Biosynthetic Ca2+/Sr2+ Exchange in the Photosystem II Oxygen-evolving Enzyme of Thermosynechococcus elongatus

Evidence for intermediate S-states as initial phase in the process of oxygen-evolving complex oxidation

We have analyzed flash-induced period-four damped oscillation of oxygen evolution and chlorophyll fluorescence with the aid of a kinetic model of photosystem II. We have shown that, for simulation of the period-four oscillatory behavior of oxygen evolution, it is essential to consider the so-called intermediate S-state as an initial phase of each of the S(n)-S(n+1), (n = 0, 1, 2, 3) transitions. The intermediate S-states are defined as [S(n)Y(Z)(ox)]-states (n = 0, 1, 2, 3) and are formed with rate constant k(iSn) approximately 1.5 x 10(6) s(-1), which was determined from comparison of theoretical predictions with experimental data. The assumed intermediate S-states shift the equilibrium in reaction P680(+)Y(Z)<-->P680Y(Z)(ox) more to the right and we suggest that kinetics of the intermediate S-states reflects a relaxation process associated with changes of the redox equilibrium in the above reaction. The oxygen oscillation is simulated without the miss and double-hit parameters, if the intermediate S-states, which are not the source of the misses or the double-hits, are included in the simulation. Furthermore, we have shown that the intermediate S-states, together with S(2)Q(A)(-) charge recombination, are prerequisites for the simulation of the period-four oscillatory behavior of the chlorophyll fluorescence.

Oxidative photosynthetic water splitting: energetics, kinetics and mechanism

This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457-476) as a framework, the energetics and kinetics of two different types of reactions comprising the overall process are discussed: (i) P680+* reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yz (ox) induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen. The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available information about the Gibbs energy differences between the individual redox states Si+1 and Si and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and on the active role of the protein in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place at the level of a complexed peroxide.

The PSII calcium site revisited

Oxidation of H2O by photosystem II is a unique redox reaction in that it requires Ca2+ as well as Cl- as obligatory activators/cofactors of the reaction, which is catalyzed by Mn atoms. The properties of the binding site for Ca2+ in this reaction resemble those of other Ca2+ binding proteins, and recent X-ray structural data confirm that the metal is in fact ligated at least in part by amino acid side chain oxo anions. Removal of Ca2+ blocks water oxidation chemistry at an early stage in the cycle of redox reactions that result in O-O bond formation, and the intimate involvement of Ca2+ in this reaction that is implied by this result is confirmed by an ever-improving set of crystal structures of the cyanobacterial enzyme. Here, we revisit the photosystem II Ca2+ site, in part to discuss the additional information that has appeared since our earlier review of this subject (van Gorkom HJ, Yocum CF In: Wydrzynski TJ, Satoh K (eds) Photosystem II: the light-driven water:plastoquinone oxidoreductase), and also to reexamine earlier data, which lead us to conclude that all S-state transitions require Ca2+.

Spare quinones in the QB cavity of crystallized photosystem II from Thermosynechococcus elongatus

The recent crystallographic structure at 3.0 A resolution of PSII from Thermosynechococcus elongatus has revealed a cavity in the protein which connects the membrane phase to the binding pocket of the secondary plastoquinone Q(B). The cavity may serve as a quinone diffusion pathway. By fluorescence methods, electron transfer at the donor and acceptor sides was investigated in the same membrane-free PSII core particle preparation from T. elongatus prior to and after crystallization; PSII membrane fragments from spinach were studied as a reference. The data suggest selective enrichment of those PSII centers in the crystal that are intact with respect to O(2) evolution at the manganese-calcium complex of water oxidation and with respect to the integrity of the quinone binding site. One and more functional quinone molecules (per PSII monomer) besides of Q(A) and Q(B) were found in the crystallized PSII. We propose that the extra quinones are located in the Q(B) cavity and serve as a PSII intrinsic pool of electron acceptors.

Influence of the Redox Potential of the Primary Quinone Electron Acceptor on Photoinhibition in Photosystem II

We report the characterization of the effects of the A249S mutation located within the binding pocket of the primary quinone electron acceptor, Q(A), in the D2 subunit of photosystem II in Thermosynechococcus elongatus. This mutation shifts the redox potential of Q(A) by approximately -60 mV. This mutant provides an opportunity to test the hypothesis, proposed earlier from herbicide-induced redox effects, that photoinhibition (light-induced damage of the photosynthetic apparatus) is modulated by the potential of Q(A). Thus the influence of the redox potential of Q(A) on photoinhibition was investigated in vivo and in vitro. Compared with the wild-type, the A249S mutant showed an accelerated photoinhibition and an increase in singlet oxygen production. Measurements of thermoluminescence and of the fluorescence yield decay kinetics indicated that the charge-separated state involving Q(A) was destabilized in the A249S mutant. These findings support the hypothesis that a decrease in the redox potential of Q(A) causes an increase in singlet oxygen-mediated photoinhibition by favoring the back-reaction route that involves formation of the reaction center chlorophyll triplet. The kinetics of charge recombination are interpreted in terms of a dynamic structural heterogeneity in photosystem II that results in high and low potential forms of Q(A). The effect of the A249S mutation seems to reflect a shift in the structural equilibrium favoring the low potential form.

No evidence from FTIR difference spectroscopy that aspartate-342 of the D1 polypeptide ligates a Mn ion that undergoes oxidation during the S0 to S1, S1 to S2, or S2 to S3 transitions in photosystem II

In the recent X-ray crystallographic structural models of photosystem II, Asp342 of the D1 polypeptide is assigned as a ligand of the oxygen-evolving Mn4 cluster. To determine if D1-Asp342 ligates a Mn ion that undergoes oxidation during one or more of the S0 --> S1, S1 --> S2, and S2 --> S3 transitions, the FTIR difference spectra of the individual S state transitions in D1-D342N mutant PSII particles from the cyanobacterium Synechocystis sp. PCC 6803 were compared with those in wild-type PSII particles. Remarkably, the data show that the mid-frequency (1800-1200 cm-1) FTIR difference spectra of wild-type and D1-D342N PSII particles are essentially identical. Importantly, the mutation alters none of the carboxylate vibrational modes that are present in the wild-type spectra. The absence of significant mutation-induced spectral alterations in D1-D342N PSII particles shows that the oxidation of the Mn4 cluster does not alter the frequencies of the carboxylate stretching modes of D1-Asp342 during the S0 --> S1, S1 --> S2, or S2 --> S3 transitions. One explanation of these data is that D1-Asp342 ligates a Mn ion that does not increase its charge or oxidation state during any of these S state transitions. However, because the same conclusion was reached previously for D1-Asp170, and because the recent X-ray crystallographic structural models assign D1-Asp170 and D1-Asp342 as ligating different Mn ions, this explanation requires that (1) the extra positive charge that develops on the Mn4 cluster during the S1 --> S2 transition be localized on the Mn ion that is ligated by the alpha-COO- group of D1-Ala344 and (2) any increase in positive charge that develops on the Mn4 cluster during the S0 --> S1 and S2 --> S3 transitions be localized on the one Mn ion that is not ligated by D1-Asp170, D1-Asp342, or D1-Ala344. In separate experiments that were conducted with l-[1-13C]alanine, we found no evidence that D1-Asp342 ligates the same Mn ion that is ligated by the alpha-COO- group of D1-Ala344.

Heteronuclear Mn-Ca/Sr complexes, and Ca/Sr EXAFS spectral comparisons with the oxygen-evolving complex of photosystem II

Heterometallic Mn-Ca and Mn-Sr complexes have been prepared and employed as model complexes for Ca and Sr EXAFS spectral comparisons with the Oxygen-Evolving Complex (OEC) of Photosystem II (PS II); these have revealed similarities that support the presence of at least one O atom bridge between the Mn and Ca/Sr in the OEC.

Probing the Functional Role of Ca2+ in the Oxygen-Evolving Complex of Photosystem II by Metal Ion Inhibition

Photosynthetic oxygen evolution in photosystem II (PSII) takes place in the oxygen-evolving complex (OEC) that is comprised of a tetranuclear manganese cluster (Mn4), a redox-active tyrosine residue (YZ), and Ca2+ and Cl- cofactors. The OEC is successively oxidized by the absorption of 4 quanta of light that results in the oxidation of water and the release of O2. Ca2+ is an essential cofactor in the water-oxidation reaction, as its depletion causes the loss of the oxygen-evolution activity in PSII. In recent X-ray crystal structures, Ca2+ has been revealed to be associated with the Mn4 cluster of PSII. Although several mechanisms have been proposed for the water-oxidation reaction of PSII, the role of Ca2+ in oxygen evolution remains unclear. In this study, we probe the role of Ca2+ in oxygen evolution by monitoring the S1 to S2 state transition in PSII membranes and PSII core complexes upon inhibition of oxygen evolution by Dy3+, Cu2+, and Cd2+ ions. By using a cation-exchange procedure in which Ca2+ is not removed prior to addition of the studied cations, we achieve a high degree of reversible inhibition of PSII membranes and PSII core complexes by Dy3+, Cu2+, and Cd2+ ions. EPR spectroscopy is used to quantitate the number of bound Dy3+ and Cu2+ ions per PSII center and to determine the proximity of Dy3+ to other paramagnetic centers in PSII. We observe, for the first time, the S2 state multiline electron paramagnetic resonance (EPR) signal in Dy3+- and Cd2+-inhibited PSII and conclude that the Ca2+ cofactor is not specifically required for the S1 to S2 state transition of PSII. This observation provides direct support for the proposal that Ca2+ plays a structural role in the early S-state transitions, which can be fulfilled by other cations of similar ionic radius, and that the functional role of Ca2+ to activate water in the O-O bond-forming reaction that occurs in the final step of the S state cycle can only be fulfilled by Ca2+ and Sr2+, which have similar Lewis acidities.

Purification, crystallization and X-ray diffraction analyses of the T. elongatus PSII core dimer with strontium replacing calcium in the oxygen-evolving complex

The core complex of photosystem II (PSII) was purified from thermophilic cyanobacterium Thermosynechococcus elongatus grown in Sr(2+)-containing and Ca(2+)-free medium. Functional in vivo incorporation of Sr(2+) into the oxygen-evolving complex (OEC) was confirmed by EPR analysis of the isolated and highly purified SrPSII complex in agreement with the previous study of Boussac et al. [J. Biol. Chem. 279 (2004) 22809-22819]. Three-dimensional crystals of SrPSII complex were obtained which diffracted to 3.9 A and belonged to the orthorhombic space group P2(1)2(1)2(1) with unit cell dimensions of a=133.6 A, b=236.6 A, c=307.8 A. Anomalous diffraction data collected at the Sr K-X-ray absorption edge identified a novel Sr(2+)-binding site which, within the resolution of these data (6.5 A), is consistent with the positioning of Ca(2+) in the recent crystallographic models of PSII [Ferreira et al. Science 303 (2004) 1831-1838, Loll et al. Nature 438 (2005) 1040-1044]. Fluorescence measurements on SrPSII crystals confirmed that crystallized SrPSII was active in transferring electrons from the OEC to the acceptor site of the reaction centre. However, SrPSII showed altered functional properties of its modified OEC in comparison with that of the CaPSII counterpart: slowdown of the Q(A)-to-Q(B) electron transfer and stabilized S(2)Q(A)(-) charge recombination.

Structure and orientation of the Mn4Ca cluster in plant photosystem II membranes studied by polarized range-extended x-ray absorption spectroscopy

X-ray absorption spectroscopy has provided important insights into the structure and function of the Mn(4)Ca cluster in the oxygen-evolving complex of Photosystem II (PS II). The range of manganese extended x-ray absorption fine structure data collected from PS II until now has been, however, limited by the presence of iron in PS II. Using a crystal spectrometer with high energy resolution to detect solely the manganese Kalpha fluorescence, we are able to extend the extended x-ray absorption fine structure range beyond the onset of the iron absorption edge. This results in improvement in resolution of the manganese-backscatterer distances in PS II from 0.14 to 0.09A(.) The high resolution data obtained from oriented spinach PS II membranes in the S(1) state show that there are three di-mu-oxo-bridged manganese-manganese distances of approximately 2.7 and approximately 2.8A in a 2:1 ratio and that these three manganese-manganese vectors are aligned at an average orientation of approximately 60 degrees relative to the membrane normal. Furthermore, we are able to observe the separation of the Fourier peaks corresponding to the approximately 3.2A manganese-manganese and the approximately 3.4A manganese-calcium interactions in oriented PS II samples and determine their orientation relative to the membrane normal. The average of the manganese-calcium vectors at approximately 3.4A is aligned along the membrane normal, while the approximately 3.2A manganese-manganese vector is oriented near the membrane plane. A comparison of this structural information with the proposed Mn(4)Ca cluster models based on spectroscopic and diffraction data provides input for refining and selecting among these models.

Reaction pattern and mechanism of light induced oxidative water splitting in photosynthesis

This mini review is an attempt to briefly summarize our current knowledge on light driven oxidative water splitting in photosynthesis. The reaction leading to molecular oxygen and four protons via photosynthesis comprises thermodynamic and kinetic constraints that require a balanced fine tuning of the reaction coordinates. The mode of coupling between electron (ET) and proton transfer (PT) reactions is shown to be of key mechanistic relevance for the redox turnover of Y(Z) and the reactions within the WOC. The WOC is characterized by peculiar energetics of its oxidation steps in the WOC. In all oxygen evolving photosynthetic organisms the redox state S(1) is thermodynamically most stable and therefore this general feature is assumed to be of physiological relevance. Available information on the Gibbs energy differences between the individual redox states S(i+1) and S(i) and on the activation energies of their oxidative transitions are used to construct a general reaction coordinate of oxidative water splitting in photosystem II (PS II). Finally, an attempt is presented to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and the active role of the protein environment in tuning the local proton activity that depends on time and redox state S(i). The O-O linkage is assumed to take place within a multistate equilibrium at the redox level of S(3), comprising both redox isomerism and proton tautomerism. It is proposed that one state, S(3)(P), attains an electronic configuration and nuclear geometry that corresponds with a hydrogen bonded peroxide which acts as the entatic state for the generation of complexed molecular oxygen through S(3)(P) oxidation by Y(Z)(ox).

Calcium exchange and structural changes during the photosynthetic oxygen evolving cycle

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

Quantitative assessment of intrinsic carbonic anhydrase activity and the capacity for bicarbonate oxidation in photosystem II

On the basis of equilibrium isotopic distribution experiments using (18)O-labeled water, it is generally accepted that water is the sole substrate for O(2) production by photosystem II (PSII). Nevertheless, recent studies indicating a direct interaction between bicarbonate and the donor side of PSII have been used to hypothesize that bicarbonate may have been a physiologically important substrate for O(2) production during the evolution of PSII [Dismukes, G. C., Klimov, V. V., Baranov, S. V., Kozlov, Y. N., DasGupta, J., and Tyryshikin, A. (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 2170-2175]. To test out this hypothesis and to determine whether contemporary oxygenic organisms have the capacity to oxidize bicarbonate, we employed special rapid-mixing isotopic experiments using (18)O/(13)C-labeled bicarbonate to quantify the inherent carbonic anhydrase activity in PSII samples and the potential flux of oxygen from bicarbonate into the photosynthetically produced O(2). The measurements were made on PSII samples prepared from spinach, Thermosynechococcus elongatus, and Arthrospira maxima. For the latter organism, a strain was used that grows naturally in an alkaline, high (bi)carbonate soda lake in Africa. The results reveal that bicarbonate is not the substrate for O(2) production in these contemporary oxygenic photoautotrophs when assayed under single turnover conditions.

Calcium ligation in photosystem II under inhibiting conditions

In oxygenic photosynthesis, PSII carries out the oxidation of water and reduction of plastoquinone. The product of water oxidation is molecular oxygen. The water splitting complex is located on the lumenal side of the PSII reaction center and contains manganese, calcium, and chloride. Four sequential photooxidation reactions are required to generate oxygen from water; the five sequentially oxidized forms of the water splitting complex are known as the Sn states, where n refers to the number of oxidizing equivalents stored. Calcium plays a role in water oxidation; removal of calcium is associated with an inhibition of the S state cycle. Although calcium can be replaced by other cations in vitro, only strontium maintains activity, and the steady-state rate of oxygen evolution is decreased in strontium-reconstituted PSII. In this article, we study the role of calcium in PSII that is limited in water content. We report that strontium substitution or 18OH2 exchange causes conformational changes in the calcium ligation shell. The conformational change is detected because of a perturbation to calcium ligation during the S1 to S2 and S2 to S3 transition under water-limited conditions.

Secondary Quinone in Photosystem II of Thermosynechococcus elongatus: Semiquinone−Iron EPR Signals and Temperature Dependence of Electron Transfer

The secondary quinone acceptor, Q(B), has been studied in photosystem II (PSII) isolated from Thermosynechococcus (T.) elongatus. Thermoluminescence indicated that Q(B) was present in this preparation. An EPR signal observed at low temperature at g = 1.9 was attributed to Fe2+ Q(B)- on the basis of the characteristic period-of-two variations in its intensity depending on the number of laser flashes given at 20 degrees C. When samples showing the Fe2+ Q(B)- signal were illuminated at 77 K, an EPR signal at g = 1.66 appeared with an amplitude proportional to that of the Fe2+ Q(B)- signal. This signal is attributed to the Q(A)- Fe2+ Q(B)- state. While these attributions have been made previously in PSII from other origins, they have remained relatively tentative since the characteristic period-of-two oscillations of Q(B) had not previously been observed. The flash experiments indicated that more than one exchangeable plastoquinone is associated with the isolated PSII. The g = 1.66 signal from the Q(A)- Fe2+ Q(B)- state was used to study the temperature dependence of electron transfer between the two quinones. Electron transfer occurred in half of the centers (after 30 s incubation) at -28 degrees C for Q(A)- to Q(B) but at -58 degrees C for Q(A)- to Q(B)-. This marked difference for the two electron transfer reactions indicates different types of rate-limiting reactions. In the better studied but homologous system, the purple bacterial reaction center, the Q(A)- to Q(B) step is limited by a gating process, while the Q(A)- to Q(B)- step is limited by protonation events. Similar reactions in PSII could give rise to the observed temperature dependence.

Functional and molecular characterization of a peritoneal dialysis model in the C57BL/6J mouse

BACKGROUND

Animal models are important for understanding the physiology and pathophysiology of peritoneal transport during peritoneal dialysis (PD). Mechanistic investigations of rat and rabbit models of PD are mostly based on intervention studies using pharmacologic agents or blocking antibodies. These models may be limited by the time-course, lack of specificity, or side effects of such interventions. Genetically modified mice could provide an attractive alternative to the above models. In this study, we have characterized PD parameters and tested the effect of gender and dialysate volume and/or osmolality in the C57BL/6J mouse.

METHODS

Mice were submitted to a 2-hour peritoneal equilibration test in order to obtain permeability parameters. The expression of the water channel aquaporin-1 (AQP1) and endothelial NO synthase (eNOS) was investigated at the protein (immunoblotting, immunostaining) and mRNA [real-time reverse-transcription-polymerase chain reaction (RT-PCR)] levels. The potential effect of gender on these parameters was also studied.

RESULTS

Exposure of mice to 2 mL of 3.86% glucose dialysate yielded equilibration curves for urea and glucose, a sodium sieving, and a net ultrafiltration (UF) that were remarkably similar to those obtained in rats. The increase in dialysate volume (from 2 mL to 3 mL and 6 mL) resulted in a higher ultrafiltration and, for the highest volume, an increase in the diffusive mass transport coefficient (MTAC) for urea. The increase in dialysate glucose concentration (from 1.36% to 3.86% and 7%) resulted in increased sodium sieving and higher UF, whereas the MTAC for urea was unchanged. In comparison with males, females had a similar peritoneal transport rate for small solutes but a significantly lower sodium sieving, reflecting a lower AQP1 mRNA and protein expression in the peritoneum.

CONCLUSION

These data demonstrate the structural and functional similarity between mouse and rat models of PD, and further emphasize the relevance of mouse models to understand PD in humans. They also suggest that gender may influence water transport and AQP1 expression in the peritoneum.

Evidence from Biosynthetically Incorporated Strontium and FTIR Difference Spectroscopy that the C-Terminus of the D1 Polypeptide of Photosystem II Does Not Ligate Calcium†

Recent FTIR studies have provided evidence that the C-terminal alpha-COO(-) group of the D1 polypeptide at D1-Ala344 is a unidentate ligand of a Mn ion in photosystem II [Chu, H.-A., Hiller, W., and Debus, R. J. (2004) Biochemistry 43, 3152-3166; Kimura, Y., Mizusawa, N., Yamanari, T., Ishii, A., and Ono, T.-A. (2005) J. Biol. Chem. 280, 2078-2083]. However, the FTIR data could not exclude Ca ligation. Furthermore, the recent approximately 3.5 A X-ray crystallographic structural model positions the alpha-COO(-) group of D1-Ala344 near a Ca ion [Ferreira, K. N., Iverson, T. M., Maghlaoui, K., Barber, J., and Iwata, S. (2004) Science 303, 1831-1838]. Therefore, to conclusively establish whether the alpha-COO(-) group of D1-Ala344 ligates Mn or Ca, the symmetric carboxylate stretching mode of the alpha-COO(-) group of D1-Ala344 was identified in the S(2)-minus-S(1) FTIR difference spectrum of PSII particles having Sr substituted for Ca. Cells of the cyanobacterium Synechocystis sp. PCC 6803 were propagated in media having Sr substituted for Ca and containing either l-[1-(13)C]alanine or unlabeled ((12)C) alanine. The S(2)-minus-S(1) FTIR difference spectra of the purified PSII particles show that substituting Sr for Ca alters several carboxylate stretching modes, including some that may correspond to one or more metal ligands, but importantly does not alter the symmetric carboxylate stretching mode of the alpha-COO(-) group of D1-Ala344. In unlabeled PSII particles, this mode appears at approximately 1356 cm(-)(1) in the S(1) state and at either approximately 1337 or approximately 1320 cm(-)(1) in the S(2) state, irrespective of whether the PSII particles contain Ca or Sr. These data are inconsistent with Ca ligation and show, therefore, that the C-terminal alpha-COO(-) group of the D1 polypeptide ligates a Mn ion. These data also show that substituting Ca with the larger Sr ion perturbs other unidentified carboxylate groups, at least one of which may ligate the Mn(4) cluster.

Cytochrome c550 in the Cyanobacterium Thermosynechococcus elongatus

Cytochrome c(550) is one of the extrinsic Photosystem II subunits in cyanobacteria and red algae. To study the possible role of the heme of the cytochrome c(550) we constructed two mutants of Thermosynechococcus elongatus in which the residue His-92, the sixth ligand of the heme, was replaced by a Met or a Cys in order to modify the redox properties of the heme. The H92M and H92C mutations changed the midpoint redox potential of the heme in the isolated cytochrome by +125 mV and -30 mV, respectively, compared with the wild type. The binding-induced increase of the redox potential observed in the wild type and the H92C mutant was absent in the H92M mutant. Both modified cytochromes were more easily detachable from the Photosystem II compared with the wild type. The Photosystem II activity in cells was not modified by the mutations suggesting that the redox potential of the cytochrome c(550) is not important for Photosystem II activity under normal growth conditions. A mutant lacking the cytochrome c(550) was also constructed. It showed a lowered affinity for Cl(-) and Ca(2+) as reported earlier for the cytochrome c(550)-less Synechocystis 6803 mutant, but it showed a shorter lived S(2)Q(B)(-) state, rather than a stabilized S(2) state and rapid deactivation of the enzyme in the dark, which were characteristic of the Synechocystis mutant. It is suggested that the latter effects may be caused by loss (or weaker binding) of the other extrinsic proteins rather than a direct effect of the absence of the cytochrome c(550).

In Situ Effects of Mutations of the Extrinsic Cytochrome c550 of Photosystem II in Synechocystis sp. PCC6803

The H(2)O oxidizing domain of the cyanobacterial photosystem II (PSII) complex contains a low potential, c-type cytochrome termed c(550) that is essential for the in vivo stability of the PSII complex. A mutant lacking cytochrome c(550) (DeltapsbV) in Synechocystis sp. PCC6803 has been further analyzed together with a construct in which the distal axial heme iron ligand, histidine 92, has been substituted with a methionine (C550-H92M). Heme staining of SDS-PAGE showed that the C550-H92M mutation did not disturb the accumulation and heme-binding properties of the cytochrome. In DeltapsbV cells, the number of charge separating PSII centers was estimated to be 56% of the wild type, but of the existing centers, 33% lacked photooxidizable Mn ions. C550-H92M did not discernibly affect the intrinsic PSII electron-transfer kinetics compared to the wild type nor did it exhibit a significant fraction of centers lacking photooxidizable Mn; however, the number of charge separating PSII centers in mutant cells was 69% of the wild type. C550-H92M lost photoautotrophic growth ability in the absence of Ca(2+), but its growth was not affected by depletion of Cl(-), which differs from DeltapsbV. Taken together, the results suggest that in the absence of cytochrome c(550) electron transfer on the donor side is retarded perhaps at the level of Y(z) to P680(+) transfer, the heme ligand. His92 is not absolutely required for assembly of functional PSII centers; however, replacement by methionine prevents normal accumulation of PSII centers in the thylakoid membranes and alters the Ca(2+) requirement of PSII. The results are discussed in terms of current understanding of the Ca(2+) site of PSII.

Site-Directed Mutagenesis of Thermosynechococcus elongatus Photosystem II: The O2-Evolving Enzyme Lacking the Redox-Active Tyrosine D

Site-directed mutagenesis in the photosystem II (PSII) oxygen-evolving enzyme was achieved in the thermophilic cyanobacterium Thermosynechococcus elongatus. PSII from this species is the focus of attention because its robustness makes it suitable for enzymological and biophysical studies. PSII, which lacks the redox-active tyrosine Tyr(D), was engineered by substituting a phenylalanine for tyrosine 160 of the D2 protein. An aim of this work was to engineer a mutant for spectroscopy, in particular, for EPR, on the active enzyme. The Tyr(D)(*) EPR signal was monitored in whole cells (i) to control the expression level of the two genes (psbD(1) and psbD(2)) encoding D2 and (ii) to assess the success of the mutagenesis. Both psbD(1) and psbD(2) could be expressed, and recombination occurred between them. The D2-Y160F mutation was introduced into psbD(1) after psbD(2) was deleted and a His-tag was attached to the CP43 protein. The effects of the Y160F mutation were characterized in cells, thylakoids, and isolated PSII. The efficiency of enzyme function under the conditions tested was unaffected. The distribution and lifetime of the redox states (S(n)() states) of the enzyme cycle were modified, with more S(0) in the dark and no rapid decay phase of S(3). Although not previously reported, these effects were expected because Tyr(D)(*) is able to oxidize S(0) and Tyr(D) is able to reduce S(2) and S(3). Slight changes in the difference spectra in the visible and infrared recorded upon the formation and reduction of the chlorophyll cation P(680)(+) and kinetic measurements of P(680)(+) reduction indicated minor structural perturbations, perhaps in the hydrogen-bonding network linking Tyr(D) and P(680), rather than electrostatic changes associated with the loss of a charge from Tyr(D)(*)(H(+)). We show here that this fully active preparation can provide spectra from the Mn(4)CaO(4) complex and associated radical species uncontaminated by Tyr(D)(*).