location, structure and protein environment of the Mn4Ca2+ cluster, which catalyses the light-driven, water-splitting reaction of photosystem II, has been revealed by X-ray crystallography. However, owing to the low resolutions of the crystal structures reported to date, and the possibility of radiation damage at the catalytic centre, the precise position of each metal ion remains unknown. To some extent, these problems have been overcome by applying spectroscopic techniques like extended X-ray absorption fine structure. Taking into account the most recent results obtained with these two X-ray-based techniques, we have attempted to refine models of the structure of the Mn4Ca2+ cluster and its protein environment.

Where water is oxidized to dioxygen: structure of the photosynthetic Mn4Ca cluster from X-ray spectroscopy

Light-driven oxidation of water to dioxygen in plants, algae, and cyanobacteria is catalyzed within photosystem II (PS II) by a Mn 4Ca cluster. Although the cluster has been studied by many different methods, its structure and mechanism have remained elusive. X-ray absorption and emission spectroscopy and extended X-ray absorption fine structure studies have been particularly useful in probing the electronic and geometric structures and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn 4Ca cluster geometry to a set of three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0- or 3.5-A-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-ray spectroscopy and crystallography are predominantly because of damage to the Mn 4Ca cluster by X-rays under conditions used for the structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn 4Ca catalytic center as it cycles through the five intermediate states known as the S i states ( i = 0-4). The electronic structure of the Mn 4Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formal oxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms, which includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structures of the Mn 4Ca cluster in the S states derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of O-O bond formation during the photosynthetic water-splitting process.

Primary charge separation in the photosystem II core from Synechocystis: a comparison of femtosecond visible/midinfrared pump-probe spectra of wild-type and two P680 mutants

It is now quite well accepted that charge separation in PS2 reaction centers starts predominantly from the accessory chlorophyll B(A) and not from the special pair P(680). To identify spectral signatures of B(A,) and to further clarify the process of primary charge separation, we compared the femtosecond-infrared pump-probe spectra of the wild-type (WT) PS2 core complex from the cyanobacterium Synechocystis sp. PCC 6803 with those of two mutants in which the histidine residue axially coordinated to P(B) (D2-His(197)) has been changed to Ala or Gln. By analogy with the structure of purple bacterial reaction centers, the mutated histidine is proposed to be indirectly H-bonded to the C(9)=O carbonyl of the putative primary donor B(A) through a water molecule. The constructed mutations are thus expected to perturb the vibrational properties of B(A) by modifying the hydrogen bond strength, possibly by displacing the H-bonded water molecule, and to modify the electronic properties and the charge localization of the oxidized donor P(680)(+). Analysis of steady-state light-induced Fourier transform infrared difference spectra of the WT and the D2-His(197)Ala mutant indeed shows that a modification of the axially coordinating ligand to P(B) induces a charge redistribution of P(680)(+). In addition, a comparison of the time-resolved visible/midinfrared spectra of the WT and mutants has allowed us to investigate the changes in the kinetics of primary charge separation induced by the mutations and to propose a band assignment identifying the characteristic vibrations of B(A).

Crystal structure of cyanobacterial photosystem II at 3.0 A resolution: a closer look at the antenna system and the small membrane-intrinsic subunits

Photosystem II (PSII) is a homodimeric protein-cofactor complex embedded in the thylakoid membrane that catalyses light-driven charge separation accompanied by the water splitting reaction during oxygenic photosynthesis. In the first part of this review, we describe the current state of the crystal structure at 3.0 A resolution of cyanobacterial PSII from Thermosynechococcus elongatus [B. Loll et al., Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II, Nature 438 (2005) 1040-1044] with emphasis on the core antenna subunits CP43 and CP47 and the small membrane-intrinsic subunits. The second part describes first the general theory of optical spectra and excitation energy transfer and how the parameters of the theory can be obtained from the structural data. Next, structure-function relationships are discussed that were identified from stationary and time-resolved experiments and simulations of optical spectra and energy transfer processes.

X-Ray spectroscopy of the photosynthetic oxygen-evolving complex

Water oxidation to dioxygen in photosynthesis is catalyzed by a Mn(4)Ca cluster with O bridging in Photosystem II (PS II) of plants, algae and cyanobacteria. A variety of spectroscopic methods have been applied to analyzing the participation of the complex. X-ray spectroscopy is particularly useful because it is element-specific, and because it can reveal important structural features of the complex with high accuracy and identify the participation of Mn in the redox chemistry. Following a brief history of the application of X-ray spectroscopy to PS II, an overview of newer results will be presented and a description of the present state of our knowledge based on this approach.

Photosystem II: structure and mechanism of the water:plastoquinone oxidoreductase

This mini-review briefly summarizes our current knowledge on the reaction pattern of light-driven water splitting and the structure of Photosystem II that acts as a water:plastoquinone oxidoreductase. The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680+*QA(-*) ; (b) reduction of plastoquinone to plastoquinol at the QB site via a two-step reaction sequence with QA(-*) as reductant and (c) oxidative water splitting into O2 and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680+* as oxidant and a redox active tyrosine YZ acting as mediator. Based on recent progress in X-ray diffraction crystallographic structure analysis the array of the cofactors within the protein matrix is discussed in relation to the functional pattern. Special emphasis is paid on the structure of the catalytic sites of PQH2 formation (QB-site) and oxidative water splitting (Mn4OxCa cluster). The energetics and kinetics of the reactions taking place at these sites are presented only in a very concise manner with reference to recent up-to-date reviews. It is illustrated that several questions on the mechanism of oxidative water splitting and the structure of the catalytic sites are far from being satisfactorily answered.

Electronic structure of the Mn4OxCa cluster in the S0 and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn-ENDOR and EPR spectroscopy

The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S0 to S4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained 55Mn hyperfine coupling constants of the S0 and S2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392-2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S0 in methanol (3%) containing samples and thereby establishes that the oxidation states of the manganese ions in S0 and S2 are, at 4 K, Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. By applying a "structure filter" that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821-825] we (i) show that this new structural model is fully consistent with EPR and 55Mn-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 A Mn-Mn distance in S0 to 2.75 A in S1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459-7471] corresponds to a deprotonation of a mu-hydroxo bridge between MnA and MnB, i.e., between the outer Mn and its neighboring Mn of the mu3-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S0 --> S1 and S1 --> S2 transitions.

Bridge over troubled water: resolving the competing photosystem II crystal structures

Density functional theory (DFT) calculations, at the Becke-Perdew/TZP level of theory, were used to investigate a set of CaMn(4)-containing clusters that model the active site of the water-oxidizing complex (WOC) of photosystem II (PSII). Metal-atom positions for three representative isomeric clusters of the formula [CaMn(4)C(9)N(2)O(16)H(10)](+)4 H(2)O are in good agreement with the disparate Mn(4) geometries of the three most recent X-ray crystal structures. Remarkably, interconversion between these three isomeric clusters is found to be facile, resulting from subtle changes in the coordination environment around the CaMn(4) centre. This result provides a clear rationalisation of the marked differences in reported crystal structures. Recent concerns have been raised regarding the opportunity for X-ray-damage-induced distortion of the metal-containing active centre during crystallographic analysis. Our calculations suggest that an even greater problem may be presented by the apparent fluxionality of the CaMn(4) skeleton within the active centre. Structural rearrangement may well precede crystallographic analysis, for example by the preferential "freezing-out" of one of several near-isoenergetic structures during the workup for crystallisation. This prospect, which our calculations cannot exclude, highlights the difficulties that will continue to be faced by experimentalists seeking unambiguous structural information on the WOC's active site.

An Activated Glutamate Residue Identified in Photosystem II at the Interface between the Manganese-stabilizing Subunit and the D2 Polypeptide

Photosystem II (PSII) catalyzes the oxidation of water during oxygenic photosynthesis. PSII is composed both of intrinsic subunits, such as D1, D2, and CP47, and extrinsic subunits, such as the manganese-stabilizing subunit (MSP). Previous work has shown that amines covalently bind to amino acid residues in the CP47, D1, and D2 subunits of plant and cyanobacterial PSII, and that these covalent reactions are prevented by the addition of chloride in plant preparations depleted of the 18- and 24-kDa extrinsic subunits. It has been proposed that these reactive groups are carbonyl-containing, post-translationally modified amino acid side chains (Ouellette, A. J. A., Anderson, L. B., and Barry, B. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 2204-2209 and Anderson, L. B., Ouellette, A. J. A., and Barry, B. A. (2000) J. Biol. Chem. 275, 4920-4927). To identify the amino acid binding site in the spinach D2 subunit, we have employed a biotin-amine labeling reagent, which can be used in conjunction with avidin affinity chromatography to purify biotinylated peptides from the PSII complex. Multidimensional chromato-graphic separation and multistage mass spectrometry localizes a novel post-translational modification in the D2 subunit to glutamate 303. We propose that this glutamate is activated for amine reaction by post-translational modification. Because the modified glutamate is located at a contact site between the D2 and manganese-stabilizing subunits, we suggest that the modification is important in vivo in stabilizing the interaction between these two PSII subunits. Consistent with this conclusion, mutations at the modified glutamate alter the steady-state rate of photosynthetic oxygen evolution.

Spin states in polynuclear clusters: the [Fe2O2] core of the methane monooxygenase active site

The ability to provide a correct description of different spin states of mono- and polynuclear transition metal complexes is essential for a detailed investigation of reactions that are catalyzed by such complexes. We study the energetics of different total and local spin states of a dinuclear oxygen-bridged iron(IV) model for the intermediate Q of the hydroxylase component of methane monooxygenase by means of spin-unrestricted Kohn-Sham density functional theory. Because it is known that the spin state total energies depend systematically on the density functional, and that this dependence is intimately connected to the exact exchange admixture of present-day hybdrid functionals, we compare total energies, local and total spin values, and Heisenberg coupling constants calculated with the established functionals BP86 and B3LYP as well as with a modified B3LYP version with an exact exchange admixture ranging from 0 to 24%. It is found that exact exchange enhances local spin polarization. As the exact exchange admixture increases, the high-spin state is energetically favored, although the Broken-Symmetry state always is the ground state. Instead of the strict linear variation of the energy splittings observed for mononuclear complexes, a slightly nonlinear dependence is found. The Heisenberg coupling constants J(Fe1Fe2) --evaluated according to three different proposals from the literature -- are found to vary from -129 to -494 cm(-1) accordingly. The experimental finding that intermediate Q has an antiferromagnetic ground state is thus confirmed.

g-Anisotropy of the S2-state manganese cluster in single crystals of cyanobacterial photosystem II studied by W-band electron paramagnetic resonance spectroscopy

The multiline signal from the S2-state manganese cluster in the oxygen evolving complex of photosystem II (PSII) was observed in single crystals of a thermophilic cyanobacterium Thermosynechococcus vulcanus for the first time by W-band (94 GHz) electron paramagnetic resonance (EPR). At W-band, spectra were characterized by the g-anisotropy, which enabled the precise determination of the tensor. Distinct hyperfine splittings (hfs's) as seen in frozen solutions of PSII at X-band (9.5 GHz) were detected in most of the crystal orientations relative to the magnetic field. In some orientations, however, the hfs's disappeared due to overlapping of a large number of EPR lines from eight crystallographic symmetry-related sites of the manganese cluster within the unit cell of the crystal. Analysis of the orientation-dependent spectral features yielded the following g-tensor components: g(x) = 1.988, g(y) = 1.981, g(z) = 1.965. The principal values suggested an approximate axial symmetry around the Mn(III) ion in the cluster.

Carbonate complexation of Mn2+ in the aqueous phase: redox behavior and ligand binding modes by electrochemistry and EPR spectroscopy

The chemical speciation of Mn2+ within cells is critical for its transport, availability, and redox properties. Herein we investigate the redox behavior and complexation equilibria of Mn2+ in aqueous solutions of bicarbonate by voltammetry and electron paramagnetic resonance (EPR) spectroscopy and discuss the implications for the uptake of Mn2+ by mangano-cluster enzymes such as photosystem II (PSII). Both the electrochemical reduction of Mn2+ to Mn0 at an Hg electrode and EPR (in the absence of a polarizing electrode) revealed the formation of 1:1 and 1:2 Mn-(bi)carbonate complexes as a function of Mn2+ and bicarbonate concentrations. Pulsed EPR spectroscopy, including ENDOR, ESEEM, and 2D-HYSCORE, were used to probe the hyperfine couplings to 1H and 13C nuclei of the ligand(s) bound to Mn2+. For the 1:2 complex, the complete 13C hyperfine tensor for one of the (bi)carbonate ligands was determined and it was established that this ligand coordinates to Mn2+ in bidentate mode with a 13C-Mn distance of 2.85 +/- 0.1 angstroms. The second (bi)carbonate ligand in the 1:2 complex coordinates possibly in monodentate mode, which is structurally less defined, and its 13C signal is broad and unobservable. 1H ENDOR reveals that 1-2 water ligands are lost upon binding of one bicarbonate ion in the 1:1 complex while 3-4 water ligands are lost upon forming the 1:2 complex. Thus, we deduce that the dominant species above 0.1 M bicarbonate concentration is the 1:2 complex, [Mn(CO3)(HCO3)(OH2)3]-.

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".

Charge separation and energy transfer in the photosystem II core complex studied by femtosecond midinfrared spectroscopy

The core of photosystem II (PSII) of green plants contains the reaction center (RC) proteins D1D2-cytb559 and two core antennas CP43 and CP47. We have used time-resolved visible pump/midinfrared probe spectroscopy in the region between 1600 and 1800 cm(-1) to study the energy transfer and charge separation events within PSII cores. The absorption difference spectra in the region of the keto and ester chlorophyll modes show spectral evolution with time constants of 3 ps, 27 ps, 200 ps, and 2 ns. Comparison of infrared (IR) difference spectra obtained for the isolated antennas CP43 and CP47 and the D1D2-RC with those measured for the PSII core allowed us to identify the features specific for each of the PSII core components. From the presence of the CP43 and CP47 specific features in the spectra up to time delays of 20-30 ps, we conclude that the main part of the energy transfer from the antennas to the RC occurs on this timescale. Direct excitation of the pigments in the RC evolution associated difference spectra to radical pair formation of PD1+PheoD1- on the same timescale as multi-excitation annihilation and excited state equilibration within the antennas CP43 and CP47, which occur within approximately 1-3 ps. The formation of the earlier radical pair ChlD1+PheoD1-, as identified in isolated D1D2 complexes with time-resolved mid-IR spectroscopy is not observed in the current data, probably because of its relatively low concentration. Relaxation of the state PD1+PheoD1-, caused by a drop in free energy, occurs in 200 ps in closed cores. We conclude that the kinetic model proposed earlier for the energy and electron transfer dynamics within the D1D2-RC, plus two slowly energy-transferring antennas C43 and CP47 explain the complex excited state and charge separation dynamics in the PSII core very well. We further show that the time-resolved IR-difference spectrum of PD1+PheoD1- as observed in PSII cores is virtually identical to that observed in the isolated D1D2-RC complex of PSII, demonstrating that the local structure of the primary reactants has remained intact in the isolated D1D2 complex.

Homogenous Water Oxidation by a Di-μ-Oxo Dimanganese Complex in the Presence of Ce

O(2) evolution was observed upon mixing aqueous [(terpy)(H(2)O)Mn(O)(2)Mn(H(2)O)(terpy)](NO(3))(3) (1, terpy = 2,2':6',6″-terpyridine) with aqueous solutions of Ce(4+). However, when the solution of 1 was incubated at pH = 1 (by dissolving in dilute HNO(3)) before mixing with Ce(4+), very small amounts of O(2) were observed. This observation of acid-induced deactivation suggests an explanation, both for the previously reported lack of O(2) evolution from aqueous solutions of 1 with Ce(4+) as oxidant, and the present observation of low amounts of O(2) production with the very acidic Ce(4+) reagent. Evidence is provided for water being the source of evolved O(2), and for the requirement of a high valent multinuclear Mn species for O(2) evolution. We test the possibility of complications in the use of cerium ammonium nitrate (CAN) in oxidation chemistry due to the presence of the oxidizable NH(4) (+) ion.

Structure of the Mn4-Ca cluster as derived from X-ray diffraction

The catalytic centre for light-induced water oxidation in photosystem II (PSII) is a multinuclear metal cluster containing four manganese and one calcium cations. Knowing the structure of this biological catalyst is of utmost importance for unravelling the mechanism of water oxidation in photosynthesis. In this review we describe the current state of the X-ray structure determination at 3.0 A resolution of the water oxidation complex (WOC) of PSII. The arrangement of metal cations in the cluster, their coordination and protein surroundings are discussed with regard to spectroscopic and mutagenesis studies. Limitations of the presently available structural data are pointed out and possible perspectives for the future are outlined, including the combination of X-ray diffraction and X-ray spectroscopy on single crystals.

Oxidative photosynthetic water splitting: energetics, kinetics and mechanism

This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457-476) as a framework, the energetics and kinetics of two different types of reactions comprising the overall process are discussed: (i) P680+* reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yz (ox) induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen. The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available information about the Gibbs energy differences between the individual redox states Si+1 and Si and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and on the active role of the protein in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place at the level of a complexed peroxide.

EPR spectroscopy of the manganese cluster of photosystem II

Electron paramagnetic resonance (EPR) spectroscopy is a valuable tool for understanding the oxidation state and chemical environment of the Mn4Ca cluster of photosystem II. Since the discovery of the multiline signal from the S2 state, EPR spectroscopy has continued to reveal details about the catalytic center of oxygen evolution. At present EPR signals from nearly all of the S-states of the Mn4Ca cluster, as well as from modified and intermediate states, have been observed. This review article describes the various EPR signals obtained from the Mn4Ca cluster, including the metalloradical signals due to interaction of the cluster with a nearby organic radical.

Perturbations at the chloride site during the photosynthetic oxygen-evolving cycle

Photosystem II (PSII) catalyzes the oxidation of water to O2 at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five Sn states. Chloride depletion inhibits O2 evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S3 to S0 (third flash) and S0 to S1 transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle.

Dynamics of electron transfer in photosystem II

Photosystem II, being a constituent of light driven photosynthetic apparatus, is a highly organized pigment-protein-lipid complex. The arrangement of PSII active redox cofactors insures efficiency of electron transfer within it. Donation of electrons extracted from water by the oxygen evolving complex to plastoquinones requires an additional activation energy. In this paper we present theoretical discussion of the anharmonic fluctuations of the protein-lipid matrix of PSII and an experimental evidence showing that the fluctuations are responsible for coupling of its donor and acceptor side. We argue that the fast collective motions liberated at temperatures higher that 200 K are crucial for the two final steps of the water splitting cycle and that one can distinguish three different dynamic regimes of PSII action which are controlled by the timescales of forward electron transfer, which vary with temperature. The three regimes of the dynamical behavior are related to different spatial domains of PSII.

Quantum mechanics/molecular mechanics structural models of the oxygen-evolving complex of photosystem II

The annual production of 260 Gtonnes of oxygen, during the process of photosynthesis, sustains life on earth. Oxygen is produced in the thylakoid membranes of green-plant chloroplasts and the internal membranes of cyanobacteria by photocatalytic water oxidation at the oxygen-evolving complex (OEC) of photosystem II (PSII). Recent breakthroughs in X-ray crystallography and advances in quantum mechanics/molecular mechanics (QM/MM) hybrid methods have enabled the construction of chemically sensible models of the OEC of PSII. The resulting computational structural models suggest the complete ligation of the catalytic center by amino acid residues, water, hydroxide and chloride, as determined from the intrinsic electronic properties of the oxomanganese core and the perturbational influence of the surrounding protein environment. These structures are found to be consistent with available mechanistic data, and are also compatible with X-ray diffraction models and extended X-ray absorption fine structure measurements. It is therefore conjectured that these OEC models are particularly relevant for the elucidation of the catalytic mechanism of water oxidation.

Deletion of PsbM in tobacco alters the QB site properties and the electron flow within photosystem II

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.

Protons bound to the Mn cluster in photosystem II oxygen evolving complex detected by proton matrix ENDOR

Protons in the vicinity of the oxygen-evolving manganese cluster in photosystem II were studied by proton matrix ENDOR. Six pairs of proton ENDOR signals were detected in both the S(0) and S(2) states of the Mn-cluster. Two pairs of signals that show hyperfine constants of 2.3/2.2 and 4.0 MHz, respectively, disappeared after D(2)O incubation in both states. The signals with 2.3/2.2 MHz hyperfine constants in S(0) and S(2) state multiline disappeared after 3 h of D(2)O incubation in the S(0) and S(1) states, respectively. The signal with 4.0 MHz hyperfine constants in S(0) state multiline disappeared after 3 h of D(2)O incubation in the S(0) state, while the similar signal in S(2) state multiline disappeared only after 24 h of D(2)O incubation in the S(1) state. The different proton exchange rates seem to be ascribable to the change in affinities of water molecules to the variation in oxidation state of the Mn cluster during the water oxidation cycle. Based on the point dipole approximation, the distances between the center of electronic spin of the Mn cluster and the exchangeable protons were estimated to be 3.3/3.2 and 2.7 A, respectively. These short distances suggest the protons belong to the water molecules ligated to the manganese cluster. We propose a model for the binding of water to the manganese cluster based on these results.

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.

Lipids in photosystem II: interactions with protein and cofactors

Photosystem II (PSII) is a homodimeric protein-cofactor complex embedded in the thylakoid membrane that catalyses light-driven charge separation accompanied by the oxidation of water during oxygenic photosynthesis. Biochemical analysis of the lipid content of PSII indicates a number of integral lipids, their composition being similar to the average lipid composition of the thylakoid membrane. The crystal structure of PSII at 3.0 A resolution allowed for the first time the assignment of 14 integral lipids within the protein scaffold, all of them being located at the interface of different protein subunits. The reaction centre subunits D1 and D2 are encircled by a belt of 11 lipids providing a flexible environment for the exchange of D1. Three lipids are located in the dimerization interface and mediate interactions between the PSII monomers. Several lipids are located close to the binding pocket of the mobile plastoquinone Q(B), forming part of a postulated diffusion pathway for plastoquinone. Furthermore two lipids were found, each ligating one antenna chlorophyll a. A detailed analysis of lipid-protein and lipid-cofactor interactions allows to derive some general principles of lipid binding pockets in PSII and to suggest possible functional properties of the various identified lipid molecules.

Participation of the C-terminal region of the D1-polypeptide in the first steps in the assembly of the Mn4Ca cluster of photosystem II

Amino acid residue D1-Asp(170) of the D1-polypeptide of photosystem II was previously shown to be implicated in the binding and oxidation of the first manganese to be assembled into the Mn(4)Ca cluster of the oxygen-evolving complex (OEC). According to recent x-ray crystallographic structures of photosystem II, D1-Glu(333) is proposed to participate with D1-Asp(170) in the coordination of Mn4 of the OEC. Other residues in the C-terminal region of the D1-polypeptide are proposed to coordinate nearby manganese of the cluster. Site-directed replacements in Synechocystis sp. PCC 6803 at D1-His(332), D1-Glu(333), D1-Asp(342), D1-Ala(344), and D1-Ser(345) were examined with regard to their ability to influence the binding and oxidation of the first manganese in manganese-depleted photosystem II core complexes. Direct and indirect measurements reveal in all mutants, but most marked in D1-Glu(333) replaced by His, an impaired ability of Mn(2+) to reduce Y(Z)., indicating a reduced ability (elevated K(m)) compared with WT to bind and oxidize the first manganese of the OEC. The effect on the K(m) of these mutations is, however, considerably weaker than some of those constructed at D1-Asp(170) (replacement by Asn, Ala, and Ser). These observations imply that the C-terminal residues ultimately involved in manganese coordination contribute to the high affinity binding at D1-Asp(170) likely through electrostatic interactions. That these residues are far from D1-Asp(170) in the primary structure of the D1-polypeptide, imply that the C terminus of the D1-polypeptide is already close to its mature conformation at the first stages of assembly of the Mn(4)Ca cluster.

Syntheses, structure, and properties of a manganese–calcium cluster containing a Mn4Ca2core

We describe here a novel, simple, efficient self-assembly method for the in situ generation of [Mn4Cl4(micro-OCH2CH2OMe)4(EtOH)4] and [Mn4(micro-Cl)Cl3(micro-OCH2CH2OMe)4(HOCH2CH2OMe)3]2 cubane-type compounds which react readily with calcium species to form cluster [Mn4Ca2Cl4(micro-OCH2CH2OMe)8], the calcium atoms attached to the Mn4 unit of flatten out the cubane inducing significant conformational changes.

Aggregation of tetranuclear Mn building blocks with alkali ion. Syntheses, crystal structures and chemical behaviors of monomer, dimer and polymer containing [Mn4(µ3-O)2]8+units

Aggregation of tetranuclear Mn(4)O(2) building blocks with alkali ion was studied. Several Mn(iii) complexes containing [Mn(4)O(2)(AcO)(7)(pyz)(2)](-) (pyz = pyrazinate) anion(s) were obtained from an assembly system containing Mn(ii), MnO(4)(-), HOAc and Hpyz (Napyz or Kpyz). These [Mn(4)O(2)](8+) complexes have monomeric (1 and 2), dimeric (4 and 5) and one-dimensional chain () structures of which alkali metal ion connects the Mn ions of adjacent [Mn(4)O(2)](8+) units through mu(1,1)- and mu(1,3)-carboxylate bridges. Complexes 2 or 3 were converted into [Mn(12)O(12)(AcO)(16)(H(2)O)(4)] in EtOH solution in the presence of HOAc. However, in MeOH solution, a coordination polymer [Mn(2)(HCOO)(4)(H(2)O)(4)](n) was obtained accompanying the oxidation of MeOH to become HCHO and HCOOH. Tracing the (1)H NMR spectra of 2 or 3 in CD(3)OD, the disappearance of the resonance signals in 3 h indicated the decomposition of the [Mn(4)O(2)](8+) cores. Complex 2 exhibits its proton NMR signals in CDCl(3) which are similar to those of its pic analogue but accompany downfield shift to various extents for all the corresponding signals. Variable-temperature magnetic susceptibilities of complexes 2-5 in the range 5-300 K were recorded and were fitted for an Mn(4)O(2) butterfly core, giving the fitting parameters J(bb) = -2.67 to -3.76 cm(-1) and J(wb) = -1.16 to -3.14 cm(-1). Small J values indicate weak antiferromagnetic coupling interactions of the Mn(iii) sites and the spin ground states are considered as S(T) = 0 based on the J(bb)/J(wb) ratio approximately 1 for these complexes. The ESR spectra were recorded for complex 2 in dual-mode at liquid-helium temperatures and no obvious signal could be found. The addition of p-cresol gives rise to the reduction of the [Mn(4)O(2)](8+), resulting in observable signals.

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.

Reaction pattern and mechanism of light induced oxidative water splitting in photosynthesis

This mini review is an attempt to briefly summarize our current knowledge on light driven oxidative water splitting in photosynthesis. The reaction leading to molecular oxygen and four protons via photosynthesis comprises thermodynamic and kinetic constraints that require a balanced fine tuning of the reaction coordinates. The mode of coupling between electron (ET) and proton transfer (PT) reactions is shown to be of key mechanistic relevance for the redox turnover of Y(Z) and the reactions within the WOC. The WOC is characterized by peculiar energetics of its oxidation steps in the WOC. In all oxygen evolving photosynthetic organisms the redox state S(1) is thermodynamically most stable and therefore this general feature is assumed to be of physiological relevance. Available information on the Gibbs energy differences between the individual redox states S(i+1) and S(i) and on the activation energies of their oxidative transitions are used to construct a general reaction coordinate of oxidative water splitting in photosystem II (PS II). Finally, an attempt is presented to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and the active role of the protein environment in tuning the local proton activity that depends on time and redox state S(i). The O-O linkage is assumed to take place within a multistate equilibrium at the redox level of S(3), comprising both redox isomerism and proton tautomerism. It is proposed that one state, S(3)(P), attains an electronic configuration and nuclear geometry that corresponds with a hydrogen bonded peroxide which acts as the entatic state for the generation of complexed molecular oxygen through S(3)(P) oxidation by Y(Z)(ox).

Density Functional Study on Geometrical Features and Electronic Structures of Di-μ-oxo-Bridged [Mn2O2(H2O)8]q+ with Mn(II), Mn(III), and Mn(IV)

We report the geometrical features and electronic structures of di-mu-oxo-bridged Mn-Mn binuclear complexes with H2O ligands [Mn2O2(H2O)8]q+ in the iso- and mixed-valence oxidation states. All of the combinations among Mn(II), Mn(III), and Mn(IV) ions are considered the oxidation states of the Mn-Mn center, and the changes in molecular structure induced by the different electron configurations of Mn-based orbitals are investigated in relation to the oxygen-evolving complex (OEC) of photosystem II. The stable geometries of complexes are determined by using the hybrid-type density functional theory for both of the highest- and lowest-spin couplings between Mn sites, and the lowest-spin-coupled states are energetically more favorable than the highest-spin-coupled states except in the case of the complexes with the Mn(II) ion. The coordination positions of H2O ligands at the Mn(II) site tend to shift from the octahedral positions in contrast to those at the Mn(III) and Mn(IV) sites. The shape of the Mn2O2 core and the distances between the Mn ions and the H2O ligands vary depending on the electron occupations of the octahedral eg orbitals on the Mn site with an antibonding nature for the Mn-ligand interactions, indicating the trend as Mn(II)-O > Mn(III)-O and Mn(IV)-O, O-Mn(II)-O > O-Mn(III)-O > O-Mn(IV)-O among the iso-valence Mn2O2 cores, and O-Mn(lower)-O < O-Mn(higher)-O within the mixed-valence Mn2O2 core, and as Mn(II)-OH2 and Mn(III)-OH2 > Mn(IV)-OH2 for the axial H2O ligand. The optimized geometries of model complexes are compared with the X-ray structure of the OEC, and it is suggested that the cubane-like Mn cluster of the active site may not contain a Mn(II) ion. The effective exchange integrals are estimated by applying the approximate spin projection to clarify the magnetic coupling between Mn sites, and the superexchange pathways through the di-mu-oxo bridge are examined on the basis of the singly occupied magnetic orbitals derived from the singlet-coupled natural orbitals in the broken-symmetry state. The comparisons of the calculated results between [Mn2O2(H2O)8]q+ in this study and [Mn2O2(NH3)8]q+ reported by McGrady et al. suggest that the symmetric pathways are dominant to the exchange coupling constant, and the crossed pathway would be less important for the former than it would for the latter in the Mn(III)-Mn(III), Mn(IV)-Mn(IV), and Mn(III)-Mn(IV) oxidation states.

Where water is oxidized to dioxygen: structure of the photosynthetic Mn4Ca cluster

The oxidation of water to dioxygen is catalyzed within photosystem II (PSII) by a Mn(4)Ca cluster, the structure of which remains elusive. Polarized extended x-ray absorption fine structure (EXAFS) measurements on PSII single crystals constrain the Mn(4)Ca cluster geometry to a set of three similar high-resolution structures. Combining polarized EXAFS and x-ray diffraction data, the cluster was placed within PSII, taking into account the overall trend of the electron density of the metal site and the putative ligands. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 angstrom-resolution x-ray structures or other previously proposed models.

PsbI affects the stability, function, and phosphorylation patterns of photosystem II assemblies in tobacco

Photosystem II (PSII) core complexes consist of CP47, CP43, D1, D2 proteins and of several low molecular weight integral membrane polypeptides, such as the chloroplast-encoded PsbE, PsbF, and PsbI proteins. To elucidate the function of PsbI in the photosynthetic process as well as in the biogenesis of PSII in higher plants, we generated homoplastomic knock-out plants by replacing most of the tobacco psbI gene with a spectinomycin resistance cartridge. Mutant plants are photoautotrophically viable under green house conditions but sensitive to high light irradiation. Antenna proteins of PSII accumulate to normal amounts, but levels of the PSII core complex are reduced by 50%. Bioenergetic and fluorescence studies uncovered that PsbI is required for the stability but not for the assembly of dimeric PSII and supercomplexes consisting of PSII and the outer antenna (PSII-LHCII). Thermoluminescence emission bands indicate that the presence of PsbI is required for assembly of a fully functional Q(A) binding site. We show that phosphorylation of the reaction center proteins D1 and D2 is light and redox-regulated in the wild type, but phosphorylation is abolished in the mutant, presumably due to structural alterations of PSII when PsbI is deficient. Unlike wild type, phosphorylation of LHCII is strongly increased in the dark due to accumulation of reduced plastoquinone, whereas even upon state II light phosphorylation is decreased in delta psbI. These data attest that phosphorylation of D1/D2, CP43, and LHCII is regulated differently.

Frequency-Domain Spectroscopic Study of the PS I−CP43‘ Supercomplex from the Cyanobacterium Synechocystis PCC 6803 Grown under Iron Stress Conditions

Absorption, fluorescence excitation, emission, and hole-burning (HB) spectra were measured at liquid helium temperatures for the PS I-CP43' supercomplexes of Synechocystis PCC 6803 grown under iron stress conditions and for respective trimeric PS I cores. Results are compared with those of room temperature, time-domain experiments (Biochemistry 2003, 42, 3893) as well as with the low-temperature steady-state experiments on PS I-CP43' supercomplexes of Synechococcus PCC 7942 (Biochim. Biophys. Acta 2002, 1556, 265). In contrast to the CP43' of Synechococcus PCC 7942, CP43' of Synechocystis PCC 6803 possesses two low-energy states analogous to the quasidegenerate states A and B of CP43 of photosystem II (J. Phys. Chem. B 2000, 104, 11805). Energy transfer between the CP43' and the PS I core occurs, to a significant degree, through the state A, characterized with a broader site distribution function (SDF). It is demonstrated that the low temperature (T = 5 K) excitation energy transfer (EET) time between the state A of CP43' (IsiA) and the PS I core in PS I-CP43' supercomplexes from Synechocystis PCC 6803 is about 60 ps, which is significantly slower than the EET observed at room temperature. Our results are consistent with fast (< or =10 ps) energy transfer from state B to state A in CP43'. Energy absorbed by the CP43' manifold has, on average, a greater chance of being transferred to the reaction center (RC) and utilized for charge separation than energy absorbed by the PS I core antenna. This indicates that energy is likely transferred from the CP43' to the RC along a well-defined path and that the "red antenna states" of the PS I core are localized far away from that path, most likely on the B7-A32 and B37-B38 dimers in the vicinity of the PS I trimerization domain (near PsaL subunit). We argue that the A38-A39 dimer does not contribute to the red antenna region.

Function of two beta-carotenes near the D1 and D2 proteins in photosystem II dimers

The antenna proteins in photosystem II (PSII) not only promote energy transfer to the photosynthetic reaction center (RC) but provide also an efficient cation sink to re-reduce chlorophyll a if the electron transfer (ET) from the Mn-cluster is inhibited. Using the newest PSII dimer crystal structure (3.0 A resolution), in which 11 beta-carotene molecules (Car) and 14 lipids are visible in the PSII monomer, we calculated the redox potentials (Em) of one-electron oxidation for all Car (Em(Car)) by solving the Poisson-Boltzmann equation. In each PSII monomer, the D1 protein harbors a previously unlocated Car (CarD1) in van der Waals contact with the chlorin ring of ChlZ(D1). Each CarD1 in the PSII dimer complex is located in the interface between the D1 and CP47 subunits, together with another four Car of the other PSII monomer and several lipid molecules. The proximity of Car bridging between CarD1 and plastoquinone/Q(A) may imply a direct charge recombination of Car+Q(A)-. The calculated Em(CarD1) and Em(ChlZ(D1)) are, respectively, 83 and 126 mV higher than Em(CarD2) and Em(ChlZ(D2)), which could explain why CarD2+ and ChlZ(D2)+ are observed rather than the corresponding CarD1+ and ChlZ(D1)+.

Lipids in photosynthetic reaction centres: structural roles and functional holes

Photosynthetic proteins power the biosphere. Reaction centres, light harvesting antenna proteins and cytochrome b(6)f (or bc(1)) complexes are expressed at high levels, have been subjected to an intensive spectroscopic, biochemical and mutagenic analysis, and several have been characterised to an informatively high resolution by X-ray crystallography. In addition to revealing the structural basis for the transduction of light energy, X-ray crystallography has brought molecular insights into the relationships between these multicomponent membrane proteins and their lipid environment. Lipids resolved in the X-ray crystal structures of photosynthetic proteins bind light harvesting cofactors, fill intra-protein cavities through which quinones can diffuse, form an important part of the monomer-monomer interface in multimeric structures and may facilitate structural flexibility in complexes that undergo partial disassembly and repair. It has been proposed that individual lipids influence the biophysical properties of reaction centre cofactors, and so affect the rate of electron transfer through the complex. Lipids have also been shown to be important for successful crystallisation of photosynthetic proteins. Comparison of the three types of reaction centre that have been structurally characterised reveals interesting similarities in the position of bound lipids that may point towards a generic requirement to reinforce the structure of the core electron transfer domain. The crystallographic data are also providing new opportunities to find molecular explanations for observed effects of different types of lipid on the structure, mechanism and organisation of reaction centres and other photosynthetic proteins.

A redox-active FKBP-type immunophilin functions in accumulation of the photosystem II supercomplex in Arabidopsis thaliana

Photosystem II (PSII) catalyzes the first of two photosynthetic reactions that convert sunlight into chemical energy. Native PSII is a supercomplex consisting of core and light-harvesting chlorophyll proteins. Although the structure of PSII has been resolved by x-ray crystallography, the mechanism underlying its assembly is poorly understood. Here, we report that an immunophilin of the chloroplast thylakoid lumen is required for accumulation of the PSII supercomplex in Arabidopsis thaliana. The immunophilin, FKBP20-2, belongs to the FK-506 binding protein (FKBP) subfamily that functions as peptidyl-prolyl isomerases (PPIases) in protein folding. FKBP20-2 has a unique pair of cysteines at the C terminus and was found to be reduced by thioredoxin (Trx) (itself reduced by NADPH by means of NADP-Trx reductase). The FKBP20-2 protein, which contains only two of the five amino acids required for catalysis, showed a low level of PPIase activity that was unaffected on reduction by Trx. Genetic disruption of the FKBP20-2 gene resulted in reduced plant growth, consistent with the observed lower rate of PSII activity determined by fluorescence (using leaves) and oxygen evolution (using isolated chloroplasts). Analysis of isolated thylakoid membranes with blue native gels and immunoblots showed that accumulation of the PSII supercomplex was compromised in mutant plants, whereas the levels of monomer and dimer building blocks were elevated compared with WT. The results provide evidence that FKBP20-2 participates specifically in the accumulation of the PSII supercomplex in the chloroplast thylakoid lumen by means of a mechanism that has yet to be determined.

Determination of μ-Oxo Exchange Rates in Di-μ-Oxo Dimanganese Complexes by Electrospray Ionization Mass Spectrometry

A time-resolved mass spectrometric technique has been used for the determination of rates of exchange of mu-O atoms with water for the complexes [(mes-terpy)2Mn2(III/IV)(mu-O)2(H2O)2](NO3)3 (1, mes-terpy = 4'-mesityl-2,2':6',2' '-terpyridine), [(bpy)4Mn2(III/IV)(mu-O)2](ClO4)3 (2, bpy = 2,2'-bipyridine), [(phen)4Mn2(III/IV)(mu-O)2](ClO4)3 (3, phen = 1,10-phenanthroline), [(bpea)2Mn2(III/IV)(mu-O)2(mu-OAc)](ClO4)2 (4, bpea = bis(2-pyridyl)ethylamine), [(bpea)2Mn2(IV/IV)(mu-O)2(mu-OAc)](ClO4)3 (4ox), [(terpy)4Mn4(IV/IV/IV/IV)(mu-O)5(H2O)2](ClO4)6 (5, terpy = 2,2':6',2''-terpyridine), and [(tacn)4Mn4(IV/IV/IV/IV)(mu-O)6]Br(3.5)(OH)0.5.6H2O (6, tacn = 1,4,7-triazacyclononane). The rate of exchange of mu-OAc bridges with free acetate in solution has been measured for complexes 4 and 4ox. These are the first measurements of rates of ligand exchange on biologically relevant high-valent Mn complexes. The data analysis method developed here is of general utility in the quantitation of isotope exchange processes by mass spectrometry. We find that the presence of labile coordination sites on Mn increases mu-O exchange rates, and that all-Mn(IV) states are more inert toward exchange than mixed Mn(III)-Mn(IV) states. The rates of mu-O exchange obtained in this work for a di-mu-oxo Mn2(III/IV) dimer with labile coordination sites are compared with the oxygen isotope incorporation rates from substrate water to evolved dioxygen measured in different S states of the oxygen evolving complex (OEC) of photosystem II (PSII). On the basis of this comparison, we propose that both substrate waters are not bound as mu-O bridges between Mn atoms in the S2 and S3 states of the OEC.

How photosynthetic reaction centers control oxidation power in chlorophyll pairs P680, P700, and P870

At the heart of photosynthetic reaction centers (RCs) are pairs of chlorophyll a (Chla), P700 in photosystem I (PSI) and P680 in photosystem II (PSII) of cyanobacteria, algae, or plants, and a pair of bacteriochlorophyll a (BChla), P870 in purple bacterial RCs (PbRCs). These pairs differ greatly in their redox potentials for one-electron oxidation, E(m). For P680, E(m) is 1,100-1,200 mV, but for P700 and P870, E(m) is only 500 mV. Calculations with the linearized Poisson-Boltzmann equation reproduce these measured E(m) differences successfully. Analyzing the origin for these differences, we found as major factors in PSII the unique Mn(4)Ca cluster (relative to PSI and PbRC), the position of P680 close to the luminal edge of transmembrane alpha-helix d (relative to PSI), local variations in the cd loop (relative to PbRC), and the intrinsically higher E(m) of Chla compared with BChla (relative to PbRC).

Absence of the PsbQ protein results in destabilization of the PsbV protein and decreased oxygen evolution activity in cyanobacterial photosystem II

We have previously reported that cyanobacterial photosystem II (PS II) contains a protein homologous to PsbQ, the extrinsic 17-kDa protein found in higher plant and green algal PS II (Kashino, Y., Lauber, W. M., Carroll, J. A., Wang, Q., Whitmarsh, J., Satoh, K., and Pakrasi, H. B. (2002) Biochemistry 41, 8004-8012) and that it has regulatory role(s) on the water oxidation machinery (Thornton, L. E., Ohkawa, H., Roose, J. L., Kashino, Y., Keren, N., and Pakrasi, H. B. (2004) Plant Cell 16, 2164-2175). In this work, the localization and the function of PsbQ were assessed using the cyanobacterium Synechocystis sp. PCC 6803. From the predicted sequence, cyanobacterial PsbQ is expected to be a lipoprotein on the luminal side of the thylakoid membrane. Indeed, experiments in this work show that upon Triton X-114 fractionation of thylakoid membranes, PsbQ partitioned in the hydrophobic phase, and trypsin digestion revealed that PsbQ was highly exposed to the luminal space of thylakoid membranes. Detailed functional assays were conducted on the psbQ deletion mutant (DeltapsbQ) to analyze its water oxidation machinery. PS II complexes purified from DeltapsbQ mutant cells had impaired oxygen evolution activity and were remarkably sensitive to NH(2)OH, which indicates destabilization of the water oxidation machinery. Additionally, the cytochrome c(550) (PsbV) protein partially dissociated from purified DeltapsbQ PS II complexes, suggesting that PsbQ contributes to the stability of PsbV in cyanobacterial PS II. Therefore, we conclude that the major function of PsbQ is to stabilize the PsbV protein, thereby contributing to the protection of the catalytic Mn(4)-Ca(1)-Cl(x) cluster of the water oxidation machinery.

Highly efficient photoactivation of Mn-depleted photosystem II by imidazole-liganded manganese complexes

The oxygen-evolving complex (OEC) of Mn-depleted photosystem II (PSII) can be reconstituted in the presence of exogenous Mn or a Mn complex under weak illumination, a process called photoactivation. Synthetic Mn complexes could provide a powerful system to analyze the assembly of the OEC. In this work, four mononuclear Mn complexes, [(terpy)2Mn(II)(OOCH3)] x 2 H2O (where terpy is 2,2':6',2''-terpyridine), Mn(II)(bzimpy)2, Mn(II)(bp)2(CH3CH2OH)2 [where bzimpy is 2,6-bis(2-benzimidazol-2-yl)pyridine] and [Mn(III)(HL)(L)(py)(CH3OH)]CH3OH (where py is pyridine) were used in photoactivation experiments. Measurements of the photoreduction of 2,6-dichorophenolindophenol and oxygen evolution demonstrate that photoactivation is more efficient when Mn complexes are used instead of MnCl2 in reconstructed PSII preparations. The most efficient recoveries of oxygen evolution and electron transport activities are obtained from a complex, [Mn(III)(HL)(L)(py)(CH3OH)]CH3OH, that contains both imidazole and phenol groups. Its recovery of the rate of oxygen evolution is as high as 79% even in the absence of the 33-kDa peptide. The imidazole ligands of the Mn complex probably accelerate P680*+ reduction and consequently facilitate the process of photoactivation. Also, the strong intermolecular hydrogen bond probably facilitates interaction with the Mn-depleted PSII via reorganization of the hydrogen-bonding network, and therefore promotes the recovery of oxygen evolution and electron transport activities.