Showing papers on "Photosynthetic reaction centre published in 1992"
TL;DR: Several possible mechanisms for proton transfer are consistent with the observed experimental results and proposed proton pathways and involve proton transfers from individual amino acid residues or internal water molecules either as single steps or in a concerted fashion.
Abstract: Proton transfer in the bacterial RC associated with the reduction of the bound QB to the dihydroquinone is an important step in the energetics of photosynthetic bacteria. The binding of two protons by the quinone is associated with the transfer of the second electron to QB at a rate of ca. 10(3) s-1 (pH 7). Mutation of three protonatable residues, GluL212, SerL223, and AspL213, located near QB to nonprotonatable residues (Gln, Ala, and Asn, respectively) resulted in large reductions (by 2 to 3 orders of magnitude) in the rate or proton transfer to QB. These mutations can be grouped into two classes: those that blocked both proton transfer and electron transfer (SerL223, and AspL213) and those that blocked only proton transfer (GluL212). These results were interpreted in terms of a pathway for proton transport in which uptake of the first proton, required for the transfer of the second electron, occurs through a pathway involving AspL213 and SerL223. Uptake of the second proton, which follows electron transfer, occurs through a pathway involving GluL212 and possibly AspL213. Acidic residues near QB affect electron transfer rates via electrostatic interactions. One residue, with a pKa of ca. 10 interacting strongly with the charge on QB (delta pKa greater than 2), was shown to be GluL212. A second residue with a pKa of ca. 6, which interacts more weakly with the charge on QB (delta pK approximately 1), could be either AspL210 or AspL213. Several possible mechanisms for proton transfer are consistent with the observed experimental results and proposed proton pathways. These involve proton transfers from individual amino acid residues or internal water molecules either as single steps or in a concerted fashion. The determination of the dominant mechanism will require evaluation of the energetics of the various steps.
315 citations
TL;DR: It is shown here that processing on the carboxy side of Ala344 also occurs in the cyanobacterium Synechocystis 6803, resulting in the removal of 16 amino acids, consistent with a role for processing in regulating the assembly of the photosynthetic manganese cluster.
Abstract: The D1 polypeptide of the photosystem II (PSII) reaction center is synthesized as a precursor polypeptide which is posttranslationally processed at the carboxy terminus. It has been shown in spinach that such processing removes nine amino acids, leaving Ala344 as the C-terminal residue [Takahashi, M., Shiraishi, T., & Asada, K. (1988) FEBS Lett. 240, 6-8; Takahashi, Y., Nakane, H., Kojima, H., & Satoh, K. (1990) Plant Cell Physiol. 31, 273-280]. We show here that processing on the carboxy side of Ala344 also occurs in the cyanobacterium Synechocystis 6803, resulting in the removal of 16 amino acids. By constructing a deletion strain of Synechocystis 6803 that lacks the three copies of the psbA gene encoding D1, we have developed a system for generating psbA mutants. Using this system, we have constructed mutants of Synechocystis 6803 that are modified in the region of the C-terminus of the D1 polypeptide. Characterization of these mutants has revealed that (1) processing of the D1 polypeptide is blocked when the residue after the cleavage site is changed from serine to proline (mutant Ser345Pro) with the result that the manganese cluster is unable to assemble correctly; (2) the C-terminal extension of 16 amino acid residues can be deleted with little consequence either for insertion of D1 into the thylakoid membrane or for assembly of D1 into a fully active PSII complex; (3) removal of only one more residue (mutant Ala344stop) results in a loss of assembly of the manganese cluster; and (4) the ability of detergent-solubilized PSII core complexes (lacking the manganese cluster) to bind and oxidize exogenous Mn2+ by the secondary donor, Z+, is largely unaffected in the processing mutants (the Ser345Pro mutant of Synechocystis 6803 and the LF-1 mutant of Scenedesmus obliquus) and the truncation mutant Ala344stop. Our results are consistent with a role for processing in regulating the assembly of the photosynthetic manganese cluster and a role for the free carboxy terminus of the mature D1 polypeptide in the ligation of one or more manganese ions of the cluster.
221 citations
TL;DR: Exposure of thylakoid membranes to ultraviolet-B radiation caused inhibition of semiquinone anion formation at Q A, inhibition of plastoquinone photoreduction and lower rates of Photosystem II electron-transport to artificial electron acceptors, suggesting a UV-B-induced damage to the primary quinone acceptor Q A and impairment in the function of plASTOquinone in the thylackoid membrane.
218 citations
TL;DR: D1-Asp170 appears to participate in the early steps associated with the assembly of the Mn cluster of cyanobacterium Synechocystis, the first reported example of an amino acid residue critical to the function andAssembly of the oxygen-evolving complex.
Abstract: Eleven site-directed mutations were constructed at aspartate 170 of the D1 polypeptide of the photosystem II (PSII) reaction center of the cyanobacterium Synechocystis sp. PCC 6803. The light-saturated rates of O2 evolution (VO2) measured in whole cells range from close to that of wild-type for Asp170Glu to zero for Asp170Ser and Ala. Those mutant strains that are best able to evolve O2 are also those that show the lowest Km in PSII core complexes for the oxidation of Mn2+ by oxidized Tyr161, the normal oxidant of the Mn cluster responsible for O2 evolution. To a first approximation, the lower the pKa of the residue at position 170, the higher the VO2 and the lower the Km. D1-Asp170 appears to participate in the early steps associated with the assembly of the Mn cluster. It is also the first reported example of an amino acid residue critical to the function and assembly of the oxygen-evolving complex.
211 citations
TL;DR: It is shown that RegA is itself responsible for differential induction of light-harvesting and reaction center gene expression relative to operons for photopigment biosynthesis and that photosynthesis growth in the absence of transactivating structural gene expression is a consequence of the superoperonal organization of the photosynthetic gene cluster.
193 citations
TL;DR: Two mutations near the monomer bacteriochlorophylls had minor changes of 25 mV or less in the donor oxidation-reduction potential, but the mutation close to the monomers on the active branch resulted in a roughly 3-fold decrease in the rate of the initial electron transfer.
Abstract: Mutations were made in four residues near the bacteriochlorophyll cofactors of the photosynthetic reaction center from Rhodobacter sphaeroides. These mutations, L131 Leu to His and M160 Leu to His, near the dimer bacteriochlorophylls, and M203 Gly to Asp and L177 Ile to Asp, near the monomer bacteriochlorophylls, were designed to result in the placement of a hydrogen bond donor group near the ring V keto carbonyl of each bacteriochlorophyll. Perturbations of the electronic structures of the bacteriochlorophylls in the mutants are indicated by additional resolved transitions in the bacteriochlorophyll absorption bands in steady-state low-temperature and time-resolved room temperature spectra in three of the resulting mutant reaction centers. The major effect of the two mutations near the dimer was an increase up to 80 mV in the donor oxidation-reduction midpoint potential. Correspondingly, the calculated free energy difference between the excited state of the primary donor and the initial charge separated state decreased by up to 55 mV, the initial forward electron-transfer rate was up to 4 times slower, and the rate of charge recombination between the primary quinone and the donor was approximately 30% faster in these two mutants compared to the wild type. The two mutations near the monomer bacteriochlorophylls had minor changes of 25 mV or less in the donor oxidation-reduction potential, but the mutation close to the monomer bacteriochlorophyll on the active branch resulted in a roughly 3-fold decrease in the rate of the initial electron transfer.
170 citations
TL;DR: A basic relationship is defined that incorporates the three parameters that effectively modulate the rate of intraprotein electron transfer, namely distance, free energy and reorganization energy, that is used to explore the minimal requirements for protein-catalyzed conversion of excited electronic states into stable charge separated states, the essence of photosynthesis.
152 citations
TL;DR: The structural and topological stability ofThylakoid components under photoinhibitory conditions was studied on Mn depleted thylakoids isolated from spinach leaves to study the effects of photoinhibition on photosystems I and II.
Abstract: The structural and topological stability of thylakoid components under photoinhibitory conditions (4,500 microE.m-2.s-1 white light) was studied on Mn depleted thylakoids isolated from spinach leaves. After various exposures to photoinhibitory light, the chlorophyll-protein complexes of both photosystems I and II were separated by sucrose gradient centrifugation and analysed by Western blotting, using a set of polyclonals raised against various apoproteins of the photosynthetic apparatus. A series of events occurring during donor side photoinhibition are described for photosystem II, including: (a) lowering of the oligomerization state of the photosystem II core; (b) cleavage of 32-kD protein D1 at specific sites; (c) dissociation of chlorophyll-protein CP43 from the photosystem II core; and (d) migration of damaged photosystem II components from the grana to the stroma lamellae. A tentative scheme for the succession of these events is illustrated. Some effects of photoinhibition on photosystem I are also reported involving dissociation of antenna chlorophyll-proteins LHCI from the photosystem I reaction center.
143 citations
TL;DR: Proton and electron transfer events in reaction centers (RCs) from Rhodobacter sphaeroides were investigated by site-directed mutagenesis of glutamic acid at position 212 and aspartic acid at 213 in the secondary quinone (QB) binding domain of the L subunit to find a significant stabilization of the QB- semiquinone.
Abstract: Proton and electron transfer events in reaction centers (RCs) from Rhodobacter sphaeroides were investigated by site-directed mutagenesis of glutamic acid at position 212 and aspartic acid at 213 in the secondary quinone (QB) binding domain of the L subunit. These residues were mutated singly to the corresponding amides (mutants L212EQ and L213DN) and together to give the double mutant (L212EQ/L213DN). In the double mutant RCs, the rate of electron transfer from the primary (QA) to the secondary (QB) acceptor quinones is fast (tau approximately 300 microseconds) and is pH independent from pH 5 to 11. The rate of recombination between the oxidized primary donor, P+, and QB- is also pH independent and much slower (tau approximately 10 s) than in the wild type (Wt), indicating a significant stabilization of the QB- semiquinone. In the double mutant, and in L213DN mutant RCs at low pH, the P+QB- decay is suggested to occur significantly via a direct recombination rather than by repopulating the P+QA- state, as in the Wt. Comparison of the behavior of Wt and the three mutant RC types leads to the following conclusions: the pK of AspL213 in the Wt is approximately 4 for the QAQB state (pKQB) and approximately 5 for the QAQB-state (pKQB-); for GluL212, pKQB approximately 9.5 and pKQB- approximately 11. In L213DN mutant RCs, pKQB of GluL212 is less than or equal to 7, indicating that the high pK values of GluL212 in the Wt are due largely to electrostatic interaction with the ionized AspL213 which contributes a shift of at least 2.5 pH units. Transfer of the second electron and all associated proton uptake to form QBH2 is drastically inhibited in double mutant and L213DN mutant RCs. At pH greater than or equal to 8, the rates are at least 10(4)-fold slower than in Wt RCs. In L212EQ mutant RCs the second electron transfer and proton uptake are biphasic. The fast phase of the electron transfer is similar to that of the Wt, but the extent of rapid transfer is pH dependent, revealing the pH dependence of the equilibrium QA(-)QB- in equilibrium with QAQBH-. The estimated limits on the pK values--pKQA-QB-less than or equal to 7.3, pKQAQB2- greater than or equal to 10.4--are similar to those derived earlier for Wt RCs [Kleinfeld et al. (1985) Biochim. Biophys. Acta 809, 291-310] and may pertain to the quinone head group, per se.(ABSTRACT TRUNCATED AT 400 WORDS)
140 citations
Book•
01 Jun 1992
TL;DR: In this paper, the effect of single electron exchange between main group or transition metal compounds and unsaturated organic substrates is discussed. But the authors focus on the effects of electron transfer on the interaction between the main group and the transition metal components.
Abstract: Mechanisms of Electron Transfer (R.D. Cannon). Electron Transfer Effect in Chemical Compounds (B. Jezowska-Trzebiatowska and W. Wojciechowski). Light Induced Electron Transfer of Metal Complexes (A. Vogler and H. Kunkely). Coordinative Aspects of Single Electron Exchange between Main Group or Transition Metal Compounds and Unsaturated Organic Substrates (W. Kaim). Electron Hopping and Delocalization in Mixed-Valence Metal-Oxygen Clusters (H. So and M.T. Pope). Electron Transfer in Semiconducting Colloids and Membranes, Applications in Artificial Photosynthesis (M. Gratzel). Electron Transfer in Photosynthetic Reaction Centers (Ch.C. Moser et al.). Exchange Interaction in Electron Transfer Proteins and their Model Compounds (W. Haase and S. Gehring). Multi-Electron Transfer Processes in Nitrogen Fixation and other Natural Systems (D.J. Lowe). Electron Transfer in Anaerobic Microorganisms (A. Kroger et al.). The Importance of Inhibitors as an Analytic Tool for the Study of the Quinol Oxidation Centre and the Quinol Oxidase Reaction (G. von Jagow and U. Brandt). Electron and Proton Transfer through the Mitochondrial Respiratory Chain (T.A. Link). Coupled Proton and Electron Transfer Pathways in the Acceptor Quinone Complex of Reaction Centers from Rhodobacter Sphaeroides (E. Takahashi et al.). The Water Oxidizing Enzyme - An Alternative Model (E.K. Pistorius). Proton Pumps, Proton Flow and Proton ATP Synthases in Photosynthesis of Green Plants (W. Junge et al.). Diffusion of Proton in Microscopic Space: Effect of Geometric Constraints and Dielectric Discontinuities (M. Gutman et al.). Protonation of the Schiff Base Chromophore in Rhodopsins (C. Sandorfy). Proton Transfer along the Hydrogen Bridge in Some Hydrogen-Bonded Molecular Complexes (H. Ratajczak). Hydrogen-Bonded Systems with Large Proton Polarizability due to Collective Proton Motion as Pathways of Protons in Biological Systems (G. Zundel). NMR Studies of Multiple Proton and Deuteron Transfers in Liquids, Crystals and Organic Glasses (H.-H. Limbach). Proton Transfer Reactions in Solutions: A Molecular Approach (D. Borgis). Recent Developments in Solitonic Model of Proton Transfer in Quasi-One-Dimensional Infinite Hydrogen-Bonded Systems (E. Kryachko). Index of Contributing Authors. Subject Index.
139 citations
TL;DR: In this paper, the catalytic mechanism of O2 reduction by cytochrome oxidase was studied in isolated mitochondria and mitoplasts by partial reversal of the reaction, which is associated with reversed electron transfer from the bimetallic center to ferricytochrome c. The results are consistent with transfer of one electron from the ferric-cupric bimetric center (O), yielding the ferryl (F) and peroxy (P) intermediates, and Em,7 values of approximately 1.1 and 1.2 V may be calculated for the
TL;DR: A three-dimensional model of the core proteins D1 and D2, including the cofactors, that form the Photosystem II reaction centre of pea (Pisum sativum), has been generated.
Abstract: A three-dimensional model of the core proteins D1 and D2, including the cofactors, that form the Photosystem II reaction centre of pea (Pisum sativum), has been generated. This model was built with a rule-based computer modelling system using the information from the crystal structures of the photosynthetic reaction centres of Rhodopseudomonas viridis and Rhodobacter sphaeroides. An alignment of the primary sequences of twenty three D1, nine D2, eight bacterial L and eight bacterial M subunits predicts strong similarity between bacterial and higher plant reaction centres, especially in the transmembrane region where the cofactors responsible for electron transport are located. The sequence to be modelled was aligned to the bacterial structures using environment-dependent substitution tables to construct matrices, improving the alignment procedure. The ancestral relationship between the bacteria and higher plant sequences allowed both the L and M subunits to be used as structural templates as they were equally related to the higher plant polypeptides. The regions with the highest predicted structural homology were used as a framework for the construction of the structurally conserved regions. The structurally conserved region of the model shows strong similarity to the bacterial reaction centre in the transmembrane helices. The stromal and lumenal loops show greater sequence variation and are therefore predicted to be the structurally variable regions in the model. The key sidechain assignments and residues that may interact with cofactors are discussed.
TL;DR: Comparison with the results of spectral simulations for tyrosyl radicals in ribonucleotide reductase from prokaryotes or eukaryotes and with in vitro radicals indicates that the spin density distribution remains that of an odd-alternant radical but that interactions with the protein can shift spin density within this basic pattern.
Abstract: The stable tyrosine radical in photosystem II, YD*, has been studied by ESR and ENDOR spectroscopies to obtain proton hyperfine coupling constants from which the electron spin density distribution can be deduced. Simulations of six previously published ESR spectra of PSII (one at Q band; five at X band, of which two were after specific deuteration and two others were of oriented membranes) can be achieved by using a single set of magnetic parameters that includes anisotropic proton hyperfine tensors, an anisotropic g tensor, and noncoincident axis systems for the g and A tensors. From the spectral simulation of the oriented samples, the orientation of the phenol head group of YD* with respect to the membrane plane has been determined. A similar orientation for YZ*, the redox-active tyrosine in PSII that mediates electron transfer between P680 and the oxygen-evolving complex, is expected. ENDOR spectra of YD* in PSII preparations from spinach and Synechocystis support the set of hyperfine coupling constants but indicate that small differences between the two species exist. Comparison with the results of spectral simulations for tyrosyl radicals in ribonucleotide reductase from prokaryotes or eukaryotes and with in vitro radicals indicates that the spin density distribution remains that of an odd-alternant radical but that interactions with the protein can shift spin density within this basic pattern. The largest changes in spin density occur at the tyrosine phenol oxygen and at the ring carbon para to the oxygen, which indicates that mechanisms exist in the protein environment for fine-tuning the chemical and redox properties of the radical species.
TL;DR: It is concluded that the PufX protein plays an important role in the structure of the functional photosynthetic unit, and its absence results in loss of efficient electron transfer from the QB site of the reaction center to the Qz site ofThe cytochrome b/c1 complex.
TL;DR: Depending on experimental conditions it is found that photo‐inhibitory treatment of photosystem II (PSII) core complexes, isolated from wheat, can generate two fragments of about 23–24 kDa that contain either the C‐terminal or N‐ terminal regions of the D1‐protein.
TL;DR: The simulations show that fluctuations of the protein matrix, which are coupled electrostatically to electron transfer, play an important role in controlling the electron transfer rates and that theprotein matrix stabilizes the separated electron pair state through rapid and temperature-independent dielectric relaxation.
Abstract: The coupling between electron transfer and protein structure and dynamics in the photosynthetic reaction center of Rhodopseudomonas viridis is investigated. For this purpose molecular dynamics simulations of the essential portions (a segment of 5797 atoms) of this protein complex have been carried out. Electron transfer in the primary event is modeled by altering the charge distributions of the chromophores according to quantum chemical calculations. The simulations show (i) that fluctuations of the protein matrix, which are coupled electrostatically to electron transfer, play an important role in controlling the electron transfer rates and (ii) that the protein matrix stabilizes the separated electron pair state through rapid (200 fs) and temperature-independent dielectric relaxation. The photosynthetic reaction center resembles a polar liquid in that the internal motions of the whole protein complex, rather than only those of specific side groups, contribute to i and ii. The solvent reorganization energy is about 4.5 kcal/mol. The simulations indicate that rather small structural rearrangements and changes in motional amplitudes accompany the primary electron transfer.
TL;DR: While virtually all of the D1 protein present in thylakoids can be phosphorylated in vitro, the steady-state level of phosphorylation in vivo varies with light intensity and did not exceed 20% of the total D1 under the conditions of this study.
TL;DR: A broad absorption band with the predicted properties was found in a previously unexplored region of the spectrum, found in reaction centers of Rhodobacter sphaeroides and several other species of bacteria that contain bacteriochlorophyll a, and near 2750 cm-1 in Rhodopseudomonas viridis.
Abstract: The primary electron donor in the reaction center of purple photosynthetic bacteria consists of a pair of bacteriochlorophylls (PL and PM). The oxidized dimer (P+) is expected to have an absorption band in the mid-IR, whose energy and dipole strength depend in part on the resonance interactions between the two bacteriochlorophylls. A broad absorption band with the predicted properties was found in a previously unexplored region of the spectrum, centered near 2600 cm-1 in reaction centers of Rhodobacter sphaeroides and several other species of bacteria that contain bacteriochlorophyll a, and near 2750 cm-1 in Rhodopseudomonas viridis. The band is not seen in the absorption spectrum of the monomeric bacteriochlorophyll cation in solution, and it is missing or much diminished in the reaction centers of bacterial mutants that have a bacteriopheophytin in place of either PL or PM. With the aid of a relatively simple quantum mechanical model, the measured transition energy and dipole strength of the band can be used to solve for the resonance interaction matrix element that causes an electron to move back and forth between PL and PM, and also for the energy difference between states in which a positive charge is localized on either PL or PM. (The absorption band can be viewed as representing a transition between supermolecular eigenstates that are obtained by mixing these basis states.) The values of the matrix element obtained in this way agree reasonably well with values calculated by using semiempirical atomic resonance integrals and the reaction center crystal structures.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: A strain of Rhodobacter sphaeroides has been constructed in which the photosynthetic reaction center is the sole pigment-protein complex and possesses assembled and functional reaction centers which can undergo photochemical charge separation and are reduced by electrons derived from the cytochrome b/c1 complex.
Abstract: A strain of Rhodobacter sphaeroides has been constructed in which the photosynthetic reaction center is the sole pigment-protein complex. The strain, named RCO1, is capable of photoheterotrophic growth and possesses assembled and functional reaction centers which can undergo photochemical charge separation and are reduced by electrons derived from the cytochrome b/c1 complex. The circular dichroism and linear dichroism of reaction centers in membranes from strain RCO1 are similar to those described previously for reaction centers isolated in detergent solution. A second strain, named RCLH11, which is devoid of the peripheral LH2 antenna complex has also been constructed. A description of the properties of these strains is presented.
TL;DR: It is concluded that photodamage of the secondary donors of the PSII reaction center occurs by two mechanisms: a rapid superoxide and tyrosine YZ+ dependent process and a slower process in which P680+/Chl+ catalyze the damages.
Abstract: The effects of photosystem II (PSII) exogenous electron donors and acceptors on the kinetics of weak light photoinhibition of NH2OH/EDTA-extracted spinach PSII membranes were examined. Under aerobic conditions, Mn2+ (approximately 1 Mn/reaction center; Km approximately 400 nM) inhibited photoinactivation and approximately 1 Mn/reaction center plus 100 microM NH2NH2 gave almost complete protection. In the absence of electron donors, strict anaerobiosis greatly inhibited photoinactivation even in the presence of an electron acceptor. Under aerobic conditions, the addition of electron acceptors (FeCN, DCIP), oxyradical scavengers, or superoxide dismutase strongly suppressed rates of photodamages. Increase in the concentrations of superoxide above those produced by illuminated NH2OH/EDTA-photosystem II membranes increased the rates of damage in the light but gave no damage in the dark. Scavengers of hydroxyl radicals and singlet oxygen did not suppress the rates of aerobic photoinhibition. These findings, along with others, lead us to conclude that photodamage of the secondary donors of the PSII reaction center occurs by two mechanisms: (1) a rapid superoxide and tyrosine YZ+ dependent process and (2) a slower process in which P680+/Chl+ catalyze the damages.
TL;DR: It is demonstrated that it is possible to achieve good photoselectivity between the primary electron donor P680 and the majority of the accessory chlorins and that the initially excited P680 excited singlet state is delocalized over at least two chlorins.
Abstract: Photosystem II reaction centers have been studied by femtosecond transient absorption spectroscopy. We demonstrate that it is possible to achieve good photoselectivity between the primary electron donor P680 and the majority of the accessory chlorins. Energy transfer can be observed in both directions between P680 and these accessory chlorins depending on which is initially excited. After excitation of either P680 or the other chlorins, the excitation energy is observed to equilibrate between the majority of these pigments at a rate of 100 +/- 50 fs-1. This energy-transfer equilibration takes place before any electron-transfer reactions and must therefore be taken into account in studies of primary electron-transfer reactions in photosystem II. We also show further evidence that the initially excited P680 excited singlet state is delocalized over at least two chlorins and that this delocalization lasts for at least 200 fs.
TL;DR: Quantitation of manganese in these particles is consistent with aspartate 170 influencing the stability or assembly of the mangane cluster, since the aspartates to asparagine mutation results in a decrease in the manGANese content per reaction center.
Abstract: To identify amino acid residues that influence the assembly or stability of the manganese cluster in photosystem II, we have generated site-directed mutations in the D1 polypeptide of the cyanobacterium, Synechocystis sp. PCC 6803. Indirect evidence has suggested that the D1 polypeptide provides some of the ligands that are required for metal binding. Mutations at position 170 of D1 were selected for characterization, since an aspartate to asparagine mutation (DN170D1) at this position completely abolishes photoautotrophic growth, while retention of a carboxylic acid at this position (aspartate to glutamate, DE170D1) supports photoautotrophic growth. Photosystem II particles were purified from control, DE170D1, and DN170D1 cells by a procedure that retains high rates of oxygen evolution activity in control particles [Noren, G.H., Boerner, R.J., & Barry, B.A. (1991) Biochemistry 30, 3943-3950]. Spectroscopic analysis shows that the tyrosine radical, Z+, which normally oxidizes the manganese cluster, is rapidly reduced in the DE170D1 mutant, but not in the DN170D1 mutant. A possible explanation of this block or dramatic decrease in the rate of electron transfer between the manganese cluster and tyrosine Z is an alteration in the properties of the metal center. Quantitation of manganese in these particles is consistent with aspartate 170 influencing the stability or assembly of the manganese cluster, since the aspartate to asparagine mutation results in a decrease in the manganese content per reaction center. Photosystem II particles from DN170D1 show a 60% decrease in the amount of specifically bound manganese per reaction center, when compared to control particles. Also, we observe a 70% decrease in the amount of specifically bound manganese per reaction center in partially purified DN170D1 particles and at least an 80% decrease in the amount of hydroxylamine-reducible manganese in DN170D1 thylakoid membranes. Single-turnover fluorescence assays and steady-state EPR measurements demonstrate that the remaining, endogenous manganese does not rapidly reduce tyrosine Z+ in the DN170D1 mutant. Additional evidence that aspartate 170 influences the assembly or stability of the metal site comes from analysis of the DE170D1 mutant. Although this mutant assembles a functional manganese cluster, as assessed by oxygen evolution and spectroscopic assays, the properties of the manganese site are perturbed.
TL;DR: Evidence is presented for a one-electron redox component that exerts strong control over the rate of photoinhibition in isolated thylakoid membranes and reveals an electron transfer pathway in photosystem II that functions to protect the reaction center against excess light energy.
Abstract: Photosystem II reaction centers in plants, algae, and cyanobacteria are susceptible to damage by excess light that irreversibly impairs activity and eventually results in the proteolytic degradation of at least one of the core proteins. The sequence of events and underlying molecular mechanisms that lead to photoinhibition are poorly understood. Here we present evidence for a one-electron redox component that exerts strong control over the rate of photosystem II photoinhibition in isolated thylakoid membranes. Monitoring the impact of various doses of visible light on the rate of water oxidation and on the variable chlorophyll fluorescence, we found that reduction of the redox component increased the rate of photoinhibition >15-fold. Anaerobic potentiometric titrations of the rate of photoinhibition revealed a redox component with a midpoint potential near 20 mV at pH 7.5. The titrations fit a Nernst equation for a one-electron reaction and were nearly pH independent. Although we have not yet identified the chemical species being titrated, a likely candidate is lowpotential cytochrome b-559. We believe this observation reveals an electron transfer pathway in photosystem II that functions to protect the reaction center against excess light energy.
TL;DR: A photosynthetic reaction center complex has been isolated from the green sulfur bacterium Chlorobium vibrioforme and the unique membrane-bound monoheme cytochrome c551 of C. vibriforme is assigned to a new class of c-type cytochromes.
TL;DR: Changes in binding of herbicides and QB to D1 as a result of the mutations revealed specific interactions between amino acid residues in D1 and the plastoquinone and distinguished between the binding sites of QB and herbicides.
Abstract: The structure-activity relationships of the plastoquinone QB binding domain in the D1 subunit of photosystem II (PSII) were investigated by characterization of mutations introduced in the D1 protein. Eight novel point mutations in the gene psbA, which encodes D1, were generated in the cyanobacterium Synechocystis PCC6803 by site-specific mutagenesis in vitro. The effects of the resulting modifications in D1 on electron transfer in PSII and on herbicide binding were analyzed. The results extend the structural analogies between the secondary quinone binding site in D1 and in subunit L of the photosynthetic reaction center in purple bacteria. The involvement of Phe255, Ser264, and Leu271 of D1 in plastoquinone binding and electron transfer in PSII was established. An indirect effect of Tyr254 on the binding of QB was demonstrated. Changes in binding of herbicides and QB to D1 as a result of the mutations revealed specific interactions between amino acid residues in D1 and the plastoquinone and distinguished between the binding sites of QB and herbicides.
TL;DR: In this paper, the authors reported a 12 ps and about 100 ps lifetime for isolated reaction centers of the purple bacterium Rb. sphaeroides and various kinetic models of the primary and secondary electron transfer processes are discussed.
TL;DR: In this paper, low-temperature steady-state emission properties have been analyzed of Photosystem II reaction center (RC) complexes isolated from spinach CP47-RC complexes after a short Triton X-100 treatment and stabilization in n-dodecyl β, d-maltoside.
TL;DR: This paper verified the extinction coefficient of the manganese-stabilizing protein by amino acid analysis, determined theManganese content of oxygen-evolving photosystem II membranes and reaction center complex using inductively coupled plasma spectrometry, and determined immunologically the amount of the Manganese Stabilizing Protein associated with Photosystem II.
TL;DR: Analysis of the sym1 mutant, mutants near P made by other groups, and interspecies variation of amino acids in the vicinity of P suggests that the protein asymmetry in the environment of the initial electron donor is important for optimizing the rate and yield of electron transfer, but is not strictly required for overall reaction center function.
Abstract: A 51 bp section of the Rhodobacter capsulatus photosynthetic reaction center M subunit gene (nucleotides M562-M612 of thepufMstructura1 sequence) encoding amino acids M 187-M203 was replaced by the homologous region of the L subunit gene. This resulted in the symmetrization of much of the amino acid environment of the reaction center initial electron donor, P. This is the first in a series of large-scale symmetry mutations and is referred to as syml. The syml mutant was able to grow photosynthetically, indicating that reaction center function was largely intact. Isolated reaction centers showed an approximately 10-nm blue shift in the QY band of P. The standard free energy change between P* and P+BphA-determined from analysis of the long-lived fluorescence from quinone-reduced reaction centers decreased from about -120 meV in the wild-type to'about -75 meV in the syml mutant. A 65-70% quantum yield of electron transfer from P* to P+QA- was observed, most of the yield loss occurring between P* and P+BphA-. The decay of the stimulated emission from P* was about 3-fold slower in this mutant than in the wild-type. Time-resolved spectral analysis of the charge-separated intermediates formed in syml reaction centers indicated that the major product was P+BphA-. A model-dependent analysis of the observed rates and electron-transfer yields gave the following microscopic rate constants for syml reaction centers (wild-type values under the same conditions are given in parentheses): (4.3) (250) 19 ps 270 ps P*BphAOA P+BphA-OA P+BPhAO-A
TL;DR: The altered GluM234 in RCs from Rb.