Kinetics of EPR signal IIvf in chloroplast Photosystem II

The risetime of EPR signal II_vf (S II_vf) has been measured in oxygen-evolving Photosystem II particles from spinach chloroplasts at pH 6.0. The EPR signal shows an instrument-limited rise upon induction ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? 3 μ}\text{s}$). These data are consistent with a model where the species Z responsible for S II_vf is the immediate electron donor to P-680⁺ in spinach chloroplasts. A new, faster decay component of S II_vf has also been detected in these experiments.     

Sulfhydryl reagents inhibit electron transport in Photosystem II of spinach chloroplasts

A 120 min incubation period with sulfhydryl reagents, such as p-chloromercuribenzoic acid, shows greater than 50% loss of electron-transport activity in Photosystem (PS) II of spinach chloroplasts. Since p-chloromercuriphenylsulfonic acid, a nonpenetrating sulfhydryl reagent, and 4,4′-dithiopyridine, a bifunctional sulfhydryl reagent, show greater inhibition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive silicomolybdate reduction than of dibromothymoquinone-insensitive indophenol reduction, it is postulated that two different sulfhydryl reagent-sensitive sites are involved in the PS II electron-transport chain of spinach chloroplasts.     

The effects of mono- and divalent salts on the O2-evolution activity and low temperature multiline EPR spectrum of Photosystem II preparations from spinach

The ability of salts to inhibit the O₂-evolution activity of PS II preparations is shown to parallel closely the Hofmeister series, suggesting that inhibition is related to the solubility of the 16, 24 and 33 kDa proteins in these salt solutions. An examination of the effect of salt inactivation on the low temperature multiline EPR signal indicates that the release of either the 16 and 24 kDa proteins, or additionally the 33 kDa protein blocks or greatly reduces the efficiency of the advancement of the water-splitting complex to the S₂-state; under some conditions, this inhibition is reversible.     

Oxygen-evolution patterns from spinach Photosystem II preparations

Patterns of O₂ evolution resulting from sequences of short flashes are reported for Photosystem (PS) II preparations isolated from spinach and containing an active, O₂-evolving system. The results can be interpreted in terms of the S-state model developed to explain the process of photosynthetic water splitting in chloroplasts and algae. The PS II samples display damped, oscillating patterns of O₂ evolution with a period of four flashes. Unlike chloroplasts, the flash yields of the preparations decay with increasing flash number due to the limited plastoquinone acceptor pool on the reducing side of PS II. The optimal pH for O₂ evolution in this system (pH 5.5–6.5) is more acidic than in chloroplasts (pH 6.5–8.0). The O₂-evolution, inactivation half-time of dark-adapted preparations was 91 min (on the rate electrode) at room temperature. Dark-inactivation half-times of 14 h were observed if the samples were aged off the electrode at room temperature. Under our conditions (experimental conditions can influence flash-sequence results), deactivation of S₃ was first order with a half-time of 105 s while that of S₂ was biphasic. The half-times for the first-order rapid phase were 17 s (one preflash) and 23 s (two preflashes). The longer S₂ phase deactivated very slowly (the minimum half-time observed was 265 s). These results indicate that deactivation from S₃ → S₂ → S₁, thought to be the dominant pathway in chloroplasts, is not the case for PS II preparations. Finally, it was demonstrated that the ratio of S₁ to S₀ can be set by previously developed techniques, that S₀ is formed mostly from activated S₃ (S₄), and that both S₀ and S₁ are stable in the dark.     

The detection,isolation and characterization of a light-harvesting complex which is specifically associated with Photosystem I

10% of the chlorophyll associated with a ‘native’ Photosystem (PS) I complex (110 chlorophylls/P-700) is chlorophyll (Chl) b. The Chl b is associated with a specific PS I antenna complex which we designate as LHC-I (i.e., a light-harvesting complex serving PS I). When the native PS I complex is degraded to the core complex by LHC-I extraction, there is a parallel loss of Chl b, fluorescence at 735 nm, together with 647 and 686 nm circular dichroism spectral properties, as well as a group of polypeptides of 24-19 kDa. In this paper we present a method by which the LHC-I complex can be dissociated from the native PS I. The isolated LHC-I contains significant amounts of Chl b (Chl $\text{a}\text{b ? 3.7}$). The long-wavelength fluorescence at 730 nm and circular dichroism signal at 686 nm observed in native PS I are maintained in this isolated complex. This isolated fraction also contains the low molecular weight polypeptides lost in the preparation of PS I core complex. We conclude that we have isolated the PS I antenna in an intact state and discuss its in vivo function.     

Purification of cytochrome b-559 from oxygen-evolving Photosystem II preparations of spinach and maize

A rapid and simple procedure is presented for the purification of chloroplast cytochrome b-559. The method is based on the protocol devised by Garewal and Wasserman (Garewal, H.S. and Wasserman, A.R. (1974) Biochemistry 13, 4063–4071), which we have modified to eliminate the requirement for a lengthy electrophoretic step. Novel features of our method include: the use of oxygen-evolving Photosystem II preparations (Kuwabara, T. and Murata, N. (1982) Plant Cell Physiol. 23, 533–539) as the starting material; isocratic elution of cytochrome b-559 from a DEAE-cellulose column (yielding the protein in a pure state); and a simple column procedure for removal of excess Triton X-100. The procedure has been applied to both spinach and maize (Zea mays L.). Purified cytochromes b-559 from these species have similar optical spectra and mobility during gel electrophoresis under native conditions. Lithium dodecyl sulfate polyacrylamide gel electrophoresis of cytochrome b-559 from both spinach and maize reveals a major polypeptide band (apparent molecular mass = 9 kDa), and two minor bands (apparent molecular masses = 10 kDa and 6 kDa).     

Oxidation-reduction properties of the electron acceptors of Photosystem II I. Redox titration of the flash-induced carotenoid band shift,of C550 and of the variable fluorescence yield in spinach chloroplasts

Redox titration of the electrochromic carotenoid band shift, detected at 50 μs after a saturating actinic flash, in spinach chloroplasts, shows that only one electron acceptor in Photosystem II participates in a transmembrane primary electron transfer. This species, the primary quinone acceptor, Q, shows only one midpoint potential (E_m,7.5) of approx. 0 V and is undoubtedly equivalent to the fluorescence quencher, Q_H. A second titration wave is observed at low potential ($\text{E}{}_{\mathrm{<mtext>m</mtext><mtext>,7.5</mtext>}}\text{? ? 240}\text{mV}$) and at greater than 3 ms after a saturating actinic flash. This wave has an action spectrum different from that of Photosystem II centers containing Q and could arise from a secondary but not primary electron transfer. A low-potential fluorescence quencher is observed in chloroplasts which largely disappears in a single saturating flash at ? 185 mV and which does not participate in a transmembrane electron transfer. This low-potential quencher (probably equivalent to fluorescence quencher, Q_L) and Q are altogether different species. Redox titration of C550 shows that if electron acceptor Q_β is indeed characterized by an E_m,7 of + 120 mV, then this acceptor does not give rise to a C550 signal upon reduction and does not participate in a transmembrane electron transfer. This titration also shows that C550 is not associated with Q_L.     

Evidence for two different herbicide-binding proteins at the reducing side of Photosystem II

The binding behaviour at the thylakoid membrane of the radioactively labelled phenolic inhibitors 2-iodo-4-nitro-6-[2′,3′-³H]isobutylphenol and 3,5-diiodo-4-hydroxy[U-¹⁴C]benzonitrile (ioxynil) has been studied. As judged from displacement experiments with other herbicides, phenolic herbicides and herbicides as represented best by 3-(3,4-dichlorophenyl)-1,1-dimethylurea have different binding sites at the reducing side of Photosystem II. The binding parameters of phenolic herbicides are not, or only slightly, affected by trypsin treatment of chloroplasts. In chloroplasts, besides free pigments, lipids, and the light-harvesting chlorophyll $\text{a}\text{b}$ protein complex, a protein of molecular weight 41 000 is radioactively labelled by the photoaffinity label 4-nitro-2-azido-6-[2′,3′-³H]isobutylphenol. The amount of radioactivity bound to the 41 kDa protein is diminished if chloroplasts are incubated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea prior to addition of the photoaffinity label, but not if the 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol is used instead. These two compounds are characteristic representatives of inhibitiors acting at the reducing or the oxidizing site of plastoquinone, respectively. Based on these results, a model for two different herbicide-binding proteins at the reducing side of Photosystem II is presented.     

EPR studies of the oxygen-evolving enzyme of Photosystem II

The light-induced EPR multiline signal is studied in O₂-evolving PS II membranes. The following results are reported: (1) Its amplitude is shown to oscillate with a period of 4, with respect to the number of flashes given at room temperature (maxima on the first and fifth flashes). (2) Glycerol enhances the signal intensity. This effect is shown to come from changes in relaxation properties rather than an increase in spin concentration. (3) Deactivation experiments clearly indicate an association with the S₂ state of the water-oxidizing enzyme. A signal at g = 4.1 with a linewidth of 360 G is also reported and it is suggested that this arises from an intermediate donor between the S states and the reaction centre. This suggestion is based on the following observations: (1) The g = 4.1 signal is formed by illumination at 200 K and not by flash excitation at room temperature, suggesting that it arises from an intermediate unstable under physiological conditions. (2) The formation of the g = 4.1 signal at 200 K does not occur in the presence of DCMU, indicating that more than one turnover is required for its maximum formation. (3) The g = 4.1 signal decreases in the dark at 220 K probably by recombination with Q^?_AFe. This recombination occurs before the multiline signal decreases, indicating that the g = 4.1 species is less stable than S₂. (4) At short times, the decay of the g = 4.1 signal corresponds with a slight increase in the multiline S₂ signal, suggesting that the loss of the g = 4.1 signal results in the disappearance of a magnetic interaction which diminishes the multiline signal intensity. (5) Tris-washed PS II membranes illuminated at 200 K do not exhibit the signal.     

Optical characterization of Photosystem II electron donors

Detailed absorbance difference spectra are reported for the Photosystem II acceptor Q, the secondary donor Z, and the donor involved in photosynthetic oxygen evolution which we call M. The spectra of Z and Q could be resolved by analysis of flash-induced kinetics of prompt and delayed fluorescence, EPR signal II_f and absorbance changes in Tris-washed system II preparations in the presence of ferricyanide and 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU). The spectrum of Z oxidation consists mainly of positive bands at 260, 300 and 390–450 nm on which a chlorophyll a band shift around 438 nm is superimposed, and is largely pH-independent as is also the case for the spectrum of Q reduction. The re-reduction of Z⁺ occurred in the millisecond time range, and could be explained by a competition between back reaction with Q^? (120 ms at pH 6.0) and reduction by ferrocyanide. When the Tris treatment is omitted the preparations evolve oxygen, and the photoreduction of Q (with DCMU present) is accompanied by the oxidation of M. The Q spectrum being known, the spectrum of the oxidation of M could be determined as well. It consists of a broad, asymmetric increase peaking near 305 nm and of a Chl a band shift, which is about the same as that accompanying Z in Tris-washed system II. Comparison with spectra of model compounds suggests that Z is a bound plastoquinol which is oxidized to the semiquinone cation and that the oxidation of M is an Mn(III) → Mn(IV) transition.     

Similarity of EPR Signal IIf rise and P-680+ decay kinetics in Tris-washed chloroplast Photosystem II preparations as a function of pH

The rise time, of Signal II_f and the decay time of P-680⁺ have been measured kinetically as a function of pH by using EPR. The Photosystem II-enriched preparations which were used as samples were derived from spinach chloroplasts, and they evolved oxygen before Tris washing. The onset kinetics of Signal II_f are in agreement, within experimental error, with the fast component of the decay of an EPR signal attributable to P-680⁺. The signal II_f rise kinetics also show good agreement with published values of the pH dependence of the decay of P-680⁺ measured optically (Conjeaud, H. and Mathis, P. (1980) Biochim. Biophys. Acta 590, 353–359). These results are consistent with a model where the species Z (or D₁) responsible for Signal II_f is the immediate electron donor to P-680⁺ in tris-washed Photosystem II fragments.     

Characterization of a resolved oxygen-evolving Photosystem II preparation from spinach thylakoids

An oxygen-evolving Photosystem (PS) II preparation was isolated after Triton X-100 treatment of spinach thylakoids in the presence of Mg²⁺. The structural and functional components of this preparation have been identified by SDS-polyacrylamide gel electrophoresis and sensitive spectrophotometric analysis. The main findings were: (1) The concentration of the primary acceptor Q of PS II was 1 per 230 chlorophyll molecules. (2) There are 6 to 7 plastoquinone molecules associated with a ‘quinone-pool’ reducible by Q. (3) The only cytochrome present in significant amounts (cytochrome b-559) occurred at a concentration of 1 per 125 chlorophyll molecules. (4) The only kind of photochemical reaction center complex present was identified by fluorescence induction kinetic analysis as PS II_α. (5) An E_m = ? 10 mV has been measured at pH 7.8 for the primary electron acceptor Q_α of PS II_α. (6) With conventional SDS-polyacrylamide gel electrophoresis, the preparation was resolved into 13 prominent polypeptide bands with relative molecular masses of 63, 55, 51, 48, 37, 33, 28, 27, 25, 22, 15, 13 and 10 kDa. The 28 kDa band was identified as the PS II light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$. In the presence of 2 M urea, however, SDS-polyacrylamide gel electrophoresis showed seven prominent polypeptides with molecular masses of 47, 39, 31, 29, 27, 26 and 13 kDa as well as several minor components. CP I under identical conditions had a molecular mass of 60–63 kDa.     

Structural and functional organization of the photosystems in spinach chloroplasts. Antenna size,relative electron-transport capacity,and chlorophyll composition

The structural and functional organization of the spinach chloroplast photosystems (PS) I, II_α and II_β was investigated. Sensitive absorbance difference spectrophotometry in the ultraviolet (?A₃₂₀) and red (?A₇₀₀) regions of the spectrum provided information on the relative concentration of PS II and PS I reaction centers. The kinetic analysis of PS II and PS I photochemistry under continuous weak excitation provided information on the number (N) of chlorophyll (Chl) molecules transferring excitation energy to PS II_α, PS II_β and PS I. Spinach chloroplasts contained almost twice as many PS II reaction centers compared to PS I reaction centers. The number N_α of chlorophyll (Chl) molecules associated with PS II_α was 234, while N_β = 100 and N_{PS I} = 210. Thus, the functional photosynthetic unit size of PS II reaction centers was different from that of PS I reaction centers. The relative electron-transport capacity of PS II was significantly greater than that of PS I. Hence, under light-limiting green excitation when both Chl a and Chl b molecules are excited equally, the limiting factor in the overall electron-transfer reaction was the turnover of PS I. The Chl composition of PS I, PS II_α and PS II_β was analyzed on the basis of a core Chl a reaction center complex component and a Chl $\text{a}\text{b-}\text{LHC}$ component. There is a dissimilar Chl $\text{a}\text{b-}\text{LHC}$ composition in the three photosystems with 77% of total Chl b associated with PS II_α only. The results indicate that PS II_α, located in the membrane of the grana partition region, is poised to receive excitation from a wider spectral window than PS II_β and PS I.     

Structural and catalytic properties of the oxygen-evolving complex. Correlation of polypeptide and manganese release with the behavior of Z+ in chloroplasts and a highly resolved preparation of the PS II complex

Treatment of intact thylakoid membranes with Triton X-100 at pH 6 produces a preparation of the PS II complex capable of high rates of O₂ evolution. The preparation contains four managanese, one cytochrome b-559, one Signal II_f and one Signal II_s per 250 chlorophylls. By selective manipulation of the preparation polypeptides of approximate molecular weights of 33, 23 and 17 kDa can be removed from the complex. Release of 23 and 17 kDa polypeptides does not release functional manganese. Under these conditions Z⁺ is not readily and directly accessible to an added donor (benzidine) and it appears as if at least some of the S-state transitions occur. Evidence is presented which indicates that benzidine does have increased access to the oxygen-evolving complex in these polypeptide depleted preparations. Conditions which release the 33 kDa species along with Mn and the 23 and 17 kDa polypeptides generate an alteration in the structure of the oxidizing side of PS II, which becomes freely accessible to benzidine. These findings are examined in relationship to alterations of normal S-state behavior (induced by polypeptide release) and a model is proposed for the organization of functional manganese and polypeptides involved in the oxygen-evolving reaction.     

Covalent modification of chloroplast Photosystem II polypeptides by p-nitrothiophenol

Illumination of the chlorophyll $\text{a}\text{b}$ light-harvesting complex in the presence of $\text{p-}\text{nitrothio}\text{[}{}^{14}\text{C]phenol}$ caused quenching of fluorescence emission at 685 nm (77 K) relative to 695 nm and covalent modification of light-harvesting complex polypeptides. Fluorescence quenching saturated with one p-nitrothiophenol bound per light-harvesting complex polypeptide (10–13 chlorophylls); $\text{1}\text{2}$ maximal quenching occurred with one p-nitrothiophenol bound per light-harvesting complex polypeptides (190–247 chlorophylls). This result provides direct evidence for excitation energy transfer between light-harvesting complex subunits which contain 4–6 polypeptides plus 40–78 chlorophylls per complex.Illumination of chloroplasts or Photosystem II (PS II) particles in the presence of $\text{p-}\text{nitrothio}\text{[}{}^{14}\text{C]phenol}$ caused inhibition of PS II activity and labeling of several polypeptides including those of 42–48 kilodaltons previously identified as PS II reaction center polypeptides. In chloroplasts, inhibition of oxygen evolution accelerated p-nitrothiophenol modification reactions; DCMU or donors to PS II decreased p-nitrothiophenol modification. These results are consistent with the hypothesis that accumulation of oxidizing equivalents on the donor side of PS II creates a ‘reactive state’ in which polypeptides of PS II are susceptible to p-nitrothiophenol modification.     

Oxidation-reduction properties of the electron acceptors of Photosystem II. II. Redox titration at various pH values of the flash-induced formation of C550 in Chlamydomonas Photosystem II particles lacking the secondary quinone electron acceptor

Redox titrations of the flash-induced formation of C550 (a linear indicator of Q^?) were performed between pH 5.9 and 8.3 in Chlamydomonas Photosystem II particles lacking the secondary electron acceptor, B. One-third of the reaction centers show a pH-dependent midpoint potential (E_m,7.5) = ? 30 mV) for redox couple $\text{Q}\text{Q}{}^{\mathrm{?}}$, which varies by ?60 mV/pH unit. Two-thirds of the centers show a pH-independent midpoint potential (E_mm = + 10 mV) for this couple. The elevated pH-independent E_m suggests that in the latter centers the environment of Q has been modified such as to stabilize the semiquinone anion, Q^?. The midpoint potentials of the centers having a pH-dependent E_m are within 20 mV of those observed in chloroplasts having a secondary electron acceptor. It appears therefore that the secondary electron acceptor exerts little influence on the E_m of $\text{Q}\text{Q}{}^{\mathrm{?}}$. An EPR signal at g 1.82 has recently been attributed to a semiquinone-iron complex which comprises Q^?. The similar redox behavior reported here for C550 and reported by others (Evans, M.C.W., Nugent, J.H.A., Tilling, L.A. and Atkinson, Y.E. (1982) FEBS Lett. 145, 176–178) for the g 1.82 signal in similar Photosystem II particles confirm the assignment of this EPR signal to Q^?. At below ?200 mV, illumination of the Photosystem II particles produces an accumulation of reduced pheophytin (Ph^?). At ?420 mV Ph^? appears with a quantum yield of 0.006–0.01 which in this material implies a lifetime of 30–100 ns for the radical pair P-680⁺Ph^?.     

Characteristics of the Photosystem II reaction centre. I. Electron acceptors

The EPR characteristics of Photosystem II electron acceptors are described, in membrane and detergent-treated preparations from a mutant of Chlamydomonas reinhardii lacking Photosystem I and photosynthetic ATPase. The relationship between the quinone-iron and pheophytin acceptors is discussed and a heterogeneity of reaction centres is demonstrated such that only a minority of reaction centres were capable of secondary electron donation at temperatures below 100 K. Only these centres were therefore able to stabilise a reduced acceptor below 100 K. Parallel experiments using a barley mutant (viridis zb63) which also lacks Photosystem I, provide similar results indicating that the C. reinhardii system can provide a general model for the Photosystem II electron acceptor complex. The similarity of the system to that of the purple photosynthetic bacteria is discussed.     

A seconds range component of the reoxidation of the primary Photosystem II acceptor,Q. Effects of bicarbonate depletion in chloroplasts

Broken chloroplasts depleted of bicarbonate (HCO^?₃) show 30–50% inhibition of the Hill reaction in low-intensity light. Also, photoreactions excited by repetitive flashes measured by oxygen evolution, ESR signal IIvf, and absorption changes at 680 and 334 nm show inhibition of 30–50%. An effect of HCO^?₃ was sought to explain these phenomena. The decay of chlorophyll a fluorescence yield in the millisecond and seconds range, following a single flash, was observed to be multiphasic with a very slow component of 1–2 s half-time. In HCO^?₃ -depleted samples this component is enhanced 2- or 3-fold. Since this occurs even after one flash, it is suggested that HCO^?₃ affects the Q^? B → QB^? reaction. In this work it is shown that 40% inhibition of oxygen flash yield is relieved to a great extent if the excitation flash rate is decreased from 2 to 0.33 Hz. A measurement of 520 nm absorption change in the presence of ferricyanide, which is proportional to Photosystem II charge separation, shows a similar inhibition that is dependent on flash rate. The maximum amplitude of variable fluorescence yield and 520 nm absorption change after a single flash are unaffected by HCO^?₃ depletion. The dark distribution of oxygen-evolution S-states is found to be shifted to a more reduced configuration in depleted samples. It is concluded that normal charge separation occurs in HCO^?₃ -depleted Photosystem II reaction centers but that a large fraction of Q^? decays so slowly that not all Q^? is reoxidized between flashes given at a rate of 1 or 2 Hz. Thus, a portion of the Photosystem II centers would be closed to photochemistry. There is a reversible effect of HCO^?₃ depletion on the oxygen-evolution system that is observed as a shift in the dark distribution of S-states.     

Measurement of the relative electron-transport capacity of Photosystem I and Photosystem II in spinach chloroplasts

The ratio of Photosystem (PS) II to PS I electron-transport capacity in spinach chloroplasts was compared from reaction-center and steady-state rate measurements. The reaction-center electron-transport capacity was based upon both the relative concentrations of the PS II_α, PS II_β and PS I centers, and the number of chlorophyll molecules associated with each type of center. The reaction-center ratio of total PS II to PS I electron-transport capacity was about 1.8:1. Steady-state electron-transport capacity data were obtained from the rate of light-induced absorbance-change measurements in the presence of ferredoxin-NADP⁺, potassium ferricyanide and 2,5-dimethylbenzoquinone (DMQ). A new method was developed for determining the partition of reduced DMQ between the thylakoid membrane and the surrounding aqueous phase. The ratio of membrane-bound to aqueous DMQH₂ was experimentally determined to be 1.3:1. When used at low concentrations (200 μM), potassium ferricyanide is shown to be strictly a PS I electron acceptor. At concentrations higher than 200 μM, ferricyanide intercepted electrons from the reducing side of PS II as well. The experimental rates of electron flow through PS II and PS I defined a PS II/PS I electron-transport capacity ratio of 1.6:1.     

Stimulation by manganese and other divalent cations of the electron donation reactions of Photosystem II

Using inside-out thylakoid membranes, it has been shown that the oxidation of water and associated reduction of dichlorophenol indophenol is partially inhibited by low concentrations of cation chelators. This inhibition correlates with a removal of two manganese ions per Photosystem II reaction centre. The chelator-induced inhibition was completely reversed by the addition of low levels of Mn²⁺ ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 20 μ}\text{M}$) and higher levels of Mg²⁺ and Ca²⁺ ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 1}\text{mM}$). Other cations were not effective, indicating that the ability to overcome the inhibition did not involve a general electrostatic screening process. The degree of inhibition by chelators was greater at lower light intensities and after treatment with glutaraldehyde. In the presence of glutaraldehyde the stimulatory effect of Mn²⁺ was lost, while pretreatment with Mn²⁺ prevented the glutaraldehyde effect. These results are discussed in terms of conformational changes of the electron donation chains involving cation- (preferentially Mn-) dependent coupling between the oxygen evolving and reaction-centre complexes of Photosystem II.