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

Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosystem II (PSII) of oxygenic photosynthetic organisms is a unique heterometallic-oxide Mn₄CaO₅-cluster that catalyzes water splitting into electrons, protons, and molecular oxygen through a five-state cycle (S_n, n = 0 ~ 4). It serves as the blueprint for the developing of the man-made water-splitting catalysts to generate solar fuel in artificial photosynthesis. Understanding the structure-function relationship of this natural catalyst is a great challenge and a long-standing issue, which is severely restricted by the lack of a precise chemical model for this heterometallic-oxide cluster. However, it is a great challenge for chemists to precisely mimic the OEC in a laboratory. Recently, significant advances have been achieved and a series of artificial Mn₄XO₄-clusters (X = Ca/Y/Gd) have been reported, which closely mimic both the geometric structure and the electronic structure, as well as the redox property of the OEC. These new advances provide a structurally well-defined molecular platform to study the structure-function relationship of the OEC and shed new light on the design of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

The Nature of the Short Oxygen-Oxygen Distance in the Mn4CaO6 Complex of Photosystem II Crystals

The O···O distance for a typical H-bond is ∼2.8 Å, whereas the radiation-damage-free structures of photosystem II (PSII), obtained using the X-ray free electron laser (XFEL), shows remarkably short O···O distances of ∼2 Å in the oxygen-evolving Mn₄CaO_5/6 complex. Herein, we report the protonation/oxidation states of the short O···O atoms in the XFEL structures using a quantum mechanical/molecular mechanical approach. The O5···O6 distance of 1.9 Å is reproduced only when O6 is an unprotonated O radical (O^•-) with Mn(IV)₃Mn(III), i.e., the S₃ state. The potential energy profile shows a barrier-less energy minimum region when O5···O6 = 1.90-2.05 Å (O^•- ↓) or 2.05-2.20 Å (O^•- ↑). Formation of such a short O5···O6 distance is not possible when O6 is OH^- with Mn(IV)₄. In the case in which the O5···O6 distance is 1.9 Å, it seems likely that the O radical species exists in the oxygen-evolving complex of the XFEL-S₃ crystals.

Synthetic control and empirical prediction of redox potentials for Co4O4 cubanes over a 1.4 V range: implications for catalyst design and evaluation of high-valent intermediates in water oxidation

The oxo-cobalt cubane unit [Co4O4] is of interest as a homogeneous oxygen-evolution reaction (OER) catalyst, and as a functional mimic of heterogeneous cobalt oxide OER catalysts. The synthesis of several new cubanes allows evaluation of redox potentials for the [Co4O4] cluster, which are highly sensitive to the ligand environment and span a remarkable range of 1.42 V. The [CoIII4O4]4+/[CoIII3CoIVO4]5+ and [CoIII3CoIVO4]5+/[CoIII2CoIV2O4]6+ redox potentials are reliably predicted by the pKas of the ligands. Hydrogen bonding is also shown to significantly raise the redox potentials, by ∼500 mV. The potential-pKa correlation is used to evaluate the feasibility of various proposed OER catalytic intermediates, including high-valent Co-oxo species. The synthetic methods and structure-reactivity relationships developed by these studies should better guide the design of new cubane-based OER catalysts.

Experimental evidence of a 3-centre, 2-electron covalent bond character of the central O–H–O fragment on the Zundel cation in crystals of Zundel nitranilate tetrahydrate

Charge density of the Zundel cation in the solid state reveals a covalent nature of its central O–H–O fragment.

Artificial synthetic Mn(IV)Ca-oxido complexes mimic the oxygen-evolving complex in photosystem II

A novel family of heteronuclear Mn(IV)Ca-oxido complexes containing Mn(IV)Ca-oxido cuboidal moieties and reactive water molecules on Ca(2+) have been synthesized and characterized to mimic the oxygen-evolving complex (OEC) of photosystem II (PSII) in nature.

Contribution of a low-barrier hydrogen bond to catalysis is not significant in ketosteroid isomerase

Low-barrier hydrogen bonds (LBHBs) have been proposed to have important influences on the enormous reaction rate increases achieved by many enzymes. Δ(5)-3-ketosteroid isomerase (KSI) catalyzes the allylic isomerization of Δ(5)-3-ketosteroid to its conjugated Δ(4)-isomers at a rate that approaches the diffusion limit. Tyr14, a catalytic residue of KSI, has been hypothesized to form an LBHB with the oxyanion of a dienolate steroid intermediate generated during the catalysis. The unusual chemical shift of a proton at 16.8 ppm in the nuclear magnetic resonance spectrum has been attributed to an LBHB between Tyr14 Oη and C3-O of equilenin, an intermediate analogue, in the active site of D38N KSI. This shift in the spectrum was not observed in Y30F/Y55F/D38N and Y30F/Y55F/Y115F/D38N mutant KSIs when each mutant was complexed with equilenin, suggesting that Tyr14 could not form LBHB with the intermediate analogue in these mutant KSIs. The crystal structure of Y30F/Y55F/Y115F/D38N-equilenin complex revealed that the distance between Tyr14 Oη and C3-O of the bound steroid was within a direct hydrogen bond. The conversion of LBHB to an ordinary hydrogen bond in the mutant KSI reduced the binding affinity for the steroid inhibitors by a factor of 8.1-11. In addition, the absence of LBHB reduced the catalytic activity by only a factor of 1.7-2. These results suggest that the amount of stabilization energy of the reaction intermediate provided by LBHB is small compared with that provided by an ordinary hydrogen bond in KSI.

The Structure of Photosystem II and the Mechanism of Water Oxidation in Photosynthesis

Oxygenic photosynthesis forms the basis of aerobic life on earth by converting light energy into biologically useful chemical energy and by splitting water to generate molecular oxygen. The water-splitting and oxygen-evolving reaction is catalyzed by photosystem II (PSII), a huge, multisubunit membrane-protein complex located in the thylakoid membranes of organisms ranging from cyanobacteria to higher plants. The structure of PSII has been analyzed at 1.9-Å resolution by X-ray crystallography, revealing a clear picture of the Mn4CaO5 cluster, the catalytic center for water oxidation. This article provides an overview of the overall structure of PSII followed by detailed descriptions of the specific structure of the Mn4CaO5 cluster and its surrounding protein environment. Based on the geometric organization of the Mn4CaO5 cluster revealed by the crystallographic analysis, in combination with the results of a vast number of experimental studies involving spectroscopic and other techniques as well as various theoretical studies, the article also discusses possible mechanisms for water splitting that are currently under consideration.

Nitranilic acid hexahydrate, a novel benchmark system of the Zundel cation in an intrinsically asymmetric environment: spectroscopic features and hydrogen bond dynamics characterised by experimental and theoretical methods

Nitranilic acid (2,5-dihydroxy-3,6-dinitro-2,5-cyclohexadiene-1,4-dione) as a strong dibasic acid in acidic aqueous media creates the Zundel cation, H5O2(+). The structural unit in a crystal comprises (H5O2)2(+) (2,5-dihydroxy-3,6-dinitro-1,4-benzoquinonate)(2-) dihydrate where the Zundel cation reveals no symmetry, being an ideal case for studying proton dynamics and its stability. The Zundel cation and proton transfer dynamics are studied by variable-temperature X-ray diffraction, IR and solid-state NMR spectroscopy, and various quantum chemical methods, including periodic DFT calculations, ab initio molecular dynamics simulation, and quantization of nuclear motion along three fully coupled internal coordinates. The Zundel cation features a short H-bond with the O···O distance of 2.433(2) Å with an asymmetric placement of hydrogen. The proton potential is of a single well type and, due to the non-symmetric surroundings, of asymmetric shape. The formation of the Zundel cation is facilitated by the electronegative NO2 groups. The employed spectroscopic techniques supported by calculations confirm the presence of a short H-bond with a complex proton dynamics.

Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å

Photosystem II is the site of photosynthetic water oxidation and contains 20 subunits with a total molecular mass of 350 kDa. The structure of photosystem II has been reported at resolutions from 3.8 to 2.9 Å. These resolutions have provided much information on the arrangement of protein subunits and cofactors but are insufficient to reveal the detailed structure of the catalytic centre of water splitting. Here we report the crystal structure of photosystem II at a resolution of 1.9 Å. From our electron density map, we located all of the metal atoms of the Mn(4)CaO(5) cluster, together with all of their ligands. We found that five oxygen atoms served as oxo bridges linking the five metal atoms, and that four water molecules were bound to the Mn(4)CaO(5) cluster; some of them may therefore serve as substrates for dioxygen formation. We identified more than 1,300 water molecules in each photosystem II monomer. Some of them formed extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen molecules. The determination of the high-resolution structure of photosystem II will allow us to analyse and understand its functions in great detail.

Conformational changes of the S2YZ* intermediate of the S2 to S3 transition in photosystem II

The paper extends earlier studies on the S(2)Y(Z)* intermediate that is trapped by illumination in the temperature range 77 K to 190 K of untreated samples poised in the S(2)...Q(A) state. X-band EPR experiments on untreated and glycerol (50% v/v) treated samples at 10 K indicate that the intermediate consists of two components. A wide one with a splitting of ca 170 G, and a narrow one characterized by a splitting of ca 120 G (untreated), or 124 G (glycerol-treated samples). Lower temperatures of illumination in the above temperature range favor the wide component, which at 10 K decays faster than the narrow one. Re-illumination at 10 K after decay of the signal trapped at 77-190 K induces only the narrow component. Rapid scan experiments in the temperature range 77-190 K reveal high resolution spectra of the isolated tyz Z* radical and no evidence of alternative radicals. The two split signals are accordingly assigned to different conformations of the S(2)Y(Z)* intermediate A point-dipole simulation of the spectra yields "effective distances" between the spin densities of Y(Z)* and the Mn(4)Ca center of 5.7 Å for the wide and 6.4 Å for the narrow component. The results are discussed on the basis of a molecular model assuming two sequential proton transfers during oxidation of tyr Z. The wide component is assigned to a transient S(2)Y(Z)* conformation, that forms during the primary proton transfer.

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

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

The dynamics of photosynthesis

Despite recent elucidation of the three-dimensional structure of major photosynthetic complexes, our understanding of light energy conversion in plant chloroplasts and microalgae under physiological conditions requires exploring the dynamics of photosynthesis. The photosynthetic apparatus is a flexible molecular machine that can acclimate to metabolic and light fluctuations in a matter of seconds and minutes. On a longer time scale, changes in environmental cues trigger acclimation responses that elicit intracellular signaling between the nucleo-cytosol and chloroplast resulting in modification of the biogenesis of the photosynthetic machinery. Here we attempt to integrate well-established knowledge on the functional flexibility of light-harvesting and electron transfer processes, which has greatly benefited from genetic approaches, with data derived from the wealth of recent transcriptomic and proteomic studies of acclimation responses in photosynthetic eukaroytes.

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.

Water splitting by Photosystem II--where do we go from here?

As this special issue shows, we know quite a lot about the workings of Photosystem II and the oxidation of water to molecular O(2). However, there are still many questions and details that remain to be answered. In this article, I very briefly outline some aspects of Photosystem II electron transport that are crucial for the efficient oxidation of water and require further studies. To fully understand Photosystem II reactions is not only a satisfying intellectual pursuit, but is also an important goal as we develop new solar technologies for the splitting of water into pure O(2) and H(2) for use as a potential fuel source. "As Students of the Past, We Send Greetings to the Students of the Future".

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.

Evidence for intermediate S-states as initial phase in the process of oxygen-evolving complex oxidation

We have analyzed flash-induced period-four damped oscillation of oxygen evolution and chlorophyll fluorescence with the aid of a kinetic model of photosystem II. We have shown that, for simulation of the period-four oscillatory behavior of oxygen evolution, it is essential to consider the so-called intermediate S-state as an initial phase of each of the S(n)-S(n+1), (n = 0, 1, 2, 3) transitions. The intermediate S-states are defined as [S(n)Y(Z)(ox)]-states (n = 0, 1, 2, 3) and are formed with rate constant k(iSn) approximately 1.5 x 10(6) s(-1), which was determined from comparison of theoretical predictions with experimental data. The assumed intermediate S-states shift the equilibrium in reaction P680(+)Y(Z)<-->P680Y(Z)(ox) more to the right and we suggest that kinetics of the intermediate S-states reflects a relaxation process associated with changes of the redox equilibrium in the above reaction. The oxygen oscillation is simulated without the miss and double-hit parameters, if the intermediate S-states, which are not the source of the misses or the double-hits, are included in the simulation. Furthermore, we have shown that the intermediate S-states, together with S(2)Q(A)(-) charge recombination, are prerequisites for the simulation of the period-four oscillatory behavior of the chlorophyll fluorescence.