Evidence from directed mutagenesis that aspartate 170 of the D1 polypeptide influences the assembly and/or stability of the manganese cluster in the photosynthetic water-splitting complex

Mutation-induced shift of the photosystem II active site reveals insight into conserved water channels

Photosystem II (PSII) is the water-plastoquinone photo-oxidoreductase central to oxygenic photosynthesis. PSII has been extensively studied for its ability to catalyze light-driven water oxidation at a Mn4CaO5 cluster called the oxygen-evolving complex (OEC). Despite these efforts, the complete reaction mechanism for water oxidation by PSII is still heavily debated. Previous mutagenesis studies have investigated the roles of conserved amino acids, but these studies have lacked a direct structural basis that would allow for a more meaningful interpretation. Here, we report a 2.14-Å resolution cryo-EM structure of a PSII complex containing the substitution Asp170Glu on the D1 subunit. This mutation directly perturbs a bridging carboxylate ligand of the OEC, which alters the spectroscopic properties of the OEC without fully abolishing water oxidation. The structure reveals that the mutation shifts the position of the OEC within the active site without markedly distorting the Mn4CaO5 cluster metal-metal geometry, instead shifting the OEC as a rigid body. This shift disturbs the hydrogen-bonding network of structured waters near the OEC, causing disorder in the conserved water channels. This mutation-induced disorder appears consistent with previous FTIR spectroscopic data. We further show using quantum mechanics/molecular mechanics methods that the mutation-induced structural changes can affect the magnetic properties of the OEC by altering the axes of the Jahn-Teller distortion of the Mn(III) ion coordinated to D1-170. These results offer new perspectives on the conserved water channels, the rigid body property of the OEC, and the role of D1-Asp170 in the enzymatic water oxidation mechanism.

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

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

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

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

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

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

The role of Ca2+ and protein scaffolding in the formation of nature's water oxidizing complex

Photosynthetic O₂ evolution is catalyzed by the Mn₄CaO₅ cluster of the water oxidation complex of the photosystem II (PSII) complex. The photooxidative self-assembly of the Mn₄CaO₅ cluster, termed photoactivation, utilizes the same highly oxidizing species that drive the water oxidation in order to drive the incorporation of Mn²⁺ into the high-valence Mn₄CaO₅ cluster. This multistep process proceeds with low quantum efficiency, involves a molecular rearrangement between light-activated steps, and is prone to photoinactivation and misassembly. A sensitive polarographic technique was used to track the assembly process under flash illumination as a function of the constituent Mn²⁺ and Ca²⁺ ions in genetically engineered membranes of the cyanobacterium Synechocystis sp. PCC6803 to elucidate the action of Ca²⁺ and peripheral proteins. We show that the protein scaffolding organizing this process is allosterically modulated by the assembly protein Psb27, which together with Ca²⁺ stabilizes the intermediates of photoactivation, a feature especially evident at long intervals between photoactivating flashes. The results indicate three critical metal-binding sites: two Mn and one Ca, with occupation of the Ca site by Ca²⁺ critical for the suppression of photoinactivation. The long-observed competition between Mn²⁺ and Ca²⁺ occurs at the second Mn site, and its occupation by competing Ca²⁺ slows the rearrangement. The relatively low overall quantum efficiency of photoactivation is explained by the requirement of correct occupancy of these metal-binding sites coupled to a slow restructuring of the protein ligation environment, which are jointly necessary for the photooxidative trapping of the first stable assembly intermediate.

Photosystem II oxygen-evolving complex photoassembly displays an inverse H/D solvent isotope effect under chloride-limiting conditions

Metal clusters play important roles in a wide variety of proteins. In cyanobacteria, algae, and plants, photosystem II uses light energy to oxidize water and release O₂ at an active site that contains 1 calcium and 4 manganese atoms. This cluster must be built within the protein environment through a process known as photoassembly. Through experiments and simulations, we found that the efficiency of photoassembly was highly dependent on protons and chloride. Surprisingly, when the solvent was switched from H₂O to deuterated water, D₂O, the yield of photoassembly was higher. These results provide insights into the stepwise mechanism of photoassembly that can inform synthesis and repair strategies being developed for artificial photosynthesis technologies.

Photosystem II (PSII) performs the solar-driven oxidation of water used to fuel oxygenic photosynthesis. The active site of water oxidation is the oxygen-evolving complex (OEC), a Mn₄CaO₅ cluster. PSII requires degradation of key subunits and reassembly of the OEC as frequently as every 20 to 40 min. The metals for the OEC are assembled within the PSII protein environment via a series of binding events and photochemically induced oxidation events, but the full mechanism is unknown. A role of proton release in this mechanism is suggested here by the observation that the yield of in vitro OEC photoassembly is higher in deuterated water, D₂O, compared with H₂O when chloride is limiting. In kinetic studies, OEC photoassembly shows a significant lag phase in H₂O at limiting chloride concentrations with an apparent H/D solvent isotope effect of 0.14 ± 0.05. The growth phase of OEC photoassembly shows an H/D solvent isotope effect of 1.5 ± 0.2. We analyzed the protonation states of the OEC protein environment using classical Multiconformer Continuum Electrostatics. Combining experiments and simulations leads to a model in which protons are lost from amino acid that will serve as OEC ligands as metals are bound. Chloride and D₂O increase the proton affinities of key amino acid residues. These residues tune the binding affinity of Mn^2+/3+ and facilitate the deprotonation of water to form a proposed μ-hydroxo bridged Mn²⁺Mn³⁺ intermediate.

Electrogenic reactions in Mn-depleted photosystem II core particles in the presence of synthetic binuclear Mn complexes

An electrometrical technique was used to investigate electron transfer between synthetic binuclear manganese (Mn) complexes, designated M - 2 and M - 3, and the redox-active neutral tyrosine radical (Y_Z^•) in proteoliposomes containing Mn-depleted photosystem II (PS II) core particles in response to single laser flashes. In the absence of Mn-containing compounds, the observed flash-induced membrane potential (ΔΨ) decay was mainly due to charge recombination between the reduced primary quinone acceptor Q_A^- and the oxidized Y_Z^•. More significant slowing down of the ΔΨ decay in the presence of lower concentrations of M - 2 and M - 3 associated with electron donation from Mn in the Mn-binding site to Y_Z^• indicates that these synthetic compounds are more effective electron donors than MnCl₂. The exponential fitting of the kinetics of additional electrogenic components of ΔΨ rise in the presence of Mn-containing compounds revealed the following relative amplitudes (A) and lifetimes (τ): for MnCl₂ - A∼ 3.5, τ∼150 μs, for M - 2 - A∼5%, τ∼1.4 ms, and for M - 3 - A∼5.5%, τ∼150 μs. This suggests that the efficiency of the manganese complexes in electron donation depends on the chemical nature of ligands. The experiments with EDTA-treated samples indicated that the ligands for M - 2 and M - 3 are required for their tight binding with the PS II reaction center. The obtained results demonstrate the importance of understanding the molecular mechanism(s) of flash-induced electrogenic reduction of the tyrosine radical Y_Z^• by synthetic Mn complexes capable of splitting water into oxygen and reducing equivalents.

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

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

Parallel expression of alternate forms of psbA2 gene provides evidence for the existence of a targeted D1 repair mechanism in Synechocystis sp. PCC 6803

The D1 protein of Photosystem II (PSII) is recognized as the main target of photoinhibitory damage and exhibits a high turnover rate due to its degradation and replacement during the PSII repair cycle. Damaged D1 is replaced by newly synthesized D1 and, although reasonable, there is no direct evidence for selective replacement of damaged D1. Instead, it remains possible that increased turnover of D1 subunits occurs in a non-selective manner due for example, to a general up-regulation of proteolytic activity triggered during damaging environmental conditions, such as high light. To determine if D1 degradation is targeted to damaged D1 or generalized to all D1, we developed a genetic system involving simultaneous dual expression of wild type and mutant versions of D1 protein. Dual D1 strains (nS345P:eWT and nD170A:eWT) expressed a wild type (WT) D1 from ectopic and a damage prone mutant (D1-S345P, D1-D170A) from native locus on the chromosome. Characterization of strains showed that all dual D1 strains restore WT like phenotype with high PSII activity. Higher PSII activity indicates increased population of PSII reaction centers with WT D1. Analysis of steady state levels of D1 in nS345P:eWT by immunoblot showed an accumulation of WT D1 only. But, in vivo pulse labeling confirmed the synthesis of both S345P (exists as iD1) and WT D1 in the dual strain. Expression of nS345P:eWT in FtsH2 knockout background showed accumulation of both iD1 and D1 proteins. This demonstrates that dual D1 strains express both forms of D1, yet only damage prone PSII complexes are selected for repair providing evidence that the D1 degradation process is targeted towards damaged PSII complexes. Since the N-terminus has been previously shown to be important for the degradation of damaged D1, the possibility that the highly conserved cysteine 18 residue situated in the N-terminal domain of D1 is involved in the targeted repair process was tested by examining site directed mutants of this and the other cysteines of the D1 protein. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Vectorial charge transfer reactions on the donor side of manganese-depleted and reconstituted photosystem 2 core complexes

The light-induced functioning of photosystem 2 (PS 2) is directly linked to the translocation of both electrons and protons across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). Generation of ΔΨ due to S-state transitions of the water oxidation complex was demonstrated for the first time in Mn-depleted and reconstituted PS 2 core complexes incorporated into liposomes. The kinetics and relative amplitudes of the electrogenic reactions in dark-adapted samples during S1→S2, S2→S3, and S4→S0 transitions in response to the first, second and third laser flashes were comparable to those obtained in the intact PS 2 core particles. These results expand current understanding of the nature and mechanisms of electrogenic (vectorial) reactions due to a charge transfer on the donor side of PS 2.

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

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

A synthetic DNA and fusion PCR approach to the ectopic expression of high levels of the D1 protein of photosystem II in Synechocystis sp. PCC 6803

A hybrid approach involving synthetic DNA, fusion PCR, and ectopic expression has been used to genetically manipulate the expression of the D1 protein of photosystem II (PSII) in the model cyanobacterium Synechocystis sp. PCC6803. Due to the toxicity of the full-length psbA gene in E. coli, a chimeric psbA2 gene locus was commercially synthesised and cloned in two halves. High-fidelity fusion PCR utilizing sequence overlap between the two synthetic gene halves allowed the production of a DNA fragment that was able to recombine the full-length psbA2 gene into the Synechocystis chromosome at an ectopic (non-native) location. This was accomplished by designing the synthetic DNA/fusion PCR product to have the psbA2 gene, with control sequences, interposed between chimeric sequences corresponding to an ectopic target chromosomal location. Additionally, a recipient strain of Synechocystis lacking all three psbA genes was produced by a combination of traditional marker replacement and markerless replacement techniques. Transformation of this multiple deletion strain by the synthetic DNA/fusion PCR product faithfully restored D1 expression in terms of its expression and PSII repair capacity. The advantages and potential issues for using this approach to rapidly introduce chimeric sequence characteristics as a general tool to produce novel genetic constructs are discussed.

Assembly and disassembly of the photosystem II manganese cluster reversibly alters the coupling of the reaction center with the light-harvesting phycobilisome

The light-driven oxidative assembly of Mn (2+) ions into the H 2O oxidation complex (WOC) of the photosystem II (PSII) reaction center is termed photoactivation. The fluorescence yield characteristics of Synechocystis sp. PCC6803 cells undergoing photoactivation showed that basal fluorescence, F 0, exhibited a characteristic decline when red, but not blue, measuring light was employed. This result was traced to a progressive increase in the coupling of the phycobilisome (PBS) to the PSII reaction center as determined by observing the changes in room temperature and 77 K fluorescence emission spectra that accompany photoactivation. The results support the hypothesis that strong energetic coupling of the PBS to the PSII reaction center depends upon the formation of an active WOC, which presumably diminishes the likelihood of photodamage to reaction centers that have either lost an intact Mn cluster or are in the process of assembling an active WOC.

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.

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

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

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.

Engineered ectopic expression of the psbA gene encoding the photosystem II D1 protein in Synechocystis sp. PCC6803

A genetic vector-recipient system was developed to engineer expression of the wild-type psbA2 gene encoding the photosystem II (PSII) D1 protein only from a non-native location (ectopic) in the Synechocystis sp. PCC6803 and the result was a new strain, designated MK1. While MK1 accumulates near normal levels of PSII under low light conditions, it is very sensitive to photoinhibition. This phenotype can be traced to impaired PSII repair capacity. Based upon the hypothesis that the non-native transcriptional activity of the re-introduced psbA gene was insufficient to sustain the translation rate necessary for normal PSII repair rates, we conducted a quantitative analysis of the relationship between psbA transcript abundance on the rate of recovery from photoinhibition. Analysis of MK1 and two other engineered strains, with intermediate levels of psbA mRNA, indicated that transcript levels are indeed limiting the engineered strains. Furthermore, transcript levels may become limiting even in the wild-type, but only under very high light conditions when the demands for D1 replacement synthesis are maximal. The work extends the original studies of Komenda and colleagues (Komenda et al. (2000) Plant Mol Biol 42(4):635-645) and sets the stage for alternative approaches to engineering non-native expression of the D1 protein.

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.

Calcium exchange and structural changes during the photosynthetic oxygen evolving cycle

PSII catalyzes the oxidation of water and reduction of plastoquinone in oxygenic photosynthesis. PSII contains an oxygen-evolving complex, which is located on the lumenal side of the PSII reaction center and which contains manganese, calcium, and chloride. Four sequential photooxidation reactions are required to generate oxygen. This process produces five Sn-states, where n refers to the number of oxidizing equivalents stored. Calcium is required for oxygen production. Strontium is the only divalent cation that replaces calcium and maintains activity. In our previous FT-IR work, we assessed the effect of strontium substitution on substrate-limited PSII preparations, which were inhibited at the S3 to S0 transition. In this work, we report reaction-induced FT-IR studies of hydrated PSII preparations, which undergo the full S-state cycle. The observed difference FT-IR spectra reflect long-lived photoinduced conformational changes in the oxygen-evolving complex; strontium exchange identifies vibrational bands sensitive to substitutions at the calcium site. During the S1' to S2' transition, the data are consistent with an electrostatic or structural perturbation of the calcium site. During the S3' to S0' and S0' to S1' transitions, the data are consistent with a perturbation of a hydrogen bonding network, which contains calcium, water, and peptide carbonyl groups. To explain our data, persistent shifts in divalent cation coordination must occur when strontium is substituted for calcium. A modified S-state model is proposed to explain these results and results in the literature.

Time-resolved vibrational spectroscopy detects protein-based intermediates in the photosynthetic oxygen-evolving cycle

Photosynthetic oxygen production by photosystem II (PSII) is responsible for the maintenance of aerobic life on earth. The production of oxygen occurs at the PSII oxygen-evolving complex (OEC), which contains a tetranuclear manganese (Mn) cluster. Photo-induced electron transfer events in the reaction center lead to the accumulation of oxidizing equivalents on the OEC. Four sequential photooxidation reactions are required for oxygen production. The oxidizing complex cycles among five oxidation states, called the S(n) states, where n refers to the number of oxidizing equivalents stored. Oxygen release occurs during the S(3)-to-S(0) transition from an unstable intermediate, known as the S(4) state. In this report, we present data providing evidence for the production of an intermediate during each S state transition. These protein-derived intermediates are produced on the microsecond to millisecond time scale and are detected by time-resolved vibrational spectroscopy on the microsecond time scale. Our results suggest that a protein-derived conformational change or proton transfer reaction precedes Mn redox reactions during the S(2)-to-S(3) and S(3)-to-S(0) transitions.

A carboxylic residue at the high-affinity, Mn-binding site participates in the binding of iron cations that block the site

The role of carboxylic residues at the high-affinity, Mn-binding site in the ligation of iron cations blocking the site [Biochemistry 41 (2000) 5854] was studied, using a method developed to extract the iron cations blocking the site. We found that specifically bound Fe(III) cations can be extracted with citrate buffer at pH 3.0. Furthermore, citrate can also prevent the photooxidation of Fe(II) cations by YZ. Participation of a COOH group(s) in the ligation of Fe(III) at the high-affinity site was investigated using 1-ethyl-3-[(3-dimethylamino)propyl] carbodiimide (EDC), a chemical modifier of carboxylic amino acid residues. Modification of the COOH groups inhibits the light-induced oxidation of exogenous Mn(II) cations by Mn-depleted photosystem II (PSII[-Mn]) membranes. The rate of Mn(II) oxidation saturates at > or = 10 microM in PSII(-Mn) membranes and > or = 500 microM in EDC-treated PSII (-Mn) samples. Intact PSII(-Mn) membranes have only one site for Mn(II) oxidation via YZ (dissociation constant, Kd = 0.64 microM), while EDC-treated PSII(-Mn) samples have two sites (Kd = 1.52 and 22 microM; the latter is the low-affinity site). When PSII(-Mn) membranes were incubated with Fe(II) before modifier treatment (to block the high-affinity site) and the blocking iron cations were extracted with citrate (pH 3.0) after modification, the membranes contained only one site (Kd = 2.3 microM) for exogenous Mn(II) oxidation by Y(Z)() radical. In this case, the rate of electron donation via YZ saturated at a Mn(II) concentration > or = 15 microM. These results indicate that the carboxylic residue participating in Mn(II) coordination and the binding of oxidized manganese cations at the HAZ site is protected from the action of the modifier by the iron cations blocking the HAZ site. We concluded that the carboxylic residue (D1 Asp-170) participating in the coordination of the manganese cation at the HAZ site (Mn4 in the tetranuclear manganese cluster [Science 303 (2004) 1831]) is also involved in the ligation of the Fe cation(s) blocking the high-affinity Mn-binding site.

The role of D1-Ala344 in charge stabilization and recombination in Photosystem II

The Ala344 residue of the D1 protein has been identified as a crucial residue of the catalytic cluster of the water-oxidizing complex, however, its function has not been fully clarified. Here we have used thermoluminescence and flash-induced chlorophyll fluorescence measurements to characterize the effect of the D1-Ala344stop mutation on the electron transport of Photosystem II in intact cells of the cyanobacterium Synechocystis 6803. Although the mutant cannot grow photoautotrophically it shows flash-induced thermoluminescence and chlorophyll fluorescence signals reflecting the stabilization of negative and positive charges on the Q(A) and Q(B) quinone electron acceptors, and stable Photosystem II donors, respectively. Decay of flash induced chlorophyll fluorescence yield is multiphasic in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), with 6 ms, 350 ms, and 26 s time constants. When cells are illuminated with repetitive flashes, fired at 1 ms intervals, the 6 ms phase is gradually decreased with the concomitant increase of the 350 ms phase. After 45 min dark adaptation of mutant cells the 6 ms and 350 ms phases were significantly decreased and a very slow decaying component was formed. Flash induced oscillation of the thermoluminescence B band, which reflects the redox cycling of the water-oxidizing complex in the wild-type cells, was completely abolished in the D1-Ala344stop mutant. The results demonstrate that low efficiency photooxidation of Mn occurs in about 60% of the PSII centers. The photooxidizable Mn is unstable in the dark, and formation of higher S states is inhibited. In addition, the Q(A) to Q(B) electron transfer step is slowed down as an indirect consequence of the donor side modification. Our data indicate that the stabilization of a Mn ion by the alpha-carboxylate chain of the D1-Ala344 residue might represent one of the final steps in the assembly of functional catalytic sites for water oxidation.

The function of D1-H332 in Photosystem II electron transport studied by thermoluminescence and chlorophyll fluorescence in site-directed mutants of Synechocystis 6803

The His332 residue of the D1 protein has been identified as the likely ligand of the catalytic Mn ions in the water oxidizing complex (Ferreira, K.N., Iverson, T.M., Maghlaoui, K., Barber, J. & Iwata, S. (2004) Science 303, 1831-1838). However, its function has not been fully clarified. Here we used thermoluminescence and flash-induced chlorophyll fluorescence measurements to characterize the effect of the D1-H333E, D1-H332D and D1-H332S mutations on the electron transport of Photosystem II in intact cells of the cyanobacterium Synechocystis 6803. Although the mutants are not photoautotrophic they all show flash-induced thermoluminescence and chlorophyll fluorescence, which originate from the S(2)Q(A) (-) and S(2)Q(B) (-) recombinations demonstrating that charge stabilization takes place in the water oxidizing complex. However, the conversion of S(2) to higher S states is inhibited and the energetic stability of the S(2)Q(A) (-) charge pair is increased by 75, 50 and 7 mV in the D1-H332D, D1-H332E and D1-H332S mutants, respectively. This is most probably caused by a decrease of E(m)(S(2)/S(1)). Concomitantly, the rate of electron donation from Mn to Tyr-Z(b) during the S(1) to S(2) transition is slowed down, relative to the wild type, 350- and 60-fold in the D1-H332E and D1-H332D mutants, respectively, but remains essentially unaffected in D1-H332S. A further effect of the D1-H332E and D1-H332D mutations is the retardation of the Q(A) to Q(B) electron transfer step as an indirect consequence of the donor side modification. Our data show that although the His residue in the D1-332 position can be substituted by other metal binding residues for binding photo-oxidisable Mn it is required for controlling the functional redox energetics of the Mn cluster.

Evidence that D1 processing is required for manganese binding and extrinsic protein assembly into photosystem II

Photosystem II (PSII) is a large membrane protein complex that catalyzes oxidation of water to molecular oxygen. During its normal function, PSII is damaged and frequently turned over. The maturation of the D1 protein, a key component in PSII, is a critical step in PSII biogenesis. The precursor form of D1 (pD1) contains a C-terminal extension, which is removed by the protease CtpA to yield PSII complexes with oxygen evolution activity. To determine the temporal position of D1 processing in the PSII assembly pathway, PSII complexes containing only pD1 were isolated from a CtpA-deficient strain of the cyanobacterium Synechocystis 6803. Although membranes from the mutant cell had nearly 50% manganese, no manganese was detected in isolated DeltactpAHT3 PSII, indicating a severely decreased manganese affinity. However, chlorophyll fluorescence decay kinetics after a single saturating flash suggested that the donor Y(Z) was accessible to exogenous Mn(2+) ions. Furthermore, the extrinsic proteins PsbO, PsbU, and PsbV were not present in PSII isolated from this mutant. However, PsbO and PsbV were present in mutant membranes, but the amount of PsbV protein was consistently less in the mutant membranes compared with the control membranes. We conclude that D1 processing precedes manganese binding and assembly of the extrinsic proteins into PSII. Interestingly, the Psb27 protein was found to be more abundant in DeltactpAHT3 PSII than in HT3 PSII, suggesting a possible role of Psb27 as an assembly factor during PSII biogenesis.

Evidence for spontaneous structural changes in a dark-adapted state of photosystem II

Photosystem II catalyzes photosynthetic water oxidation in plants, green algae, and cyanobacteria. The manganese-containing active site cycles through a series of five oxidation states, S(n), where n refers to the number of oxidizing equivalents stored. In this report, reaction-induced Fourier transform infrared and electron paramagnetic resonance spectra of the S(1)-to-S(2) transition are presented. These data suggest that changes in carboxylate ligation to manganese, changes in secondary structure, and/or changes in polarity occur during dark adaptation in the S(1) state. These spontaneous structural changes are attributed to a S(1)' intermediate, at the same oxidation level as S(1), in the process of photosynthetic water oxidation.

Sucrose and glycerol effects on photosystem II

Photosystem II catalyzes the oxidation of water and the reduction of plastoquinone. The active site cycles among five oxidation states, which are called the S(n) states. PSII purification procedures include the use of the cosolvents, sucrose and/or glycerol, to stabilize water splitting activity and for cryoprotection. In this study, the effects of sucrose and glycerol on PSII were investigated. Sucrose addition was observed to stimulate the steady-state rate of oxygen evolution in the range from 0 to 1.35 M. Glycerol addition was observed to stimulate oxygen evolution in the range from 0 to 30%. Both cosolvents were observed to be inhibitory at higher concentrations. Sucrose addition was shown to have no effect on the rate of Q(A)(-) oxidation or on the K(M) for exogenous acceptor. PSII was then treated to remove extrinsic proteins. In these samples, sucrose addition stimulated activity, but glycerol addition was inhibitory at concentrations higher than approximately 0.5 M. This inhibitory effect of glycerol at relatively low concentrations is attributed to glycerol binding to the active site, when extrinsic subunits are not present. Reaction induced FTIR spectra, associated with the S(1) to S(2) transition of the water-oxidizing complex, exhibited significant differences throughout the 1,800-1,200 cm(-1) region, when glycerol- and sucrose-containing samples were compared. These measurements suggest a cosolvent-induced shift in the pK(A) of an aspartic or glutamic acid side chain, as well as structural changes at the active site. These structural alterations are attributed to a change in preferential hydration of the oxygen-evolving complex.

Two functionally distinct manganese clusters formed by introducing a mutation in the carboxyl terminus of a photosystem II reaction center polypeptide, D1, of the green alga Chlamydomonas reinhardtii

To study the function of the carboxyl-terminal domain of a photosystem II (PSII) reaction center polypeptide, D1, chloroplast mutants of the green alga Chlamydomonas reinhardtii have been generated in which Leu-343 and Ala-344 have been simultaneously or individually replaced by Phe and Ser, respectively. The mutants carrying these replacements individually, L343F and A344S, showed a wild-type phenotype. In contrast, the double mutant, L343FA344S, evolved O2 at only 20-30% of the wild-type rate and was unable to grow photosynthetically. In this mutant, PSII accumulated to 60% of the wild-type level, indicating that the O2-evolving activity per PSII was reduced to approximately half that of the wild-type. However, the amount of Mn atom detected in the thylakoids suggested that a normal amount of Mn cluster was assembled. An investigation of the kinetics of flash-induced fluorescence yield decay revealed that the electron transfer from Q(-)(A) to Q(B) was not affected. When a back electron transfer from Q(-)(A) to a donor component was measured in the presence of 3-(3,4-dichlorophenol)-1,1-dimethylurea, a significantly slower component of the Q(-)(A) oxidation was detected in addition to the normal component that corresponds to the back electron transfer from the Q(-)(A) to the S(2)-state of the Mn cluster. Thermoluminescence measurements revealed that L343FA344S cells contained two functionally distinct Mn clusters. One was equivalent to that of the wild-type, while the other was incapable of water oxidation and was able to advance the transition from the S(1)-state to the S(2)-state. These results suggested that a fraction of the Mn cluster had been impaired by the L343FA344S mutation, leading to decreased O2 evolution. We concluded that the structure of the C-terminus of D1 is critical for the formation of the Mn cluster that is capable of water oxidation, in particular, transition to higher S-states.

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

The combination of site-directed mutagenesis, isotopic labeling, new magnetic resonance techniques and optical spectroscopic methods have provided new insights into cofactor coordination and into the mechanism of electron transport and proton-coupled electron transport in photosystem II. Site-directed mutations in the D1 polypeptide of this photosystem have implicated a number of histidine and carboxylate residues in the coordination and assembly of the manganese cluster, responsible for photosynthetic water oxidation. Many of these are located in the carboxy-terminal region of this polypeptide close to the processing site involved in its maturation. This maturation is a required precondition for cluster assembly. Recent proposals for the mechanism of water oxidation have directly implicated redox-active tyrosine Y(Z) in this mechanism and have emphasized the importance of the coupling of proton and electron transfer in the reduction of Y(Z)(radical) by the Mn cluster. The interaction of both homologous redox-active tyrosines Y(Z) and Y(D) with their respective homologous proton acceptors is discussed in an effort to better understand the significance of such coupling.

Amino acid residues that modulate the properties of tyrosine Y(Z) and the manganese cluster in the water oxidizing complex of photosystem II

The catalytic site for photosynthetic water oxidation is embedded in a protein matrix consisting of nearly 30 different polypeptides. Residues from several of these polypeptides modulate the properties of the tetrameric Mn cluster and the redox-active tyrosine residue, Y(Z), that are located at the catalytic site. However, most or all of the residues that interact directly with Y(Z) and the Mn cluster appear to be contributed by the D1 polypeptide. This review summarizes our knowledge of the environments of Y(Z) and the Mn cluster as obtained from the introduction of site-directed, deletion, and other mutations into the photosystem II polypeptides of the cyanobacterium Synechocystis sp. PCC 6803 and the green alga Chlamydomonas reinhardtii.

Metallo-radical hypothesis for photoassembly of (Mn)4-cluster of photosynthetic oxygen evolving complex

A new hypothetical mechanism is proposed for photoassembly of the (Mn)4-cluster of the photosynthetic oxygen evolving complex (OEC). In this process, a neutral radical of Y(Z) tyrosine plays a role in oxidizing Mn2+ associated with an apo-OEC, and also in abstracting a proton from a water molecule bound to the Mn2+ ion, together with D1-His190. This is in a similar fashion to the metallo-radical mechanism proposed for photosynthetic water oxidation by the (Mn)4-cluster. The model insists that a common mechanism participates in the photoassembly of the (Mn)4-cluster and the photosynthetic water oxidation.

Kinetic characterization of His-tagged CP47 photosystem II in Synechocystis sp. PCC6803

Recently, construction of strains of Synechocystis sp. PCC6803 having a His(6) extension (His-tag) of the carboxyl terminus of the CP47 protein has been reported (T.M. Bricker et al, Biochim. Biophys. Acta 1409 (1998) 50; M.J. Reifler et al., in: Garab, Pusztai (Eds.) Proc. XIth International Congress on Photosynthesis, 1998). While these initial reports suggest a minimal impact of the His-tag upon Photosystem (PS) II function, a more thorough analysis of the kinetic properties of the modified complex is essential. This communication reports on a more detailed kinetic analysis to assess possible perturbations of PS II due to the genetic addition of the His-tag on the CP47 protein. It was found that: (1) Patterns of flash O(2) yield exhibited normal period four oscillations and the associated fits of the Kok-Joliot S-state cycling parameters were virtually identical to the wild type; (2) O(2) release kinetics during the S(3)-S(0) transition were experimentally indistinguishable from the wild type; (3) S-state decay measurements indicate slightly faster decays of the S(2) and S(3) states compared to the wild type; (4) fluorescence measurements indicate that the kinetics of the forward reaction of electron transfer from Q(A)(-) to Q(B) and back-reactions of Q(A)(-) with PS II electron donors are similar in the His-tag and wild-type strains. It is therefore concluded that the addition of the His-tag results in a minimal perturbation of PS II function.

Is the manganese stabilizing 33 kDa protein of photosystem II attaining a 'natively unfolded' or 'molten globule' structure in solution?

This study compares the properties of the extrinsic 33 kDa subunit acting as 'manganese stabilizing protein' (MSP) of the water oxidizing complex with characteristic features of proteins that are known to attain a 'natively unfolded' or a 'molten globule' structure. The analysis leads to the conclusion that the MSP in solution is most likely a 'molten globule' with well defined compact regions of beta structure. The possible role of these structural peculiarities of MSP in solution for its function as important constituent of the WOC is discussed.

The PsbY protein is not essential for oxygenic photosynthesis in the cyanobacterium Synechocystis sp. PCC 6803

A tetra-manganese cluster in the photosystem II (PSII) pigment-protein complex plays a critical role in the photosynthetic oxygen evolution process. PsbY, a small membrane-spanning polypeptide, has recently been suggested to provide a ligand for manganese in PSII (A.E. Gau, H.H. Thole, A. Sokolenko, L. Altschmied, R.G. Herrmann, E.K. Pistorius [1998] Mol Gen Genet 260: 56-68). We have constructed a mutant strain of the cyanobacterium Synechocystis sp. PCC 6803 with an inactivated psbY gene (sml0007). Southern-blot and polymerase chain reaction analysis showed that the mutant had completely segregated. However, the DeltapsbY mutant cells grew normally under photoautotrophic conditions. Moreover, growth of the wild-type and mutant cells were similar under high-light photoinhibition conditions, as well as in media without any added manganese, calcium, or chloride, three required inorganic cofactors for the oxygen-evolving complex of PSII. Analysis of steady-state and flash-induced oxygen evolution, fluorescence induction, and decay kinetics, and thermoluminescence profiles demonstrated that the DeltapsbY mutant cells have normal photosynthetic activities. We conclude that the PsbY protein in Synechocystis 6803 is not essential for oxygenic photosynthesis and does not provide an important binding site for manganese in the oxygen-evolving complex of PSII.

A Difference Fourier transform infrared study of tyrosyl radical Z* decay in photosystem II

Photosystem II (PSII) contains a redox-active tyrosine, Z* Difference Fourier transform infrared (FTIR) spectroscopy can be used to obtain structural information about this species, which is a neutral radical, Z*, in the photooxidized form. Previously, we have used isotopic labeling, inhibitors, and site-directed mutagenesis to assign a vibrational line at 1478 cm(-1) to Z*; these studies were performed on highly resolved PSII preparations at pH 7.5, under conditions where Q(A)(-) and Q(B)(-) make no detectable contribution to the vibrational spectrum (Kim, Ayala, Steenhuis, Gonzalez, Razeghifard, and Barry. 1998. Biochim. Biophys. Acta. 1366:330-354). Here, time-resolved infrared data associated with the reduction of tyrosyl radical Z* were acquired from spinach core PSII preparations at pH 6.0. Electron paramagnetic resonance spectroscopy and fluorescence control experiments were employed to measure the rate of Q(A)(-) and Z* decay. Q(B)(-) did not recombine with Z* under these conditions. Difference FTIR spectra, acquired over this time regime, exhibited time-dependent decreases in the amplitude of a 1478 cm(-1) line. Quantitative comparison of the rates of Q(A)(-) and Z* decay with the decay of the 1478 cm(-1) line supported the assignment of a 1478 cm(-1) component to Z*. Comparison with difference FTIR spectra obtained from PSII samples, in which tyrosine is labeled, supported this conclusion and identified other spectral components assignable to Z* and Z. To our knowledge, this is the first kinetic study to use quantitative comparison of kinetic constants in order to assign spectral features to Z*.

Alterations in carboxylate ligation at the active site of photosystem II

Photosystem II (PSII) is the photosynthetic enzyme catalyzing the oxidation of water and reduction of plastoquinone (Q). This reaction occurs at a catalytic site containing four manganese atoms and cycling among five oxidation states, the Sn states, where n refers to the number of oxidizing equivalents stored. Biochemical and spectroscopic techniques have been used previously to conclude that aspartate 170 in the D1 subunit influences the structure and function of the PSII active site (Boerner, R. J., Nguyen, A. P., Barry, B. A., and Debus, R. J. (1992) Biochemistry 31, 6660-6672). Substitution of glutamate for aspartate 170 resulted in an assembled manganese cluster, which was capable of enzymatic turnover, but at lower steady-state oxygen evolution rates. Here, we obtained the difference (light-minus-dark) Fourier transform IR spectrum associated with the S2Q--minus-S1Q transition by illumination of oxygen-evolving wild-type and DE170D1 PSII preparations at 200 K. These spectra are known to be dominated by contributions from carboxylic acid and carboxylate residues that are close to or ligating the manganese cluster. Substitution of glutamate for aspartate 170 results in alterations in the S2Q--minus-S1Q spectrum; the alterations are consistent with a change in carboxylate coordination to manganese or calcium. In particular, the spectra are consistent with a shift from bridging/bidentate carboxylates in wild-type PSII to unidentate carboxylate ligation in DE170D1 PSII.

Infrared spectroscopic identification of the C-O stretching vibration associated with the tyrosyl Z. and D. radicals In photosystem II

Photosystem II (PSII) is a multisubunit complex, which catalyzes the photo-induced oxidation of water and reduction of plastoquinone. Difference Fourier-transform infrared (FT-IR) spectroscopy can be used to obtain information about the structural changes accompanying oxidation of the redox-active tyrosines, D and Z, in PSII. The focus of our work is the assignment of the 1478 cm-1 vibration, which is observable in difference infrared spectra associated with these tyrosyl radicals. The first set of FT-IR experiments is performed with continuous illumination. Use of cyanobacterial strains, in which isotopomers of tyrosine have been incorporated, supports the assignment of a positive 1478/1477 cm-1 mode to the C-O stretching vibration of the tyrosyl radicals. In negative controls, the intensity of this spectral feature decreases. The negative controls involve the use of inhibitors or site-directed mutants, in which the oxidation of Z or D is eliminated, respectively. The assignment of the 1478/1477 cm-1 mode is also based on control EPR and fluorescence measurements, which demonstrate that no detectable Fe+2QA- signal is generated under FT-IR experimental conditions. Additionally, the difference infrared spectrum, associated with formation of the S2QA- state, argues against the assignment of the positive 1478 cm-1 line to the C-O vibration of QA-. In the second set of FT-IR experiments, single turnover flashes are employed, and infrared difference spectra are recorded as a function of time after photoexcitation. Comparison to kinetic transients generated in control EPR experiments shows that the decay of the 1477 cm-1 line precisely parallels the decay of the D. EPR signal. Taken together, these two experimental approaches strongly support the assignment of a component of the 1478/1477 cm-1 vibrational lines to the C-O stretching modes of tyrosyl radicals in PSII. Possible reasons for the apparently contradictory results of Hienerwadel et al. (1996) Biochemistry 35, 15,447-15,460 and Hienerwadel et al. (1997) Biochemistry 36, 14,705-14,711 are discussed. Copyright 1998 Elsevier Science B.V. All rights reserved.

The protein environment surrounding tyrosyl radicals D. and Z. in photosystem II: a difference Fourier-transform infrared spectroscopic study

Photosystem II contains two redox-active tyrosine residues, termed D and Z, which have different midpoint potentials and oxidation/reduction kinetics. To understand the functional properties of redox-active tyrosines, we report a difference Fourier-transform infrared (FT-IR) spectroscopic study of these species. Vibrational spectra associated with the oxidation of each tyrosine residue are acquired; electron paramagnetic resonance (EPR) and fluorescence experiments demonstrate that there is no detectable contribution of Q(A)- to these spectra. Vibrational lines are assigned to the radicals by isotopic labeling of tyrosine. Global 15N labeling, 2H exchange, and changes in pH identify differences in the reversible interactions of the two redox-active tyrosines with N-containing, titratable amino acid side chains in their environments. To identify the amino acid residue that contributes to the spectrum of D, mutations at His189 in the D2 polypeptide were examined. Mutations at this site result in substantial changes in the spectrum of tyrosine D. Previously, mutations at the analogous histidine, His190 in the D1 polypeptide, were shown to have no significant effect on the FT-IR spectrum of tyrosine Z (Bernard, M. T., et al. 1995. J. Biol. Chem. 270:1589-1594). A disparity in the number of accessible, proton-accepting groups could influence electron transfer rates and energetics and account for functional differences between the two redox-active tyrosines.

Chemical complementation identifies a proton acceptor for redox-active tyrosine D in photosystem II

Through the use of site-directed mutagenesis and chemical rescue, we have identified the proton acceptor for redox-active tyrosine D in photosystem II (PSII). Effects of chemical rescue on the tyrosyl radical were monitored by EPR spectroscopy. We also have acquired the Fourier-transform infrared (FT-IR) spectrum associated with the oxidation of tyrosine D and concomitant protonation of the acceptor. Mutant and isotopically labeled PSII samples are used to assign vibrational lines in the 3,600-3,100 cm-1 region to N-H modes of His-189 in the D2 polypeptide. When His-189 in D2 is changed to a leucine (HL189D2) in PSII, dramatic alterations of both EPR and FT-IR spectra are observed. When imidazole is introduced into HL189D2 samples, results from both EPR and FT-IR spectroscopy argue that imidazole is functionally reconstituted into an accessible pocket and that imidazole acts as a chemical mimic for His-189. Small perturbations of EPR and FT-IR spectra are consistent with access to this pocket in wild-type PSII, as well. Structures of the analogous site in bacterial reaction centers suggest that an accessible pocket, large enough to contain imidazole, is bordered by tyrosine D and His-189 in the D2 polypeptide. These data provide evidence that His-189 in the D2 polypeptide of PSII acts as a proton acceptor for redox-active tyrosine D and that proton transfer to the imidazole ring facilitates the efficient oxidation/reduction of tyrosine D.

The lumenal loop connecting transmembrane helices I and II of the D1 polypeptide is important for assembly of the photosystem two complex

Current structural models indicate that the D1 and D2 polypeptides of the Photosystem two reaction center complex (PS II RC) each span the thylakoid membrane five times. In order to assess the importance of the lumenal extrinsic loop that connects transmembrane helices I and II of D1 we have constructed five deletion mutants and two double mutants in the cyanobaterium Synechocystic sp. PCC 6803. Four of the deletion mutants (Δ59-65, Δ69-74, Δ79-86 and Δ109-110) are obligate photoheterotrophs unable to accumulate D1 in the membrane as assayed by immunoblotting experiments or pulse-labelling experiments using [(35)S]-methionine. In contrast deletion mutant Δ100 which lacks A100 behaved very similarly to the WT control strain in terms of photoautotrophic growth rate, saturated rates of oxygen evolution, flash-induced oxygen evolution, fluorescence induction and decay, and thermoluminescence. Δ100 is the first example of an internal deletion on the lumenal side of the D1 polypeptide that is benign to photosystem two function. Double mutant D103G/E104A also behaves similarly to the WT control strain leading to the conclusion that residues D103 and E104 are unlikely to be involved in ligating the metal ions Mn or Ca(2+), which are needed for photosynthetic oxygen evolution. Double mutant, G109A/G110A, was constructed to assess the significance of this GlyGly motif which is also conserved in the L subunit of purple bacterial reaction centres. The G109A/G110A mutant is able to evolve oxygen at approximately 50-70% of WT rates but is unable to grow phatoautotrophically apparently because of an enhanced sensitivity to photoinactivation than the WT control strain. A photoautotropic revertant was isolated from this strain and shown to result from a mutation that restored the WT codon at position 109. Pulse-chase experiments in cells using [(35)S]-methionine showed that resistance to photoinhibition in the revertant correlated with an enhanced rate of incorporation of D1 into the membrane compared to mutant G109A/G110A. The sensitivity to photoinhibition shown by the G109A/G110A mutant is therefore consistent with a perturbation to the D1 repair cycle possibly at the level of D1 synthesis or incorporation of D1 into the PS II complex.

Electron paramagnetic resonance characterization of tyrosine radical, M+, in site-directed mutants of photosystem II(t)

Photosystem II contains two well-characterized tyrosine radicals, D(.) and Z(.). Z is an electron carrier between the primary chlorophyll donor and the manganese catalytic site and is essential for enzymatic function. On the other hand, D forms a stable radical with no known role in oxygen evolution. D(.) and Z(.) give rise to similar, but not identical, room temperature electron paramagnetic resonance (EPR) signals, which can be distinguished by their decay kinetics. A third room temperature EPR signal has also been observed in site-directed mutants in which a nonredox active amino acid is substituted at the D or Z site. This four-line EPR signal has been shown to have a tyrosine origin by isotopic labeling (Boerner and Barry, 1994, J. Biol. Chem. 269:134-137), but such an EPR signal has never before been observed from a tyrosyl radical. The radical giving rise to this third unique signal has been named M+. Here we provide kinetic evidence that this signal arises from a third redox active tyrosine, distinct from tyrosine D and Z, in the photosystem II reaction center. Isotopic labeling and EPR spectroscopy provide evidence that M is a covalently modified tyrosine.