A model for the mechanism of chloride activation of oxygen evolution in photosystem II

Genes encoding the photosystem II proteins are under purifying selection: an insight into the early evolution of oxygenic photosynthesis

The molecular evolution concerns coding sequences (CDSs) of genes and may affect the structure and function of proteins. Non-uniform use of synonymous codons during translation, known as codon usage bias (CUB), depends on the balance between mutations bias and natural selection. We estimated different CUB indices, i.e. the effective number of codons (ENC), G + C content in the 3rd codon positions (GC₃), and codon adaptation index for CDSs of intrinsic proteins of photosystem II (PSII), such as psbA (D1), psbD (D2), psbB (CP47), psbC (CP43), and CDSs of the extrinsic protein psbO (PsbO). These genes occur in all organisms that perform oxygenic photosynthesis (OP) on Earth: cyanobacteria, algae and plants. Comparatively, a similar analysis of codon bias for CDSs of L and M subunits that constitute the core proteins of the type II reaction centre (RCII) in anoxygenic bacteria was performed. Analysis of CUB indices and determination of the number of synonymous (d_S) and nonsynonymous substitutions (d_N) in all analysed CDSs indicated that the crucial PSII and RCII proteins were under strong purifying (negative) selection in course of evolution. Purifying selection was also estimated for CDSs of atpA, the α subunit of ATP synthase, an enzyme that was most likely already present in the last universal common ancestor (LUCA). The data obtained point to an ancient origin of OP, even in the earliest stages of the evolution of life on Earth.

A sixty-year tryst with photosynthesis and related processes: an informal personal perspective

After briefly describing my early collaborative work at the University of Allahabad, that had laid the foundation of my research life, I present here some of our research on photosynthesis at the University of Illinois at Urbana-Champaign, randomly selected from light absorption to NADP⁺ reduction in plants, algae, and cyanobacteria. These include the fact that (i) both the light reactions I and II are powered by light absorbed by chlorophyll (Chl) a of different spectral forms; (ii) light emission (fluorescence, delayed fluorescence, and thermoluminescence) by plants, algae, and cyanobacteria provides detailed information on these reactions and beyond; (iii) primary photochemistry in both the photosystems I (PS I) and II (PS II) occurs within a few picoseconds; and (iv) most importantly, bicarbonate plays a unique role on the electron acceptor side of PS II, specifically at the two-electron gate of PS II. Currently, the ongoing research around the world is, and should be, directed towards making photosynthesis better able to deal with the global issues (such as increasing population, dwindling resources, and rising temperature) particularly through genetic modification. However, basic research is necessary to continue to provide us with an understanding of the molecular mechanism of the process and to guide us in reaching our goals of increasing food production and other chemicals we need for our lives.

Proteomics research on the effects of applying selenium to apple leaves on photosynthesis

Untreated and Se-enriched apple leaves (Malus domestica Borkh. cv. 'Red Fuji') were used as the experimental materials. Proteomes of the differentially prepared tissues were compared through two-dimensional electrophoresis analysis and mass spectrum identification. There were 505 more protein spots in the proteome of the Se-enriched leaves than in the control leaves. Forty-seven protein spots were significantly differentially expressed (P < 0.05), among those, 32 protein spots were up-regulated while 12 protein points were down-regulated, and three new protein spots were found with the relative molecular masses of 31, 29, 26 kDa. Twenty-three protein spots with good shape and significant expression were selected for mass spectrometry analysis. These spots were excised from the gel and analyzed by a matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS). Peptide mass fingerprints (PMF) of all the proteins were submitted to NCBInr for protein identification, and 10 differential proteins were positively identified. Biological information of the identified proteins was found via http://www.uniprot.org/. There were three (1475, 1479, 1527) ribulose-1,5-bisphosphate carboxylase/oxygenase large subunits (Rubisco), two ribulose-1,5-bisphosphate carboxylases (346, 486) belonging to the Rubisco large chain family, one photosystem I reaction center subunit II (297), one chloroplast oxygen-evolving enhancer protein 1 (619), one Os12g0127100 protein whose function was unknown (927), one monodehydroascorbate reductase (1451), and one polyphenol oxidase V (1596). The major subcellular location for these proteins was the chloroplast, and they play important roles in photosynthesis and stress resistance for plants.

The extrinsic proteins of Photosystem II

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

Current status of the role of Cl(-) ion in the oxygen-evolving complex

This minireview summarizes the current state of knowledge concerning the role of Cl(-) in the oxygen-evolving complex (OEC) of photosystem II (PSII). The model that proposes that Cl(-) is a Mn ligand is discussed in light of more recent work. Studies of Cl(-) specificity, stoichiometry, kinetics, and retention by extrinsic polypeptides are discussed, as are the results that fail to detect Cl(-) ligation to Mn and results that show a lack of a requirement for Cl(-) in PSII-catalyzed H(2)O oxidation. Mutagenesis experiments in cyanobacteria and higher plants that produce evidence for a correlation between Cl(-) retention and stable interactions among intrinsic and extrinsic polypeptides are summarized, and spectroscopic data on the interaction between PSII and Cl(-) are discussed. Lastly, the question of the site of Cl(-) action in PSII is discussed in connection with the current crystal structures of the enzyme.

Acetate binding at the photosystem II oxygen evolving complex: an S(2)-state multiline signal ESEEM study

Previously, using acetate deuterated in the methyl hydrogen positions, we showed that acetate binds in close proximity to the Mn cluster/Y(.)(z) tyrosine dual spin complex in acetate-inhibited photosystem II (PSII) preparations exhibiting the "split" EPR signal arising from the S(2)-Y(.)(z) interaction [Force, D. A.; Randall, D. W.; Britt, R. D. Biochemistry 1997, 36, 12062-12070]. By using paramagnetic NO to quench the paramagnetism of Y(.)(z), we are able to observe the ESEEM spectrum of deuterated acetate interacting with only the Mn cluster. A good fit of the ESEEM data indicates two (2)H dipolar hyperfine couplings of 0.097 MHz and one of 0.190 MHz. Modeling of these dipolar interactions, using our "dangler" 3 + 1 model for the S(2)-state of the Mn cluster, reveals distances consistent with direct ligation of acetate to the Mn cluster. As acetate inhibition is competitive with the essential cofactor Cl(-), this suggests that Cl(-) ligates directly to the Mn cluster. The effect of acetate binding on the structure of the Mn cluster is investigated by comparing the Mn-histidine coupling in NO/acetate-treated PSII and untreated PSII using ESEEM. We find that the addition of acetate and NO does not affect the histidine ligation to the Mn cluster. We also investigate the ability of acetate to access Y(.)(z) in Mn-depleted PSII, a PSII preparation expected to be more solvent accessible than intact PSII. We detect no coupling between Y(.)(z) and acetate. We have previously shown that small alcohols such as methanol can ligate to the Mn cluster with ease, while larger alcohols such as 2-propanol, as well as DMSO, are excluded [Force, D. A.; Randall, D. W.; Lorigan, G. A.; Clemens, K. L.; Britt, R. D. J. Am. Chem. Soc. 1998, 120, 13321-13333]. We probe the effect of acetate binding on the ability of methanol and DMSO to bind to the Mn cluster. We find that methanol is able to bind to the Mn cluster in the presence of acetate. We detect no DMSO binding in the presence of acetate. Thus, acetate binding does not increase the affinity or accessibility for DMSO binding at the Mn cluster. We also explore the possibility that the acetate binding site is also a binding site for substrate water. By comparing the ratioed three-pulse ESEEM spectra of a control, untreated PSII sample in 50% D(2)O to an NO/acetate-treated PSII sample in 50% D(2)O, we find that the binding of acetate to the oxygen evolving complex of photosystem II displaces deuterons bound very closely to the Mn cluster.

Modeling of the D1/D2 proteins and cofactors of the photosystem II reaction center: implications for herbicide and bicarbonate binding

A three-dimensional model of the photosystem II (PSII) reaction center from the cyanobacterium Synechocystis sp. PCC 6803 was generated based on homology with the anoxygenic purple bacterial photosynthetic reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis, for which the X-ray crystallographic structures are available. The model was constructed with an alignment of D1 and D2 sequences with the L and M subunits of the bacterial reaction center, respectively, and by using as a scaffold the structurally conserved regions (SCRs) from bacterial templates. The structurally variant regions were built using a novel sequence-specific approach of searching for the best-matched protein segments in the Protein Data Bank with the "basic local alignment search tool" (Altschul SF, Gish W, Miller W, Myers EW, Lipman DJ, 1990, J Mol Biol 215:403-410), and imposing the matching conformational preference on the corresponding D1 and D2 regions. The structure thus obtained was refined by energy minimization. The modeled D1 and D2 proteins contain five transmembrane alpha-helices each, with cofactors (4 chlorophylls, 2 pheophytins, 2 plastoquinones, and a non-heme iron) essential for PSII primary photochemistry embedded in them. A beta-carotene, considered important for PSII photoprotection, was also included in the model. Four different possible conformations of the primary electron donor P680 chlorophylls were proposed, one based on the homology with the bacterial template and the other three on existing experimental suggestions in literature. The P680 conformation based on homology was preferred because it has the lowest energy. Redox active tyrosine residues important for P680+ reduction as well as residues important for PSII cofactor binding were analyzed. Residues involved in interprotein interactions in the model were also identified. Herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was also modeled in the plastoquinone QB binding niche using the structural information available from a DCMU-binding bacterial reaction center. A bicarbonate anion, known to play a role in PSII, but not in anoxygenic photosynthetic bacteria, was modeled in the non-heme iron site, providing a bidentate ligand to the iron. By modifying the previous hypothesis of Blubaugh and Govindjee (1988, Photosyn Res 19:85-128), we modeled a second bicarbonate and a water molecule in the QB site and we proposed a hypothesis to explain the mechanism of QB protonation mediated by bicarbonate and water. The bicarbonate, stabilized by D1-R257, donates a proton to QB2- through the intermediate of D1-H252; and a water molecule donates another proton to QB2-. Based on the discovery of a "water transport channel" in the bacterial reaction center, an analogous channel for transporting water and bicarbonate is proposed in our PSII model. The putative channel appears to be primarily positively charged near QB and the non-heme iron, in contrast to the polarity distribution in the bacterial water transport channel. The constructed model has been found to be consistent with most existing data.

Fluorescence and Fourier-transform infrared spectroscopic studies on the role of disulfide bond in the calcium binding in the 33 kDa protein of Photosystem II

The 33 kDa protein of Photosystem II has one intrachain disulfide bond. Fluorescence spectroscopy shows that the major groups in the protein that bind to Ca(2+) should be the carboxylic side groups of glutamic acid and/or aspartic acid. Fluorescence and Fourier-transform infrared (FTIR) spectroscopic studies indicate that the conformation of the 33 kDa protein is altered upon reduction, while the reduced protein still retains the secondary structure. FTIR spectroscopy also shows that the metal ions induce a relative decrease of unordered structure and β-sheet, and a substantial increase of α-helix in both the intact and the reduced 33 kDa protein. This indicates that the addition of cations results in a much more compact structure and that both the intact and the reduced 33 kDa proteins have the ability to bind calcium. The above results may suggest that the disulfide bridge is not essential for calcium binding.

Lumenal side histidine mutations in the D1 protein of Photosystem II affect donor side electron transfer in Chlamydomonas reinhardtii

Site-directed mutants of the D1 protein generated in Chlamydomonas reinhardtii have been characterized to determine whether specific lumenal side histidine residues participate in or directly influence electron transfer. Histidine 195 (H195), a conserved residue located near the amino-terminal end of the D1 transmembrane alpha-helix containing the putative P680 chlorophyll ligand H198, was changed to asparagine (H195N), aspartic acid (H195D), and tyrosine (H195Y). These H195 mutants displayed essentially wild-type rates of electron transfer from the water-oxidizing complex to 2,6-dichlorophenolindophenol. Flash-induced chlorophyll a (Chl a) fluorescence yield rise and decay measurements for Mn-depleted membranes of the H195Y and H195D mutants, however, revealed modified YZ to P680+ electron transfer kinetics. The rate of the variable Chl a fluorescence rise was reduced approximately 10-fold in H195Y and H195D relative to the wild type. In addition, the rate of Chl a fluorescence decay in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea was approximately 50-fold more rapid in H195D than in the wild type. These results can be accommodated by a change in the midpoint potential of YZ+/YZ which is apparent only upon the removal of the Mn cluster. In addition, we have generated a histidine to phenylalanine substitution at histidine 190 (H190), a conserved residue located near the lumenal thylakoid surface of D1 in close proximity to the secondary donor YZ. The H190F mutant is characterized by an inability to oxidize water associated with the loss of the Mn cluster and severely altered donor side kinetics. These and other results suggest that H190 may participate in redox reactions leading to the assembly of the Mn cluster.

Water cleavage by solar radiation-an inspiring challenge of photosynthesis research

Solar energy exploitation by photosynthetic water cleavage is of central relevance for the development and sustenance of all higher forms of living matter in the biosphere. The key steps of this process take place within an integral protein complex referred to as Photosystem II (PS II) which is anisotropically incorporated into the thylakoid membrane. This minireview concentrates on mechanistic questions related to i) the generation of strongly oxidizing equivalents (holes) at a special chlorophyll a complex (designated as P680) and ii) the cooperative reaction of four holes with two water molecules at a manganese containing unit WOC (water oxidizing complex) resulting in the release of molecular oxygen and four protons. The classical work of Pierre Joliot and Bessel Kok and their coworkers revealed that water oxidation occurs via a sequence of univalent oxidation steps including intermediary redox states Si (i = number of accumulated holes within the WOC). Based on our current stage of knowledge, an attempt is made a) to identify the nature of the redox states Si, b) to describe the structural arrangement of the (four) manganese centers and their presumed coordination and ligation within the protein matrix, and c) to propose a mechanism of photosynthetic water oxidation with special emphasis on the key step, i.e. oxygen-oxygen bond formation. It is assumed that there exists a dynamic equilibrium in S3 with one state attaining the nuclear geometry and electronic configuration of a complexed peroxide. This state is postulated to undergo direct oxidation to complexed dioxygen by univalent electron abstraction with YZ (ox) and simultaneous internal ligand to metal charge transfer.Key questions on the mechanism will be raised. The still fragmentary answers to these questions not only reflect our limited knowledge but also illustrate the challenges for future research.

Studies of the slowly exchanging chloride in Photosystem II of higher plants

(36)Cl(-) was used to study the slow exchange of chloride at a binding site associated with Photosystem II (PS II). When PS II membranes were labeled with different concentrations of (36)Cl(-), saturation of binding at about I chloride/PS II was observed. The rate of binding showed a clear dependence on the concentration of chloride approaching a limiting value of about 3·10(-4) s(-1) at high concentrations, similar to the rate of release of chloride from labeled membranes. These rates were close to that found earlier for the release of chloride from PS II membranes isolated from spinach grown on (36)Cl(-), which suggests that we are observing the same site for chloride binding. The similarity between the limiting rate of binding and the rate of release of chloride suggests that the exchange of chloride with the surrounding medium is controlled by an intramolecular process. The binding of chloride showed a pH-dependence with an apparent pKa of 7.5 and was very sensitive to the presence of the extrinsic polypeptides at the PS II donor side. The binding of chloride was competitively inhibited by a few other anions, notably Br(-) and NO3 (-). The slowly exchanging Cl(-) did not show any significant correlation with oxygen evolution rate or yield of EPR signals from the S2 state. Our studies indicate that removal of the slowly exchanging chloride lowers the stability of PS II as indicated by the loss of oxygen evolution activity and S2 state EPR signals.

Photosynthetic water oxidation: The protein framework

Approximately 20 protein subunits are associated with the PS II complex, not counting subunits of peripheral light-harvesting antenna complexes. However, it is not yet established which proteins specifically are involved in the water-oxidation process. Much evidence supports the concept that the D1/D2 reaction center heterodimer not only plays a central role in the primary photochemistry of Photosystem II, but also is involved in electron donation to P680 and in ligation of the manganese cluster. This evidence includes (a) the primary donor to P680 has been shown to be a redox-active tyrosyl residue (Tyr161) in the D1 protein, and (b) site-directed mutagenesis and computer-assisted modeling of the reaction center heterodimer have suggested several sites with a possible function in manganese ligation. These include Asp170, Gln165 and Gln189 of the D1 protein and Glu69 of the D2 protein as well as the C-terminal portion of the mature D1 protein. Also, hydrophilic loops of the chlorophyll-binding protein CP43 that are exposed at the inner thylakoid surface could be essential for the water-splitting process.In photosynthetic eukaryotes, three lumenal extrinsic proteins, PS II-O (33 kDa), PS II-P (23 kDa) and PS II-Q (16 kDa), influence the properties of the manganese cluster without being involved in the actual catalysis of water oxidation. The extrinsic proteins together may have multiple binding sites to the integral portion of PS II, which could be provided by the D1/D2 heterodimer and CP47. A major role for the PS II-O protein is to stabilize the manganese cluster. Most experimental evidence favors a connection of the PS II-P protein with binding of the Cl(-) and Ca(2+) ions required for the water oxidation, while the PS II-Q protein seems to be associated only with the Cl(-) requirement. The two latter proteins are not present in PS II of prokaryotic organisms, where their functions may be replaced by a 10-12 kDa subunit and a newly discovered low-potential cytochrome c-550.

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

To identify amino acid residues that influence the assembly or stability of the manganese cluster in photosystem II, we have generated site-directed mutations in the D1 polypeptide of the cyanobacterium, Synechocystis sp. PCC 6803. Indirect evidence has suggested that the D1 polypeptide provides some of the ligands that are required for metal binding. Mutations at position 170 of D1 were selected for characterization, since an aspartate to asparagine mutation (DN170D1) at this position completely abolishes photoautotrophic growth, while retention of a carboxylic acid at this position (aspartate to glutamate, DE170D1) supports photoautotrophic growth. Photosystem II particles were purified from control, DE170D1, and DN170D1 cells by a procedure that retains high rates of oxygen evolution activity in control particles [Noren, G.H., Boerner, R.J., & Barry, B.A. (1991) Biochemistry 30, 3943-3950]. Spectroscopic analysis shows that the tyrosine radical, Z+, which normally oxidizes the manganese cluster, is rapidly reduced in the DE170D1 mutant, but not in the DN170D1 mutant. A possible explanation of this block or dramatic decrease in the rate of electron transfer between the manganese cluster and tyrosine Z is an alteration in the properties of the metal center. Quantitation of manganese in these particles is consistent with aspartate 170 influencing the stability or assembly of the manganese cluster, since the aspartate to asparagine mutation results in a decrease in the manganese content per reaction center. Photosystem II particles from DN170D1 show a 60% decrease in the amount of specifically bound manganese per reaction center, when compared to control particles. Also, we observe a 70% decrease in the amount of specifically bound manganese per reaction center in partially purified DN170D1 particles and at least an 80% decrease in the amount of hydroxylamine-reducible manganese in DN170D1 thylakoid membranes. Single-turnover fluorescence assays and steady-state EPR measurements demonstrate that the remaining, endogenous manganese does not rapidly reduce tyrosine Z+ in the DN170D1 mutant. Additional evidence that aspartate 170 influences the assembly or stability of the metal site comes from analysis of the DE170D1 mutant. Although this mutant assembles a functional manganese cluster, as assessed by oxygen evolution and spectroscopic assays, the properties of the manganese site are perturbed.

Structure model of core proteins in photosystem I inferred from the comparison with those in photosystem II and bacteria; an application of principal component analysis to detect the similar regions between distantly related families of proteins

A principal component analysis based on the physico-chemical properties of amino acid residues is developed to assign similar regions between distantly related families of proteins, taking account of the species diversities in respective families. The most important advantage of this analysis should be that it reflects different physico-chemical properties and thus can predict more detailed structural properties, including the transmembrane helices, than the hydropathy analysis. Its first application reconfirms the similarity between the core proteins of photosynthetic reaction center in purple bacteria and those of photosystem II, indicating that the low percentage of identical amino acid residues estimated previously between them is due to much allowance for amino acid substitutions in purple bacteria. The application of this analysis to the core proteins of photosystem I reveals that any of these proteins includes two domains, each showing high similarity to the amino acid sequences of core proteins in photosystem II and purple bacteria. A core structure model of A1 and A2 proteins folded into four layers of sheets of transmembrane helices is proposed to provide a molecular basis for the electron pathway suggested by spectroscopic experiments as well as for the interaction sites with plastocyanin, 9 kDa protein and LHC proteins.

The carboxyl modifier 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) inhibits half of the high-affinity Mn-binding site in photosystem II membrane fragments

The diphenylcarbazide(DPC)/Mn2+ assay [Hsu, B.-D., Lee, J.-Y., & Pan, R.-L. (1987) Biochim. Biophys. Acta 890, 89-96] was used to assess the amount of the high-affinity Mn-binding site in manganese-depleted photosystem II (PS II) membrane fragments from spinach and Scenedesmus obliquus. The assay mechanism at high DPC concentration was shown to involve noncompetitive inhibition of only half of the control level of DPC donation to PS II by micromolar concentrations of Mn at pH 6.5 (i.e., one of two DPC donation sites is inhibited). At low DPC concentration both DPC and Mn2+ donate to PS II additively. Treatment with the carboxyl amino acid modifier 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) inhibited half of the high-affinity Mn-binding site in spinach and Scenedesmus WT PS II membranes and all of the available site in Scenedesmus LF-1 mutant PS II membranes. A similar EDC concentration dependence was observed in all cases. Addition of 2 mM MnCl2 to the 10 mM EDC modification buffer provided complete protection for the Mn-binding site from modification. This protection was specific for Mn2+; six other divalent cations were ineffective. We conclude that EDC modifies that half of the high-affinity Mn-binding site that is insensitive to the histidine modifier diethyl pyrocarbonate (DEPC) [Seibert, M., Tamura, N., & Inoue, Y. (1989) Biochim. Biophys. Acta 974, 185-191] and directly affects ligands that bind Mn. The effects of EDC and DEPC that influence the high-affinity site are mutually exclusive and are specific to the lumenal side of the PS II membrane. Removal of the two more loosely bound of the four functional Mn from PS II membranes uncovers that part of the high-affinity site associated with carboxyl but not histidyl residues. We suggest that carboxyl residues on reaction center proteins are associated with half of the high-affinity Mn-binding site in PS II and are involved along with histidine residues in binding Mn functional in the O2-evolving process.

Purification of highly active oxygen-evolving photosystem II from Chlamydomonas reinhardtii

A method is described for the isolation and purification of active oxygen-evolving photosystem II (PS II) membranes from the green alga Chlamydomonas reinhardtii. The isolation procedure is a modification of methods evolved for spinach (Berthold et al. 1981). The purity and integrity of the PS II preparations have been assesssed on the bases of the polypeptide pattern in SDS-PAGE, the rate of oxygen evolution, the EPR multiline signal of the S2 state, the room temperature chlorophyll a fluorescence yield, the 77 K emission spectra, and the P700 EPR signal at 300 K. These data show that the PS II characteristics are increased by a factor of two in PS II preparations as compared to thylakoid samples, and the PS I concentration is reduced by approximately a factor ten compared to that in thylakoids.

Is there a direct chloride cofactor requirement in the oxygen-evolving reactions of photosystem II?

The dark incubation at room temperature of photosystem II (PS II) membrane fragments in a chloride-free medium at pH 6.3 slowly leads to large chloride-restorable and non-restorable O2 evolution activity losses with time as compared with control samples incubated in the presence of 10 mM NaCl. The chloride requirement in O2 evolution generated under these conditions reveals a complex interplay among various experimental parameters, including the source of the plant material, the times of incubation, the sample concentration, the chloride concentration, as well as those treatments which are believed to specifically displace chloride from PS II such as alkaline pH pretreatment and Na2SO4 addition. The results indicate that secondary, structural changes within the PS II complex are an important factor in determining the influence of chloride on the O2 evolution activity and raise the question whether or not chloride ions actually play a direct cofactor role in the water-oxidizing reactions leading to O2 evolution.

Current perceptions of Photosystem II

In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385-398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q400 component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field.

Chloride binding proteins: mechanistic implications for the oxygen-evolving complex of Photosystem II

Chloride plays a key role in activating the photosynethetic oxygen-evolving complex (OEC) of Photosystem II, but the OEC is only one of many enzymes affected by this anion. Some of the mechanistic features of Cl(-) involvement in water-splitting resemble those of other proteins whose structure and chemistry are known in detail. An overview of the similarities and differences between these Cl(-)-binding systems is presented.

Chloride binding to photosystem II in the dark is in slow exchange

We have studied the 35Cl- NMR line broadening in the presence of photosystem II (PS II) membranes from spinach in the dark. In the presence of NH3 (which other work has shown to competitively inhibit chloride binding to PS II) we observed no decrease in 35 Cl- linewidths. We conclude that binding of Cl- to the O2 evolving center in PS II in the dark (previously demonstrated by EPR) is in slow exchange on the NMR timescale. We assign the observed line broadening to interaction with non-specific binding sites and with free paramagnetics.

The extrinsic 33 kDa polypeptide of the oxygen-evolving complex of photosystem II is a putative calcium-binding protein and is encoded by a multi-gene family in pea

The extrinsic 33 kDa polypeptide of the water-oxidizing complex has been extracted from pea photosystem II particles by washing with alkaline-Tris and purified by ion-exchange chromatography. The N-terminal amino acid sequence has been determined, and specific antisera have been raised in rabbits and used to screen a pea leaf cDNA library in λgt11. Determination of the nucleotide sequence of positive clones revealed an essentially full-length cDNA for the 33 kDa polypeptide, the deduced amino acid sequence showing it to code for a mature protein of 248 amino acids with an N-terminal transit peptide of 81 amino acids. The protein showed a high degree of conservation with previously reported sequences for the 33 kDa protein from other species and the sequence contained a putative Ca(2+)-binding site with homology to mammalian intestinal calcium-binding proteins. Northern analysis of total pea RNA indicated a message of approximately 1.4 kb, in good agreement with the size of the cDNA obtained at 1.3 kbp. Southern blots of genomic DNA probed with the labelled cDNA give rise to several bands suggesting that the 33 kDa polypeptide is coded by a multi-gene family.

The effect of chloride on the thermal inactivation of oxygen evolution

The effect of Cl depletion on the sensitivity of the oxygen-evolving complex of Photosystem II (PS II) to heat treatment was examined by a parallel study of the Hill activity (H2O→2,6-dichlorophenolindophenol), Cl(-) binding (by (35)Cl-NMR) and Mn release (by EPR). The extent of thermal inactivation in spinach thylakoids was found to depend on the degree of Cl(-) depletion in the sample. In partially Cl(-)-depleted thylakoids, mild heating (38°C, 3 min) was found to eliminate inflections in plots of both Hill activity versus [Cl(-)] (at low light intensity) and excess (35)Cl-NMR linewidth versus [Cl(-)] (in the dark). In PS II membranes, the same treatment reduced the differences between the linewidth maxima and minima, particularly in the region of 0.3 mM and 7.0 mM Cl(-), as compared to unheated membranes. These results indicate that mild heating affects the Cl(-)-binding domains within the oxygen-evolving complex, OEC, EPR measurements of the temperature dependence of Mn release from heated thylakoids show that Mn release begins to correlate with the loss of Hill activity only at higher temperatures, where the OEC is already substantially inactivated. We conclude from these studies that the Cl(-)-binding domains of the OEC constitute a principal site of damage by heat treatment.

Chloride relations of photosystem II membrane preparations depleted of, and resupplied with, their 17 and 23 kDa extrinsic polypeptides

Photosystem II membranes prepared from thylakoids of Phytolacca americana chloroplasts were depleted of their intrinsic 17 and 23 kDa polypeptides, and the effects of a reconstitution of these polypeptides on the Cl(-) requirements of O2 evolution activity were analyzed. It was found that the activating effectiveness of limiting amounts of added Cl(-) was increased several fold by an addition of the 23 kDa polypeptide. When it was supplemented by the 17 kDa species, only a small additional increase occurred, but Cl(-) retention in Cl(-) free media was enhanced greatly. Addition of the 17 kDa polypeptide alone was without effect because it is known that it cannot bind to its native site unless the 23 kDa polypeptide is in place.Optimal enhancements of the effectiveness of activating added Cl(-) were observed when the assays were done in the presence of the reconstituting polypeptides. When the reconstituting treatment with the polypeptides, and the assay of the Cl(-) relations, were separated, it was advantageous to have Cl(-) present in the reconstituting medium, and not to add Ca(2+), another cofactor of photosynthetic water oxidation. Those requirements are attributed to the labilizing effects Cl(-) free conditions and divalent cations have on the association of especially the 23 kDa polypeptides with the water oxidizing complex, and to a possible aggregation of the membranes under the influence of Ca(2+) which might have impeded proper polypeptide binding.

A model for the mechanism of chloride activation of oxygen evolution in photosystem II

A hypothesis is proposed to explain the function of Cl(-) in activating the oxygenevolving complex (OEC) of photosystem II (PS II), based on the results of recent (35)Cl-NMR studies. The putative mechanism involves Cl(-) binding to two types of sites. An intrinsic site is suggested to be composed of three histidyl residues (His 332 and His 337 from D1 and His 337 D2). It is proposed that Cl(-) binding to this site accelerates the abstraction of H(+) from water by raising the pKa's of the histidine imidazole groups. Cl(-) binding also stimulates the transfer of H(+) from this intrinsic site to a set of extrinsic sites on the 33 kD extrinsic polypeptide. The extrinsic Cl(-) binding sites are suggested to involve four protein domains that are linked together by salt-bridge contacts. Chloride and H(+) donated from the intrinsic site attack these intramolecular salt-bridges in a defined sequence, thereby exposing previously inaccessible Cl(-) and H(+) binding sites and stimulating the oxidation of water. This hypothesis also proposes a possible structure for the Mn active site within the D1/D2 complex. Specific amino-acid residues that are likely to participate as Mn lignads are identified on the lumenal portions of the D1 and D2 proteins that are different from those in the L and M subunits of photosynthetic bacteria; the choice of these residues is based on the metal coordination chemistry of these residues, their location within the polypeptide chain, the regularity of their spacing, and their conservation through evolution. The catalytic Mn-binding residues are suggested to be D-61, E-65, E-92, E-98, D-103; D-308, E-329, E-342 and E-333 in D1, and H-62, E-70, H-88, E-97, D-101; E-313, D-334, E-338 and E-345 in D2. Finally, this hypothesis identifies sites on both D2 and the 33 kD extrinsic polypeptide that might be involved in high- and low-affinity Ca(2+) binding.