Electron transfer events in chloride-depleted photosystem II

Requirement of Chloride for the Downhill Electron Transfer Pathway from the Water-Splitting Center in Natural Photosynthesis

In photosystem II (PSII), Cl^- is a prerequisite for the second flash-induced oxidation of the Mn₄CaO₅ cluster (the S₂ to S₃ transition). We report proton transfer from the substrate water molecule via D1-Asp61 and electron transfer via redox-active D1-Tyr161 (TyrZ) to the chlorophyll pair in Cl^--depleted PSII using a quantum mechanical/molecular mechanical approach. The low-barrier H-bond formation between the substrate water molecule and D1-Asp61 remained unaffected upon the depletion of Cl^-. However, the binding site, D2-Lys317, formed a salt bridge with D1-Asp61, leading to the inhibition of the subsequent proton transfer. Remarkably, the redox potential (E_m) of S₂/S₃ increased significantly, making electron transfer from S₂ to TyrZ energetically uphill, as observed in Ca²⁺-depleted PSII. The uphill electron transfer pathway was induced by the significant increase in E_m(S₂/S₃) caused by the loss of charge compensation for D2-Lys317 upon the depletion of Cl^-, whereas it was induced by the significant decrease in E_m(TyrZ) caused by the rearrangement of the water molecules at the Ca²⁺ binding moiety upon the depletion of Ca²⁺.

pH optimum of the photosystem II H₂O oxidation reaction: effects of PsbO, the manganese-stabilizing protein, Cl- retention, and deprotonation of a component required for O₂ evolution activity

Hydroxide ion inhibits Photosystem II (PSII) activity by extracting Cl(-) from its binding site in the O(2)-evolving complex (OEC) under continuous illumination [Critchley, C., et al. (1982) Biochim. Biophys. Acta 682, 436]. The experiments reported here examine whether two subunits of PsbO, the manganese-stabilizing protein, bound to eukaryotic PSII play a role in protecting the OEC against OH(-) inhibition. The data show that the PSII binding properties of PsbO affect the pH optimum for O(2) evolution activity as well as the Cl(-) affinity of the OEC that decreases with an increasing pH. These results suggest that PsbO functions as a barrier against inhibition of the OEC by OH(-). Through facilitation of efficient retention of Cl(-) in PSII [Popelkova, H., et al. (2008) Biochemistry 47, 12593], PsbO influences the ability of Cl(-) to resist OH(-)-induced release from its site in the OEC. Preventing inhibition by OH(-) allows for normal (short) lifetimes of the S(2) and S(3) states in darkness [Roose, J. L., et al. (2011) Biochemistry 50, 5988] and for maximal steady-state activity by PSII. The data presented here indicate that activation of H(2)O oxidation occurs with a pK(a) of ∼6.5, which could be a function of deprotonation of one or more amino acid residues that reside near the OEC active site on the D1 and CP43 intrinsic subunits of the PSII reaction center.

Structural-functional role of chloride in photosystem II

Chloride binding in photosystem II (PSII) is essential for photosynthetic water oxidation. However, the functional roles of chloride and possible binding sites, during oxygen evolution, remain controversial. This paper examines the functions of chloride based on its binding site revealed in the X-ray crystal structure of PSII at 1.9 Å resolution. We find that chloride depletion induces formation of a salt bridge between D2-K317 and D1-D61 that could suppress the transfer of protons to the lumen.

Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography

The chloride ion, Cl(-), is an essential cofactor for oxygen evolution of photosystem II (PSII) and is closely associated with the Mn(4)Ca cluster. Its detailed location and function have not been identified, however. We substituted Cl(-) with a bromide ion (Br(-)) or an iodide ion (I(-)) in PSII and analyzed the crystal structures of PSII with Br(-) and I(-) substitutions. Substitution of Cl(-) with Br(-) did not inhibit oxygen evolution, whereas substitution of Cl(-) with I(-) completely inhibited oxygen evolution, indicating the efficient replacement of Cl(-) by I(-). PSII with Br(-) and I(-) substitutions were crystallized, and their structures were analyzed. The results showed that there are 2 anion-binding sites in each PSII monomer; they are located on 2 sides of the Mn(4)Ca cluster at equal distances from the metal cluster. Anion-binding site 1 is close to the main chain of D1-Glu-333, and site 2 is close to the main chain of CP43-Glu-354; these 2 residues are coordinated directly with the Mn(4)Ca cluster. In addition, site 1 is located in the entrance of a proton exit channel. These results indicate that these 2 Cl(-) anions are required to maintain the coordination structure of the Mn(4)Ca cluster as well as the proposed proton channel, thereby keeping the oxygen-evolving complex fully active.

Decoupling of the processes of molecular oxygen synthesis and electron transport in Ca2+-depleted PSII membranes

Extraction of Ca(2+) from the O(2)-evolving complex (OEC) of photosystem II (PSII) membranes with 2 M NaCl in the light (PSII(-Ca/NaCl)) results in 90% inhibition of the O(2)-evolution reaction. However, electron transfer from the donor to acceptor side of PSII, measured as the reduction of the exogenous acceptor 2,6-dichlorophenolindophenol (DCIP) under continuous light, is inhibited by only 30%. Thus, calcium extraction from the OEC inhibits the synthesis of molecular O(2) but not the oxidation of a substrate we term X, the source of electrons for DCIP reduction. The presence of electron transfer across PSII(-Ca/NaCl) membranes was demonstrated using fluorescence induction kinetics, a method that does not require an artificial acceptor. The calcium chelator, EGTA (5 mM), when added to PSII(-Ca/NaCl) membranes, does not affect the inhibition of O(2) evolution by NaCl but does inhibit DCIP reduction up to 92% (the reason why electron transport in Ca(2+)-depleted materials has not been noticed before). Another chelator, sodium citrate (citrate/low pH method of calcium extraction), also inhibits both O(2) evolution and DCIP reduction. The role of all buffer components (including bicarbonate and sucrose) as possible sources of electrons for PSII(-Ca/NaCl) membranes was investigated, but only the absence of chloride anions strongly inhibited the rate of DCIP reduction. Substitution of other anions for chloride indicates that Cl(-) serves its well-known role as an OEC cofactor, but it is not substrate X. Multiple turnover flash experiments have shown a period of four oscillations of the fluorescence yield (both the maximum level, F(max), and the fluorescence level measured 50 s after an actinic flash in the presence of DCMU) in native PSII membranes, reflecting the normal function of the OEC, but the absence of oscillations in PSII(-Ca/NaCl) samples. Thus, PSII(-Ca/NaCl) samples do not evolve O(2) but do transfer electrons from the donor to acceptor sides and exhibit a disrupted S-state cycle. We explain these results as follows. In Ca(2+)-depleted PSII membranes, obtained without chelators, the oxidation of the OEC stops after the absorption of three quanta of light (from the S1 state), which should convert the native OEC to the S4 state. An one-electron oxidation of the water molecule bound to the Mn cluster then occurs (the second substrate water molecule is absent due to the absence of calcium), and the OEC returns to the S3 state. The appearance of a sub-cycle within the S-state cycle between S3-like and S4-like states supplies electrons (substrate X is postulated to be OH(-)), explains the absence of O(2) production, and results in the absence of a period of four oscillation of the normal functional parameters, such as the fluorescence yield or the EPR signal from S2. Chloride anions probably keep the redox potential of the Mn cluster low enough for its oxidation by Y(Z)(*).

Azide as a probe of proton transfer reactions in photosynthetic oxygen evolution

In oxygenic photosynthesis, photosystem II (PSII) is the multisubunit membrane protein responsible for the oxidation of water to O2 and the reduction of plastoquinone to plastoquinol. One electron charge separation in the PSII reaction center is coupled to sequential oxidation reactions at the oxygen-evolving complex (OEC), which is composed of four manganese ions and one calcium ion. The sequentially oxidized forms of the OEC are referred to as the S(n) states. S(1) is the dark-adapted state of the OEC. Flash-induced oxygen production oscillates with period four and occurs during the S(3) to S(0) transition. Chloride plays an important, but poorly understood role in photosynthetic water oxidation. Chloride removal is known to block manganese oxidation during the S(2) to S(3) transition. In this work, we have used azide as a probe of proton transfer reactions in PSII. PSII was sulfate-treated to deplete chloride and then treated with azide. Steady state oxygen evolution measurements demonstrate that azide inhibits oxygen evolution in a chloride-dependent manner and that azide is a mixed or noncompetitive inhibitor. This result is consistent with two azide binding sites, one at which azide competes with chloride and one at which azide and chloride do not compete. At pH 7.5, the K(i) for the competing site was estimated as 1 mM, and the K(i)' for the uncompetitive site was estimated as 8 mM. Vibrational spectroscopy was then used to monitor perturbations in the frequency and amplitude of the azide antisymmetric stretching band. These changes were induced by laser-induced charge separation in the PSII reaction center. The results suggest that azide is involved in proton transfer reactions, which occur before manganese oxidation, on the donor side of chloride-depleted PSII.

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

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

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.

Tyrosine-Z in oxygen-evolving photosystem II: a hydrogen-bonded tyrosinate

In oxygen-evolving photosystem II (PSII), a tyrosine residue, D1Tyr161 (YZ), serves as the intermediate electron carrier between the catalytic Mn cluster and the photochemically active chlorophyll moiety P680. A more direct catalytic role of YZ, as a hydrogen abstractor from bound water, has been postulated. That YZox appears as a neutral (i.e. deprotonated) radical, YZ*, in EPR studies is compatible with this notion. Data based on electrochromic absorption transients, however, are conflicting because they indicate that the phenolic proton remains on or near to YZox. In Mn-depleted PSII the electron transfer between YZ and P680+ can be almost as fast as in oxygen-evolving material, however, only at alkaline pH. With an apparent pK of about 7 the fast reaction is suppressed and converted into an about 100-fold slower one which dominates at acid pH. In the present work we investigated the optical difference spectra attributable to the transition YZ --> YZox as function of the pH. We scanned the UV and VIS range and used Mn-depleted PSII core particles and also oxygen-evolving ones. Comparing these spectra with published in vitro and in vivo spectra of phenolic compounds, we arrived at the following conclusions: In oxygen-evolving PSII YZ resembles a hydrogen-bonded tyrosinate, YZ(-).H(+).B. The phenolic proton is shifted toward a base B already in the reduced state and even more so in the oxidized state. The retention of the phenolic proton in a hydrogen-bonded network gives rise to a positive net charge in the immediate vicinity of the neutral radical YZ*. It may be favorable both for the very rapid reduction by YZ of P680+ and for electron (not hydrogen) abstraction by YZ* from the Mn-water cluster.

S-state dependence of chloride binding affinities and exchange dynamics in the intact and polypeptide-depleted O2 evolving complex of photosystem II

The Cl- binding properties in the successive oxidation states of the O2 evolving complex of photosystem II were investigated by measurements of UV absorbance changes, induced by a series of saturating flashes, that monitor manganese oxidation state transitions. In dark-adapted, intact photosystem II, Cl- can be replaced by NO3- in minutes, in an exchange reaction that depends on the NO3- concentration and that is not rate-limited by dissociation of Cl- from its binding site. Preillumination of dark-adapted photosystem II by one or two flashes accelerated the NO3- substitution reaction by an order of magnitude. A quantitative analysis of the Cl- concentration dependence of UV absorbance changes, measured in photosystem II preparations depleted of extrinsic 17 and 23 kDa polypeptides, shows that the Cl- binding properties of photosystem II change with the oxidation state of the oxygen evolving complex. Although the affinity for the individual S-states could not be determined with precision, it is shown that the affinity is an order of magnitude lower in the S2 state than in the S1 state. Comparison of the results obtained using intact photosystem II and preparations depleted of the 17 and 23 kDa extrinsic polypeptides suggests that these proteins constitute a diffusion barrier, which prevents fast equilibration of the Cl- binding site with the medium, but does not change the Cl- affinity of the binding site.

Kinetic and spectroscopic properties of the YZ* radical in Ca2+- and Cl--depleted photosystem II preparations

Depletion of Ca2+ and/or Cl- ions from PSII membranes blocks the electron-transfer reactions that precede O2 evolution on the oxidizing side of the enzyme. Illumination of these inhibited preparations at 273 K generates a paramagnetic species that is detectable by low-temperature (T < 20 K) EPR as a signal in the g = 2 region, 90-230 G wide, depending on the treatment that PSII has undergone. This signal has recently been assigned to YZ* in magnetic interaction with the manganese cluster in its S2 state [Gilchrist et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9545-9549]. This view, however, is not universal, owing, in part, to the fact that its spectroscopic properties depend on the preparation and the experimental conditions used for its study and, in part, to uncertainties as to the room temperature behavior of YZ* in inhibited preparations. Here, we report time-resolved and conventional EPR data showing that, at room temperature and at 273 K, YZ* can be accumulated in its 20 G form in high yields in both Ca2+-depleted and acetate-inhibited preparations, and that the kinetics of its decay match the decay kinetics of the low-temperature signal generated in corresponding samples. The properties of the YZ* signal, however, are shown to depend on the polypeptide content, the temperature, and the electron donors and acceptors present in the sample under examination. Our results support assignment of the EPR signal in inhibited preparations to S2 YZ* and demonstrate a protective role of the 17 and 23 kDa extrinsic polypeptides for the manganese cluster against externally added reductants.

Thermoluminescence measurements on chloride-depleted and calcium-depleted photosystem II

Photosystem II (PSII) in which O2 evolution was inhibited by depletion of either chloride or calcium ions was studied by thermoluminescence (TL) and luminescence (L) measurements in the presence and absence of 3-(3',4'-dichlorophenyl)-1,1-dimethyl urea (DCMU). Cl--depleted PSII gives rise to TL and L signals which are similar to those in untreated controls i.e., DCMU shifts the TL band from 30 degreesC to 8 degreesC and suppresses the L component with t1/2=10-15 s. In Ca2+-depleted PSII a TL-band at around 50 degreesC and a slow luminescence decay (t1/2=60 s) is observed. Under these conditions, DCMU does not lead to a downshift of the peak temperature of the TL-band nor does it accelerate the decay kinetics of the luminescence. This indicates that in Ca2+-depleted PSII the QA/QB electron transfer is inhibited prior to the addition of DCMU while in Cl--depleted PSII QA/QB electron transfer seems unaffected. These results are consistent with previous fluorescence measurements which showed that the midpoint potential of the redox couple QA/QA- is unchanged in Cl--depleted PSII compared to the control while in Ca2+-depleted PSII it is shifted towards a more positive value [A. Krieger, A.W. Rutherford, Biochim. Biophys. Acta, 1319 (1997) 91-98]. In the literature there are several conflicting reports concerning the TL in Ca2+ and Cl--depleted material so we attempted to understand the origin of some of these discrepancies. We find that in the absence of cryoprotectants, excitation of TL at low temperatures leads to an upshift of TL-bands in Cl--depleted PSII, both in the presence and absence of DCMU, while the peak temperature of TL-bands in control and Ca2+-depleted PSII are downshifted. When TL is excited at 20 degreesC or at low temperature in the presence of a cryoprotectant then there was no shift of the peak temperature of TL-bands. These unexpected results suggest that the formation of the charge pair triggers modifications in its environment and that the exact nature of these modifications differs depending on the temperature of excitation. It seems that once these modifications have occurred at a given temperature they remain 'locked in' being unaffected by subsequent temperature changes until charge recombination has occurred. Copyright 1998 Elsevier Science B.V.

The tetranuclear manganese cluster in photosystem II: location and magnetic properties of the S2 state as determined by saturation-recovery EPR spectroscopy

The spin-lattice relaxation enhancement of the dark-stable tyrosine radical, YD., by the S2 state of the O2-evolving complex (OEC) of photosystem II (PSII) has been measured by using saturation-recovery EPR spectroscopy. Two forms of the S2 state have been compared: the multiline EPR signal species in untreated PSII and the altered multiline EPR signal species in NH3-treated PSII. Previous work has shown that the non-single-exponential spin-lattice relaxation kinetics of YD. in S2-state PSII result from a dipole-dipole interaction with the Mn4 cluster of the OEC. By taking into account the temperature variation of the effective magnetic moment of the S2-state multiline EPR signal form of the OEC, we provide a quantitative analysis of its temperature-dependent enhancement of the spin-lattice relaxation of YD.. Different spin states of the Mn4 cluster in the S2 state are responsible for the effect at different temperature regimes: for T </= 10 K, it is the ground spin state (S = 1/2); for T >/= 30 K, it is the first excited spin state; and at intermediate temperatures, the contributions of the two spin states are comparable. The relaxation enhancement of YD. is equivalent for both forms of the S2-state multiline EPR signal examined, indicating that the magnetic properties of the Mn4 cluster are very similar in the S2 state for both untreated and NH3-treated PSII. EPR progressive microwave-power saturation has also been used to assess the spin-lattice relaxation properties of the Mn4 cluster giving the altered S2-state multiline EPR signal in the NH3 derivative of PSII. The Orbach mechanism is shown to provide the dominant relaxation pathway; the energy difference between the ground and first excited spin states is estimated to be 30 +/- 2 cm-1, which is very similar to the value found for the S2-state multiline EPR signal species in untreated PSII. Below 4 K, the effectiveness of the S2-state multiline EPR signal species as a spin relaxation enhancer of YD. drops dramatically. This is interpreted to occur because of temperature-dependent 55Mn nuclear spin-lattice relaxation which causes averaging of the effective Larmor frequency of the S2-state multiline EPR signal species during the time scale for spin-lattice relaxation of YD.; because the line shape of the S2-state multiline EPR signal is dominated by isotropic 55Mn nuclear hyperfine splittings, such nuclear relaxation processes allow frequencies in near resonance with that of YD. to be accessed, thereby producing a greater relaxation enhancement. By using a dipolar model that includes the line shapes of both the YD. and S2-state multiline EPR signals, the spin-lattice relaxation enhancement of YD. is analyzed to obtain a lower limit of 22 A for the distance between YD. and the OEC. Together with recent studies showing a close proximity of the Mn4 cluster to YZ., these results provide further support for an asymmetric location of the Mn4 cluster with respect to the two redox-active tyrosines in PSII.

The photosynthetic oxygen evolving complex requires chloride for its redox state S2-->S3 and S3-->S0 transitions but not for S0-->S1 or S1-->S2 transitions

The Cl- requirement in the redox cycle of the oxygen-evolving complex (OEC) was determined by measurements of flash-induced UV absorbance changes in Cl(-)-depleted and Cl(-)-reconstituted photosystem II membranes. On the first flash after dark adaptation the spectrum and amplitude of those changes, known to reflect the oxidation of MnIII to MnIV on the S1-->S2 transition, were the same in the presence or absence of Cl-. On the second and later flashes, however, absorbance changes in Cl(-)-depleted samples revealed only electron transfer from tyrosine to quinone which reversed slowly in the dark by charge recombination and did not produce the S3-state. A rapid method was developed to remove Cl- after producing the S3-state by two flashes. The lifetime of the S3-state was found to be unaffected by Cl(-)-depletion, in contrast to the 20-fold stabilization of the S2 lifetime by Cl- removal, and the Cl(-)-depleted S3-state did not proceed to S0 on flash illumination. However, when the same Cl(-)-depletion procedure was applied after producing the S0-state by three flashes, further advance to S2 by two additional flashes was not impaired by the absence of Cl-. The requirement for Cl- only on the S2-->S3 and S3-->S0 transitions can be rationalized by the hypothesis that Cl- is required for electron transfer between manganese ions within the oxygen-evolving complex.

The role of the extrinsic 33 kDa protein in Ca2+ binding in photosystem II

The role of the 33 kDa protein in Ca2+ binding was studied by comparing the EPR properties of photosystem II in the presence and absence of the 33 kDa protein and Ca2+. When the removal of the 33 kDa protein was carried out in the dark, a normal manganese multiline EPR signal could be observed when the S2 state was generated. In addition, the split S3 signal could not be generated by illumination at 273 K. Exposure of the 33 kDa protein-less photosystem II to room light did not lead to any change in the EPR properties of the S2 state, but the split S3 state signal at around g = 2 could then be generated, indicating that Ca2+ was released from this preparation during the exposure to light. Treatment of photosystem II lacking the 33 kDa protein with EGTA in the light led to a modification of the S2 state characterized by a dark-stable multiline EPR signal. Much lower EGTA concentrations were required in order to obtain this modification in the absence of the 33 kDa protein than was required when the 33 kDa protein was present. This indicates that the manganese cluster was more accessible to chelator binding when the 33 kDa protein was absent. When 33 kDa protein-less photosystem II was treated with EGTA in the dark, no modification of the multiline EPR signal of the S2 state of the manganese cluster occurred, nor was Ca2+ released as monitored by the inability to generate the split S3 signal. These chelator- and Ca(2+)-binding properties occurring in PSII lacking the 33 kDa protein are very similar to those observed previously in NaCl-washed PSII in which the 33 kDa protein is present (reviewed in Boussac & Rutherford, 1994a). It is concluded that the 33 kDa protein has little or no direct role in binding the Ca2+ ion which is required for oxygen evolution.

Involvement of the donor tyrosine-D1 (Y d) in Photosystem II electron transport in the green alga, Chlamydobotrys stellata

The light-induced oxidation of the accessory donor tyrosine-D (YD) has been studied by measurements of the EPR Signal IIslow at room temperature in the autotrophically and photoheterotrophically cultivated alga Chlamydobotrys stellata. After illumination and dark adaptation, YD Signal IIslow was observed only in autotrophic algae, i.e. under conditions of a linear photosynthetic electron transfer from water to NADP(+). The addition of artificial electron acceptors phenyl-p-benzoquinone (PPQ) or dichloro-p-benzoquinone (DCQ) to the autotrophic cells caused an almost negligible increase of this signal. When photosynthetic electron flow and oxygen evolution were diminished by removal of the carbon source CO2 and addition of acetate (photoheterotrophy), a pronounced YD Signal IIslow was seen only in presence of DCQ or PPQ. Several possibilities are discussed to explain the absence of YD Signal IIslow in photoheterotrophic Chl. stellata such as the existence of a cyclic PS II electron flow very effectively reducing P680 and thereby preventing the possibility of YD oxidation. Artificial electron acceptors withdraw electrons from this cycle thus keeping the primary quinone acceptor, QA, oxidized and thereby diminishing the reduction of P680 (+) by cyclic PSII. This leads to the appearance of the YD Signal IIslow also in the photoheterotrophically grown algae.

Properties of the chloride-depleted oxygen-evolving complex of photosystem II studied by electron paramagnetic resonance

The effects of different Cl- depletion treatments in photosystem II (PS-II)-enriched membranes have been investigated by electron paramagnetic resonance (EPR) spectroscopy and by measurements of oxygen-evolving activity. The results indicated that the oxygen-evolving complex of PS-II exhibits two distinct Cl(-)-dependent properties. (1) After Cl(-)-free washes at pH 6.3, a reversibly altered distribution of structural states of PS-II was observed, manifested as the appearance of a g = 4 EPR signal from the S2 state in a significant fraction of centers (20-40%) at the expense of the S2 multiline signal. In addition, small but significant changes in the shape of the S2 multiline EPR signal were observed. Reconstitution of Cl- to Cl(-)-free washed PS-II rapidly reversed the observed effects of the Cl(-)-free washing. The anions, SO4(2-) and F-, which are often used during Cl- depletion treatments, had no effect on the S2 EPR properties of PS-II under these conditions in the absence or presence of Cl-. Flash experiments and measurements of oxygen evolution versus light intensity indicated that the two structural states observed after the removal of Cl- at pH 6.3 originated from oxygen-evolving centers exhibiting a lowered quantum yield of water oxidation. (2) Depletion of Cl- in PS-II by pH 10 treatment reversibly inhibited the oxygen-evolving activity to approximately 15%. The pH 10 treatment depleted the Cl- from a site which is considered to be equivalent to that studied in most earlier work on Cl(-)-depleted PS-II. The S2 state in pH 10/Cl(-)-depleted PS-II was reversibly modified to a state from which no S2 multiline EPR signal was generated and which exhibited an intense S2 g = 4 EPR signal corresponding to at least 40% of the centers but possibly to a much larger fraction of centers. The state responsible for the intense S2 g = 4 signal generated under these conditions is unlike that observed after removal of Cl- from PS-II at pH 6.3, in that this state was more stable in the dark, showing a half-decay time of approximately 1.5 h at 0 degrees C, and was unable to undergo further charge accumulation. Nevertheless, a fraction of centers, probably different from those exhibiting the S2 g = 4 signal, was able to advance to the formal S3 state, giving rise to a narrow EPR signal around g = 2. Addition of the anions SO4(2-) or F- to pH 10/Cl(-)-depleted PS-II affected the properties of PS-II, resulting in EPR properties of the S2 state similar to those reported earlier following Cl- depletion treatment of PS-II in the presence of these anions. Surprisingly, after addition of F-, the g = 4 EPR signal showed a damped flash-dependent oscillation. In addition, a narrow signal around g = 2, corresponding to the formal S3 state, also showed a damped flash-dependent oscillation pattern. The presence of oscillating EPR signals (albeit damped) in F(-)-treated pH 10/Cl(-)-depleted PS-II indicates functional enzyme turnover. This was confirmed by measurements of the oxygen-evolving activity versus light intensity which indicated that in approximately 45% of oxygen-evolving centers the enzyme turnover was slowed by a factor of 2. The distinct Cl- depletion effects in PS-II observed under the two different Cl- depletion treatments are considered to reflect the presence of two distinct Cl(-)-binding sites in PS-II.