Kinetic models for the electron donors of photosystem II of photosynthesis

Regulation of light energy conversion between linear and cyclic electron flow within photosystem II controlled by the plastoquinone/quinol redox poise

Ultrapurified Photosystem II complexes crystalize as uniform microcrystals (PSIIX) of unprecedented homogeneity that allow observation of details previously unachievable, including the longest sustained oscillations of flash-induced O₂ yield over > 200 flashes and a novel period-4.7 water oxidation cycle. We provide new evidence for a molecular-based mechanism for PSII-cyclic electron flow that accounts for switching from linear to cyclic electron flow within PSII as the downstream PQ/PQH₂ pool reduces in response to metabolic needs and environmental input. The model is supported by flash oximetry of PSIIX as the LEF/CEF switch occurs, Fourier analysis of O₂ flash yields, and Joliot-Kok modeling. The LEF/CEF switch rebalances the ratio of reductant energy (PQH₂) to proton gradient energy (H⁺_o/H⁺_i) created by PSII photochemistry. Central to this model is the requirement for a regulatory site (Q_C) with two redox states in equilibrium with the dissociable secondary electron carrier site Q_B. Both sites are controlled by electrons and protons. Our evidence fits historical LEF models wherein light-driven water oxidation delivers electrons (from Q_A^-) and stromal protons through Q_B to generate plastoquinol, the terminal product of PSII-LEF in vivo. The new insight is the essential regulatory role of Q_C. This site senses both the proton gradient (H⁺_o/H⁺_i) and the PQ pool redox poise via e^-/H⁺ equilibration with Q_B. This information directs switching to CEF upon population of the protonated semiquinone in the Qc site (Q^-H⁺)_C, while the WOC is in the reducible S2 or S3 states. Subsequent photochemical primary charge separation (P⁺Q_A^-) forms no (QH₂)_B, but instead undergoes two-electron backward transition in which the Q_C protons are pumped into the lumen, while the electrons return to the WOC forming (S1/S2). PSII-CEF enables production of additional ATP needed to power cellular processes including the terminal carboxylation reaction and in some cases PSI-dependent CEF.

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

What are the oxidation states of manganese required to catalyze photosynthetic water oxidation?

Photosynthetic O(2) production from water is catalyzed by a cluster of four manganese ions and a tyrosine residue that comprise the redox-active components of the water-oxidizing complex (WOC) of photosystem II (PSII) in all known oxygenic phototrophs. Knowledge of the oxidation states is indispensable for understanding the fundamental principles of catalysis by PSII and the catalytic mechanism of the WOC. Previous spectroscopic studies and redox titrations predicted the net oxidation state of the S(0) state to be (Mn(III))(3)Mn(IV). We have refined a previously developed photoassembly procedure that directly determines the number of oxidizing equivalents needed to assemble the Mn(4)Ca core of WOC during photoassembly, starting from free Mn(II) and the Mn-depleted apo-WOC complex. This experiment entails counting the number of light flashes required to produce the first O(2) molecules during photoassembly. Unlike spectroscopic methods, this process does not require reference to synthetic model complexes. We find the number of photoassembly intermediates required to reach the lowest oxidation state of the WOC, S(0), to be three, indicating a net oxidation state three equivalents above four Mn(II), formally (Mn(III))(3)Mn(II), whereas the O(2) releasing state, S(4), corresponds formally to (Mn(IV))(3)Mn(III). The results from this study have major implications for proposed mechanisms of photosynthetic water oxidation.

Two tyrosines that changed the world: Interfacing the oxidizing power of photochemistry to water splitting in photosystem II

Photosystem II (PSII), the thylakoid membrane enzyme which uses sunlight to oxidize water to molecular oxygen, holds many organic and inorganic redox cofactors participating in the electron transfer reactions. Among them, two tyrosine residues, Tyr-Z and Tyr-D are found on the oxidizing side of PSII. Both tyrosines demonstrate similar spectroscopic features while their kinetic characteristics are quite different. Tyr-Z, which is bound to the D1 core protein, acts as an intermediate in electron transfer between the primary donor, P(680) and the CaMn₄ cluster. In contrast, Tyr-D, which is bound to the D2 core protein, does not participate in linear electron transfer in PSII and stays fully oxidized during PSII function. The phenolic oxygens on both tyrosines form well-defined hydrogen bonds to nearby histidine residues, His(Z) and His(D) respectively. These hydrogen bonds allow swift and almost activation less movement of the proton between respective tyrosine and histidine. This proton movement is critical and the phenolic proton from the tyrosine is thought to toggle between the tyrosine and the histidine in the hydrogen bond. It is found towards the tyrosine when this is reduced and towards the histidine when the tyrosine is oxidized. The proton movement occurs at both room temperature and ultra low temperature and is sensitive to the pH. Essentially it has been found that when the pH is below the pK(a) for respective histidine the function of the tyrosine is slowed down or, at ultra low temperature, halted. This has important consequences for the function also of the CaMn₄ complex and the protonation reactions as the critical Tyr-His hydrogen bond also steer a multitude of reactions at the CaMn₄ cluster. This review deals with the discovery and functional assignments of the two tyrosines. The pH dependent phenomena involved in oxidation and reduction of respective tyrosine is covered in detail. This article is part of a Special Issue entitled: Photosystem II.

Molecular mechanism of water oxidation in photosynthesis based on the functioning of manganese in two different environments

We present a model of photosynthetic water oxidation that utilizes the property of higher-valent Mn ions in two different environments and the characteristic function of redox-active ligands to explain all known aspects of electron transfer from H(2)O to Z, the electron donor to P680, the photosystem II reaction center chlorophyll a. There are two major features of this model. (i) The four functional Mn atoms are divided into two groups of two Mn each: [Mn] complexes in a hydrophobic cavity in the intrinsic 34-kDa protein; and (Mn) complexes on the hydrophilic surface of the extrinsic 33-kDa protein. The oxidation of H(2)O is carried out by two [Mn] complexes, and the protons are transferred from a [Mn] complex to a (Mn) complex along the hydrogen bond between their respective ligand H(2)O molecules. (ii) Each of the two [Mn] ions binds one redox-active ligand (RAL), such as a quinone (alternatively, an aromatic amino acid residue). Electron transfer occurs from the reduced RAL to the oxidized Z. When the experimental data concerning atomic structure of the water-oxidizing center (WOC), electron transfer between the WOC and Z, the electronic structure of the WOC, the proton-release pattern, and the effect of Cl(-) are compared with the predictions of the model, satisfactory qualitative and, in many instances, quantitative agreements are obtained. In particular, this model clarifies the origin of the observed absorption-difference spectra, which have the same pattern in all S-state transitions, and of the effect of Cl(-)-depletion on the S states.

Radical pair state in photosystem II

A stable light-induced EPR signal is reported in photosystem II particles and in chloroplasts at 5 K. Characteristic spectral features indicate that the signal arises from dipole-dipole interactions of a radical pair triplet state. From its dependence on potential, its relationship to the spin-polarized triplet state, and the redox state of the pheophytin acceptor (Ph) and because it is present in Tris-washed chloroplasts but not in untreated chloroplasts, we conclude that the signal is formed when the reaction center is in the state D(+)P(680)Ph(-) (P(680) is the primary chlorophyll donor and D(+) is an oxidized donor to P(680)). The low-temperature photochemical sequence is thought to occur as follows. (i) Donation from D to the P(680) (+)Ph(-) state occurs at liquid helium temperature in low quantum yield; this reaction is reversible at temperatures above 5 K. (ii) In normal chloroplasts, donation occurs to the D(+)P(680)Ph(-) state, but this does not occur in Tris-washed chloroplasts or in the photosystem II particles at 77 K or lower. (iii) Illumination, at 200 K, of photosystem particles or Tris-washed chloroplasts results in donation to the D(+)P(680)Ph(-) state from another donor. From experiments in the absence of redox mediators and the temperatures dependence of the splitting of the signal, it is suggested that the state D(+)P(680)Ph(-) itself may be the origin of the radical pair triplet signal. The signal has been simulated by assuming the presence of at least two distinct radical pairs that differ slightly in the distance separating the radicals of the pairs. The distance between the radicals of the pair is calculated to be 6-7 A.

Stoichiometry, inhibitor sensitivity, and organization of manganese associated with photosynthetic oxygen evolution

Chloroplast thylakoid membranes isolated in the presence of EDTA retain high rates of O(2) evolution (>/=340 mumol.h(-1).mg chlorophyll(-1)) but contain no Mn(2+) that is detectable by electron paramagnetic resonance (EPR) at room temperature. The total Mn(2+) content of these preparations is 4.6 per 400 chlorophylls; 0.6 Mn(2+) can be released by addition of Ca(2+), a treatment that does not affect O(2) evolution. The remaining Mn(2+) (4 per 400 chlorophylls) appears to be functionally associated with O(2) evolution activity. Inhibition by Tris, NH(2)OH, or heat will release a small fraction of Mn(2+) from these membranes ( approximately 25% with Tris, for example). Addition of Ca(2+) further enhances Mn(2+) release so that for Tris and for NH(2)OH, 2 and 3, respectively, Mn(2+) per 400 chlorophylls are extracted from the O(2)-evolving complex. Based on the microwave power-saturation properties of the EPR signal IIf, which arises from an intermediate electron carrier in the water splitting process, it appears that one of the four Mn(2+) associated with photosystem II is uniquely sensitive to Tris. A new model is proposed for the organization and inhibitor sensitivity of manganese in the O(2)-evolving complex.

On the chlorophyll a fluorescence yield in chloroplasts upon excitation with twin turnover flashes (TTF) and high frequency flash trains

Chlorophyll fluorescence is routinely taken as a quantifiable measure of the redox state of the primary quinone acceptor Q(A) of PSII. The variable fluorescence in thylakoids increases in a single turnover flash (STF) from its low dark level F (o) towards a maximum F (m) (STF) when Q(A) becomes reduced. We found, using twin single turnover flashes (TTFs) that the fluorescence increase induced by the first twin-partner is followed by a 20-30% increase when the second partner is applied within 20-100 micros after the first one. The amplitude of the twin response shows a period-of-four oscillation associated with the 4-step oxidation of water in the Kok cycle (S states) and originates from two different trapped states with a life time of 0.2-0.4 and 2-5 ms, respectively. The oscillation is supplemented with a binary oscillation associated with the two-electron gate mechanism at the PSII acceptor side. The F(t) response in high frequency flash trains (1-4 kHz) shows (i) in the first 3-4 flashes a transient overshoot 20-30% above the F (m) (STF) = 3*F (o) level reached in the 1st flash with a partial decline towards a dip D in the next 2-3 ms, independent of the flash frequency, and (ii) a frequency independent rise to F (m) = 5*F (o) in the 3-60 ms time range. The initial overshoot is interpreted to be due to electron trapping in the S(0) fraction with Q(B)-nonreducing centers and the dip to the subsequent recovery accompanying the reoxidation of the double reduced acceptor pair in these RCs after trapping. The rise after the overshoot is, in agreement with earlier findings, interpreted to indicate a photo-electrochemical control of the chlorophyll fluorescence yield of PSII. It is anticipated that the double exciton and electron trapping property of PSII is advantageous for the plant. It serves to alleviate the depression of electron transport in single reduced Q(B)-nonreducing RCs, associated with electrochemically coupled proton transport, by an increased electron trapping efficiency in these centers.

Flash-induced oxygen evolution in photosynthesis: simple solution for the extended S-state model that includes misses, double-hits, inactivation, and backward-transitions

Flash-induced oxygen evolution in higher plants, algae, and cyanobacteria exhibits damped period-four oscillations. To explain such oscillations, Kok suggested a simple phenomenological S-state model, in which damping is due to empirical misses and double-hits. Here we developed an analytical solution for the extended Kok model that includes misses, double-hits, inactivation, and backward-transitions. The solution of the classic Kok model (with misses and double-hits only) can be obtained as a particular case of this solution. Simple equations describing the flash-number dependence of individual S-states and oxygen evolution in both cases are almost identical and, therefore, the classic Kok model does not have a significant advantage in its simplicity over the extended version considered in this article. Developed equations significantly simplify the fitting of experimental data via standard nonlinear regression analysis and make unnecessary the use of many previously developed methods for finding parameters of the model. The extended Kok model considered here can provide additional insight into the effect of dark relaxation between flashes and inactivation.

Site-directed photosystem II mutants with perturbed oxygen-evolving properties. 1. Instability or inefficient assembly of the manganese cluster in vivo

Several site-directed photosystem II mutants with substitutions at Asp-170 of the D1 polypeptide were characterized by noninvasive methods in vivo. In several mutants, including some that evolve oxygen, a significant fraction of photosystem II reaction centers are shown to lack photooxidizable Mn ions. In this fraction of reaction centers, either the high-affinity site from which Mn ions rapidly reduce the oxidized secondary electron donor, YZ+, is devoid of Mn ions or the Mn ion(s) bound at this site are unable to reduce YZ+. It is concluded that the Mn clusters in these mutants are unstable or are assembled inefficiently in vivo. Mutants were constructed in the unicellular cyanobacterium Synechocystis sp. PCC 6803. The in vivo characterization procedures employed in this study involved measuring changes in the yield of variable chlorophyll a fluorescence following a saturating flash or brief illumination given in the presence of the electron transfer inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, or following each of a series of saturating flashes given in the absence of this inhibitor. These procedures are easily applied to mutants that evolve little or no oxygen, facilitate the characterization of mutants with labile oxygen-evolving complexes, permit photosystem II isolation efforts to be concentrated on mutants having the stablest Mn clusters, and guide systematic spectroscopic studies of isolated photosystem II particles to mutants of particular interest.

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.

Oxygen evolution in photosynthesis: from unicycle to bicycle

Flash-induced oxygen evolution in the thylakoids of plants and algae exhibits damped oscillations with period four. These are well described by the S-state model of Kok et al. [Kok, B., Forbush, B. & McGloin, M. (1970) Photochem. Photobiol. 11, 457-475], with damping provided by empirical misses and double hits in the reaction center of photosystem II. Here we apply a mechanistic interpretation of misses as mainly determined by reaction centers that are inactive at the time of the flash due to the presence of either P+ or QA, according to the electron transfer equilibria on the donor and acceptor sides of the reaction center. Calculation of misses on this basis, using known or estimated values of the equilibrium constants for electron transfer between the S states and tyrosine Yz, between Yz and P680, as well as between the acceptor plastoquinones, allows a natural description of the flash number dependence of oxygen evolution. The calculated misses are different for each flash-induced reaction center transition. Identification of this mechanism underlying the miss factor for each transition leads to the recognition of two different reaction sequence cycles of photosystem II, with different transition probabilities, producing an intrinsic heterogeneity of photosystem II activity.

The role of redox-active amino acids in the photosynthetic water-oxidizing complex

The protein environment can dramatically affect the EPR line shape of tyrosine radicals. The alterations can be caused by: (1) a change in methylene geometry caused by different protein steric constraints; (2) a change in spin density caused by a change in protein environment; or (3) covalent modification of the tyrosine. Any or all of these effects may also be important, in some cases, in control of oxidation potential and chemical reactivity. The new signal that has been observed in the YF161D1 PS II mutant has an approximate 1:3:3:1 lineshape. There is no precedent for a 1:3:3:1 EPR signal from a tyrosine in a powder sample. However, as described above, given the diversity of signals from tyrosine radicals, it is impossible to exclude the possibility that the signal arises from tyrosine on this basis.

Oxygen activation and the conservation of energy in cell respiration

Many of the membrane-associated oxidases that catalyse respiratory reduction of O2 to water simultaneously couple this exergonic reaction to the translocation of protons across the inner mitochondrial membrane, or the cell membrane in prokaryotes, a process by which metabolic energy is conserved for subsequent synthesis of ATP. The molecular mechanism of O2 reduction and its linkage to H+ translocation are now emerging. The bimetallic haem iron-copper reaction centre in this family of enzymes is the critical structure for catalysis of both these processes.

Electron-transfer reactions in manganese-depleted photosystem II

We have used flash-detection optical and electron paramagnetic resonance spectroscopy to measure the kinetics and yield per flash of the photooxidation of cytochrome b559 and the yield per flash of the photooxidation of the tyrosine residue YD in Mn-depleted photosystem II (PSII) membranes at room temperature. The initial charge separation forms YZ+ QA-. Following this, cytochrome b559 is oxidized on a time scale of the same order and with the same pH dependence as is observed for the decay of YZ+; under the conditions of our experiments, the decay of YZ+ is determined by the lifetime of YZ+ QA-. In order to explain this observation, we have constructed a model for electron donation in which YZ+ and P680+ are in redox equilibrium and cytochrome b559 and YD are oxidized via P680+. Using our results, together with data from earlier investigations of the kinetics of electron transfer from YZ to P680+ and charge recombination of YZ+ QA-, we have obtained the first global fit for electron donation in Mn-depleted PSII that accounts for the data over the pH range from 5 to 7.5. From these calculations, we have obtained the intrinsic rate constants of all the electron-donation reactions in Mn-depleted PSII. These rate constants allow us to calculate the free energy difference between YZ+ P680 and YZ P680+, which is found to increase by 47 +/- 4 mV/pH from pH 5 to 6 and is observed to increase more slowly per pH unit for pH greater than 6. An important conclusion of our experimental work is that the rates of photooxidation of cytochrome b559 and YD are determined by the lifetime of the oxidizing equivalent on YZ/P680. Extension of our model to oxygen-evolving PSII samples leads to the prediction that the kinetics and yields of electron donation from cytochrome b559 and YD to P680+ will depend on the S2- or S3-state lifetime.

Studies on the electron transfer from Tyr-161 of polypeptide D-1 to P680(+) in PS II membrane fragments from spinach

The functional connection between redox component Y z identified as Tyr-161 of polypeptide D-1 (Debus et al. 1988) and P680(+) was analyzed by measurements of laser flash induced absorption changes at 830 nm in PS II membrane fragments from spinach. It was found that neither DCMU nor the ADRY agent 2-(3-chloro-4-trifluoromethyl) anilino-3,5-dinitrothiophene (ANT 2p) affects the rate of P680(+) reduction by Y z under conditions where the catalytic site of water oxidation stays in the redox state S1. In contrast to that, a drastic retardation is observed after mild trypsin treatment at pH=6.0. This effect which is stimualted by flash illumination can be largely reversed by Ca(2+). The above mentioned data lead to the following conclusions: (a) the segment of polypeptide D-1 containing Tyr-161 and coordination sites of P680 is not allosterically affected by structural changes due to DCMU binding at the QB-site which is also located in D-1. (b) ANT 2p as a strong protonophoric uncoupler and ADRY agent does not modify the reaction coordinate of P680(+) reduction by Y z , and (c) Ca(2+) could play a functional role for the electronic and vibrational coupling between the redox groups Y z and P680. The electron transport from Y z to P680(+) is discussed within the framework of a nonadiabatic process. Based on thermodynamic considerations the reorganization energy is estimated to be in the order of 0.5 V.

Comparative thermoluminescence study of autotrophically and photoheterotrophically cultivated Chlamydobotrys stellata

Thermoluminescence (TL) from autotrophically and photoheterotrophically cultivated Chlamydobotrys stellata was measured. Strong TL was emitted at 30°C after acetatenutrition of the alga. DCMU enhanced this band, as also did ferricyanide. It also appeared after poisoning of the alga with NH2OH or ANT-2p. These observations suggest that an alternative donor to photosystem II and not the water-splitting system is responsible for the TL + 30 band.

Evidence for a cytochrome f-Rieske protein subcomplex in the cytochrome b6f system from spinach chloroplasts

The cytochrome b6f complex of spinach chloroplasts was prepared with minor modification according to the method of E. Hurt and G. Hauska (1981) Eur. J. Biochem. 117, 591-599) replacing, however, the final ultracentrifugation step by hydroxyapatite chromatography as suggested by M. F. Doyle and C.-A Yu (1985) Biochem. Biophys. Res. Commun. 131, 700-706). The purified complex was partially dissociated by treatment with 4 M urea or 0.1% sodium dodecyl sulfate (SDS) in the absence of reducing agents. A binary subcomplex consisting of cytochrome f and the Rieske iron-sulfur protein was observed under these conditions by three different methods: (a) hydroxyapatite chromatography; (b) extraction with an isopropanol/water/trifluoroacetic acid mixture; and (c) gel filtration in the presence of low SDS concentrations. The subcomplex dissociated into its components by treatment with mercaptoethanol. These results suggest a close interaction of the cytochrome f with the Rieske protein involving SH groups which under reducing conditions leads to complete dissociation of the subcomplex.

Photoreduction of QA, QB, and cytochrome b-559 in an oxygen-evolving photosystem II preparation from the thermophilic cyanobacterium Synechococcus sp

Light-induced absorption changes in an oxygen-evolving photosystem II (PS II) preparation from the thermophilic cyanobacterium Synechococcus sp. were analyzed using continuous illumination which caused the reduction of both QA (first stable quinone electron acceptor) and QB (second quinone electron acceptor of photosystem II). In this photosystem II preparation in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) the amount of QA was estimated to be 1 per 42 chlorophylls. In the absence of DCMU, plastoquinone (1.68 per QA) was photoreduced to plastohydroquinone within a few seconds, indicating that QB is reduced and protonated during this period. An electrochromic band shift centered around 685 nm was observed with and without DCMU. The extent of this band shift caused by QB reduction per electron was about a third or half of that caused by QA reduction. A significant amount of cytochrome b-559 (0.86 per QA) was photoreduced. Only 60% of the photoreduction of cytochrome b-559 was inhibited by a DCMU concentration that inhibited electron transfer beyond QB, indicating that the site of the reduction of cytochrome b-559 is located before the QB site and possibly on the donor side of PS II.

Tyrosine radicals are involved in the photosynthetic oxygen-evolving system

In addition to the reaction-center chlorophyll, at least two other organic cofactors are involved in the photosynthetic oxygen-evolution process. One of these cofactors, called "Z," transfers electrons from the site of water oxidation to the reaction center of photosystem II. The other species, "D," has an uncertain function but gives rise to the stable EPR signal known as signal II. Z+. and D+. have identical EPR spectra and are generally assumed to arise from species with the same chemical structure. Results from a variety of experiments have suggested that Z and D are plastoquinones or plastoquinone derivatives. In general, however, the evidence to support this assignment is indirect. To address this situation, we have developed more direct methods to assign the structure of the Z+./D+. radicals. By selective in vivo deuteration of the methyl groups of plastoquinone in cyanobacteria, we show that hyperfine couplings from the methyl protons cannot be responsible for the partially resolved structure seen in the D+. EPR spectrum. That is, we verify by extraction and mass spectrometry that quinones are labeled in algae fed deuterated methionine, but no change is observed in the line shape of signal II. Considering the spectral properties of the D+. radical, a tyrosine origin is a reasonable alternative. In a second series of experiments, we have found that deuteration of tyrosine does indeed narrow the D+. signal. Extraction and mass spectral analysis of the quinones in these cultures show that they are not labeled by tyrosine. These results eliminate a plastoquinone origin for D+.; we conclude instead that D+., and most likely Z+., are tyrosine radicals.

The tetranuclear manganese complex of Photosystem II

A polynuclear manganese complex functions in Photosystem II both to accumulate oxidizing equivalents and to bind water and catalyze its four-electron oxidation. Recent electron paramagnetic resonance (EPR) spectroscopic studies of the manganese complex show that four manganese ions are required to account for its magnetic properties. The exchange couplings between manganese ions in the S2 state are characteristic of a Mn4O4 "cubane"-like structure. Based on this structure for the manganese complex in the S2 state, as well as a consideration of the known properties of the manganese complex in Photosystem II and the coordination chemistry of manganese, structures are proposed for the five intermediate oxidation states of the manganese complex. A molecular mechanism for the formation of an O-O bond and the displacement of O2 from the S4 state is suggested.

Electron transfer in the water-oxidizing complex of Photosystem II

An overview is presented of secondary electron transfer at the electron donor side of Photosystem II, at which ultimately two water molecules are oxidized to molecular oxygen, and the central role of manganese in catalyzing this process is discussed. A powerful technique for the analysis of manganese redox changes in the water-oxidizing mechanism is the measurement of ultraviolet absorbance changes, induced by single-turnover light flashes on dark-adapted PS II preparations. Various interpretations of these ultraviolet absorbance changes have been proposed. Here it is shown that these changes are due to a single spectral component, which presumably is caused by the oxidation of Mn(III) to Mn(IV), and which oscillates with a sequence +1, +1, +1, -3 during the so-called S0----S1----S2----S3----S0 redox transitions of the oxygen-evolving complex. This interpretation seems to be consistent with the results obtained with other techniques, such as those on the multiline EPR signal, the intervalence Mn(III)-Mn(IV) transition in the infrared, and EXAFS studies. The dark distribution of the S states and its modification by high pH and by the addition of low concentrations of certain water analogues are discussed. Finally, the patterns of proton release and of electrochromic absorbance changes, possibly reflecting the change of charge in the oxygen-evolving system, are discussed. It is concluded that nonstoichiometric patterns must be considered, and that the net electrical charge of the system probably is the highest in state S2 and the lowest in state S1.

Flash-induced oxygen yield sequences during the life cycle of Chlorella

(i) The pattern of O2 flash yields in the first 4 hours of the life cycle cannot be described by the simple Kok model without additional assumptions. (ii) The miss coefficient α in the mature cells in significantly higher than that in the autospores, its change occurring at the expense of the single-hit coefficient β. Computer simulation yielded α values of 0.29 and 0.23 and β values of 0.66 and 0.72 in the light and dark, respectively. (iii) The onset of light at the beginning of the cycle drastically changes the equilibrium distribution of the S states in the dark; the ratio S0/S1 increases from 30/70 to 50/50 in 1 h, and is restored not earlier than in the 6th hour. (iv) In the presence of 1 mmol/l p-benzoquinone, the alga shows pronounced and long-lasting oscillations in the O2 yield sequences, independently of the time of the life cycle. This means that the O2-evolving system itself is always present and equally efficient throughout the life cycle. Limits imposed on its activity (mainly in the first 4 hours) are clearly of an external nature. The redox potential of the inner thylakoid space is presumably involved.