Modulation of Quantum Yield of Primary Radical Pair Formation in Photosystem II by Site-Directed Mutagenesis Affecting Radical Cations and Anions

Merry, S.; Nixon, Peter J.; Barter, Laura M. C.; Schilstra, Maria J.; Porter, George; Barber, James; Durrant, James R.; Klug, David R.

doi:10.1021/bi980502d

Cited by 89 publications

(90 citation statements)

References 29 publications

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“…The differences between the in vivo and in vitro measurements may be a consequence of the loss of the Mn cluster which is known to influence the rate of reduction of P + by Y Z (82,91) and the difference in reduction potential between Y Z •/Y Z and P + /P (92). The differences with the results of Merry et al (43) may be due to their use of reaction center preparations as well as the early times at which their measurements are made (60 ps) as compared to our own which are on the seconds time scale. The slower measurements here allow ample time for protein-based relaxations to occur.…”

Section: Discussioncontrasting

confidence: 78%

“…This possibility has been previously proposed by Rutherford and co-workers (32,109) and by Dekker and van Grondelle (33). From the results of Merry et al (43) it is clear that mutations that affect the reduction potentials P A + / P A and the Pheo -/Pheo also have an impact on the yield of charge separation at 60 ps following the excitation pulse. Thus radical pair states that include P A + and Pheo -(mostly P A + Pheo -) are already present at this time and are in equilibrium with the excited state of the reaction center.…”

Section: Location Of 3 P As a Function Of Temperaturementioning

confidence: 51%

“…For this reason we place P + B A -and/or B A + Pheo -equipotential with B A * ( Figure 9B). It would be of interest to examine these pathways further by examining the influence of the reduction potential of P + /P on the quantum yield of charge separation at low temperature as has been done at room temperature by Merry et al (43).…”

Section: Location Of 3 P As a Function Of Temperaturementioning

confidence: 99%

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Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

et al. 2001

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Site-directed mutations were introduced to replace D1-His198 and D2-His197 of the D1 and D2 polypeptides, respectively, of the photosystem II (PSII) reaction center of Synechocystis PCC 6803. These residues coordinate chlorophylls P A and P B which are homologous to the special pair Bchlorophylls of the bacterial reaction centers that are coordinated respectively by histidines L-173 and M-200 (202). P A and P B together serve as the primary electron donor, P, in purple bacterial reaction centers. In PS II, the site-directed mutations at D1 His198 affect the P + -P-absorbance difference spectrum. The bleaching maximum in the Soret region (in WT at 433 nm) is blue-shifted by as much as 3 nm. In the D1 His198Gln mutant, a similar displacement to the blue is observed for the bleaching maximum in the Q y region (672.5 nm in WT at 80 K), whereas features attributed to a band shift centered at 681 nm are not altered. In the Y Z •-Y Z -difference spectrum, the band shift of a reaction center chlorophyll centered in WT at 433-434 nm is shifted by 2-3 nm to the blue in the D1-His198Gln mutant. The D1-His198Gln mutation has little effect on the optical difference spectrum, 3 P-1 P, of the reaction center triplet formed by P + Pheo -charge recombination (bleaching at 681-684 nm), measured at 5-80 K, but becomes visible as a pronounced shoulder at 669 nm at temperatures g150 K. Measurements of the kinetics of oxidized donor-Q A -charge recombination and of the reduction of P + by redox active tyrosine, Y Z , indicate that the reduction potential of the redox couple P + /P can be appreciably modulated both positively and negatively by ligand replacement at D1-198 but somewhat less so at D2-197. On the basis of these observations and others in the literature, we propose that the monomeric accessory chlorophyll, B A , is a long-wavelength trap located at 684 nm at 5 K. B A * initiates primary charge separation at low temperature, a function that is increasingly shared with P A * in an activated process as the temperature rises. Charge separation from B A * would be potentially very fast and form P A + B A -and/or B A + Pheo -as observed in bacterial reaction centers upon direct excitation of B A ) Proc. Natl. Acad Sci. 96, 2054-2059. The cation, generated upon primary charge separation in PSII, is stabilized at all temperatures primarily on P A , the absorbance spectrum of which is displaced to the blue by the mutations. In WT, the cation is proposed to be shared to a minor extent (∼20%) with P B , the contribution of which can be modulated up or down by mutation. The band shift at 681 nm, observed in the P + -P difference spectrum, is attributed to an electrochromic effect of P A + on neighboring B A . Because of its low-energy singlet and therefore triplet state, the reaction center triplet state is stabilized on B A at e80 K but can be shared with P A at >80 K in a thermally activated process.

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Section: Discussioncontrasting

confidence: 78%

Section: Location Of 3 P As a Function Of Temperaturementioning

confidence: 51%

Section: Location Of 3 P As a Function Of Temperaturementioning

confidence: 99%

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Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

et al. 2001

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“…The E m (P680/P680 ϩ ) value was estimated to be less positive than ϩ1,210 mV. The E m (Phe a/Phe a Ϫ ) value for the PS II complexes from T. elongatus WT*, where only the psbA 3 gene expressed, was Ϫ505 mV, whereas a site-directed mutagenesis study with Synechocystis PCC 6803 (56) showed that substitution of Glu at position 130 of D1 with Gln decreased the free energy difference ⌬G CS , corresponding to a shift in the E m (Phe a/Phe a Ϫ ) value by ϩ33 mV. Such a mutational effect on the energetics was also confirmed by thermoluminescence measurements (57).…”

Section: Discussionmentioning

confidence: 93%

Spectroelectrochemical determination of the redox potential of pheophytina, the primary electron acceptor in photosystem II

Kato

Sugiura

Oda

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

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Thin-layer cell spectroelectrochemistry, featuring rigorous potential control and rapid redox equilibration within the cell, was used to measure the redox potential Em(Phe a/Phe a ؊ ) of pheophytin (Phe) a, the primary electron acceptor in an oxygen-evolving photosystem (PS) II core complex from a thermophilic cyanobacterium Thermosynechococcus elongatus. Interferences from dissolved O 2 and water reductions were minimized by airtight sealing of the sample cell added with dithionite and mercury plating on the gold minigrid working electrode surface, respectively. The result obtained at a physiological pH of 6.5 was E m(Phe a/Phe a ؊ ) ‫؍‬ ؊505 ؎ 6 mV vs. SHE, which is by Ϸ100 mV more positive than the values measured Ϸ30 years ago at nonphysiological pH and widely accepted thereafter in the field of photosynthesis research. Using the P680* ؊ Phe a free energy difference, as estimated from kinetic analyses by previous authors, the present result would locate the E m(P680/P680 ؉ ) value, which is one of the key parameters but still resists direct measurements, at approximately ؉1,210 mV. In view of these pieces of information, a renewed diagram is proposed for the energetics in PS II.charge separation ͉ photosynthesis ͉ spectroelectrochemistry ͉ water oxidation I n the photosynthetic primary process of higher plants, algae, and cyanobacteria, photosystem (PS) I and PS II cooperate in series to convert photon energy into chemical energy through light-induced charge separation and subsequent electron transfers. PS II appears to catalyze water oxidation at a pentanuclear Mn 4 Ca cluster that accumulates oxidizing equivalents (see recent reviews in refs. 1-5). It is generally supposed that the water oxidation is triggered by charge separation between the primary electron donor, P680, and the primary electron acceptor, pheophytin (Phe) a. The initial radical pair P680 ϩ Phe a Ϫ formed by the charge separation, drives forward electron transfer from Phe a Ϫ to the first plastoquinone Q A , and hole transfer from P680 ϩ to the Mn 4 Ca cluster through a redox-active tyrosine residue denoted Y Z , thus preventing charge recombination. Recent X-ray crystallography clarified the arrangement of protein subunits and cofactors in PS II with 2.9-3.7-Å resolution (6-9), visualizing the electron transfer pathway.To date, the nature of P680 remains controversial. Available experimental data and theoretical analyses suggest that P680 is assignable to the pigment cluster of the four chlorophyll a molecules (denoted P D1 , P D2 , Chl D1 , and Chl D2 ) in the PS II reaction center (1, 5). Further uncertainty surrounds the value of the redox potential E m (P680/P680 ϩ ). Although the E m (P680/ P680 ϩ ) value is an essential parameter to draw a whole picture of the PS II energetics, its direct measurement has never been attained (10) because water, present as dominant (solvent) molecules in most sample solutions, is oxidized first heavily and tends to mask the subsequent oxidation of the target entity P680 at a higher potential. The E m ...

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“…The importance of H-bonds to cofactors in the bacterial RC has been well studied (13). As shown previously in the case of Photosystem II (14), the loss of such a H-bond should shift the Chl's reduction potential to more negative values, thus altering the rate, and possibly the yield, of ET to the targeted ec3 Chl.…”

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confidence: 91%

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Est

Lucas

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

101

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Photosystem I has two branches of cofactors down which lightdriven electron transfer (ET) could potentially proceed, each consisting of a pair of chlorophylls (Chls) and a phylloquinone (PhQ). Forward ET from PhQ to the next ET cofactor (F X) is described by two kinetic components with decay times of Ϸ20 and Ϸ200 ns, which have been proposed to represent ET from PhQ B and PhQA, respectively. Immediately preceding each quinone is a Chl (ec3), which receives a H-bond from a nearby tyrosine. To decrease the reduction potential of each of these Chls, and thus modify the relative yield of ET within the targeted branch, this H-bond was removed by conversion of each Tyr to Phe in the green alga Chlamydomonas reinhardtii. Together, transient optical absorption spectroscopy performed in vivo and transient electron paramagnetic resonance data from thylakoid membranes showed that the mutations affect the relative amplitudes, but not the lifetimes, of the two kinetic components representing ET from PhQ to F X. The mutation near ec3 A increases the fraction of the faster component at the expense of the slower component, with the opposite effect seen in the ec3B mutant. We interpret this result as a decrease in the relative use of the targeted branch. This finding suggests that in Photosystem I, unlike type II reaction centers, the relative efficiency of the two branches is extremely sensitive to the energetics of the embedded redox cofactors.Chlamydomonas ͉ directionality ͉ photosynthetic reaction center ͉ pump-probe spectroscopy ͉ transient EPR P hotosynthetic reaction centers (RCs) are the membrane proteins responsible for the capture and storage of light energy in photosynthetic organisms. As with all of the RCs, Photosystem I (PS1) has a C2 symmetrical structure with two virtually identical branches of redox cofactors extending across the membrane (see Fig. 1). The x-ray crystal structure of PS1 from Thermosynechococcus elongatus (1) has allowed identification of amino acid residues interacting with the electron transfer (ET) cofactors, permitting the use of site-directed mutagenesis to investigate to what extent ET occurs in the two branches. Many of these studies (2-10) point toward the possibility that ET takes place in both branches of cofactors (A-branch and Bbranch; see Fig. 1). The data consistently show that the major fraction occurs in the A-branch and that the slow phase of ET from phylloquinone (PhQ) to F x is associated with this branch. The involvement of the B-branch is less clear, but there is mounting evidence to support the assignment of the fast component of PhQ to F x ET to this branch. If ET does indeed occur in both branches in PS1, this behavior would be remarkably different from the type II RCs. In purple bacterial RCs, for example, it is well known that initial ET is biased almost exclusively toward the A-branch, probably because the B-branch quinone is a mobile electron and proton carrier in this type of RC. The strong biasing of the ET in the type II RCs makes it difficult to investigate the fac...

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Modulation of Quantum Yield of Primary Radical Pair Formation in Photosystem II by Site-Directed Mutagenesis Affecting Radical Cations and Anions

Cited by 89 publications

References 29 publications

Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

Spectroelectrochemical determination of the redox potential of pheophytina, the primary electron acceptor in photosystem II

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

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