The effect of o-phenanthroline on the midpoint potential of the primary electron acceptor of photosystem II

Cytoplasmic Localization of Sulfide:Quinone Oxidoreductase and Persulfide Dioxygenase of Cupriavidus pinatubonensis JMP134

Heterotrophic bacteria have recently been reported to oxidize sulfide to sulfite and thiosulfate by using sulfide:quinone oxidoreductase (SQR) and persulfide dioxygenase (PDO). In chemolithotrophic bacteria, both SQR and PDO have been reported to function in the periplasmic space, with SQR as a peripheral membrane protein whose C terminus inserts into the cytoplasmic membrane and PDO as a soluble protein. Cupriavidus pinatubonensis JMP134, best known for its ability to degrade 2,4-dichlorophenoxyacetic acid and other aromatic pollutants, has a gene cluster of sqr and pdo encoding C. pinatubonensis SQR (CpSQR) and CpPDO2. When cloned in Escherichia coli, the enzymes are functional. Here we investigated whether they function in the periplasmic space or in the cytoplasm in heterotrophic bacteria. By using sequence analysis, biochemical detection, and green fluorescent protein (GFP)/PhoA fusion proteins, we found that CpSQR was located on the cytoplasmic side of the membrane and CpPDO2 was a soluble protein in the cytoplasm with a tendency to be peripherally located near the membrane. The location proximity of these proteins near the membrane in the cytoplasm may facilitate sulfide oxidation in heterotrophic bacteria. The information may guide the use of heterotrophic bacteria in bioremediation of organic pollutants as well as H₂S.IMPORTANCE Sulfide (H₂S, HS^-, and S^2-), which is common in natural gas and wastewater, causes a serious malodor at low levels and is deadly at high levels. Microbial oxidation of sulfide is a valid bioremediation method, in which chemolithotrophic bacteria that use sulfide as the energy source are often used to remove sulfide. Heterotrophic bacteria with SQR and PDO have recently been reported to oxidize sulfide to sulfite and thiosulfate. Cupriavidus pinatubonensis JMP134 has been extensively characterized for its ability to degrade organic pollutants, and it also contains SQR and PDO. This paper shows the localization of SQR and PDO inside the cytoplasm in the vicinity of the membrane. The information may provide guidance for using heterotrophic bacteria in sulfide bioremediation.

The nonheme iron in photosystem II

Photosystem II (PSII), the light-driven water:plastoquinone (PQ) oxidoreductase of oxygenic photosynthesis, contains a nonheme iron (NHI) at its electron acceptor side. The NHI is situated between the two PQs QA and QB that serve as one-electron transmitter and substrate of the reductase part of PSII, respectively. Among the ligands of the NHI is a (bi)carbonate originating from CO2, the substrate of the dark reactions of oxygenic photosynthesis. Based on recent advances in the crystallography of PSII, we review the structure of the NHI in PSII and discuss ideas concerning its function and the role of bicarbonate along with a comparison to the reaction center of purple bacteria and other enzymes containing a mononuclear NHI site.

Redox titration of fluorescence yield of photosystem II

The variable fluorescence yield of photosystem II is dependent on the redox state of the fluorescence quencher molecule or the primary electron acceptor of the system. We have carried out redox titrations of fluorescence yield of a photochemically active photosystem-II reaction-center particle and have measured the redox potential of the photosystem-II primary acceptor.During reductive titrations using dithionite as the reductant, only a single quenching transition was observed. For instance, at pH 7.0, the midpoint potential of the fluorescence transition is -325 mV, and those at a pH between 6.0 and 7.5 are consistent with a pH dependence of about 60 mV/pH unit. At a given pH, the midpoint potential of the transition closely corresponds to that of the most negative transition previously measured in unfractionated chloroplasts (both by chemical reductive titration). Oxidative titrations using ferricyanide as the oxidant yielded hysteresis in the titration curves.Similar changes in fluorescence yield were observed in redox titrations by electrochemical reduction or oxidation. Electrochemical reductive and oxidative titrations yielded reversible transitions, contrary to the hysteresis observed during chemical oxidative titration. From coulometric-titration data, we have estimated that most likely one electron is involved in the redox transition of the fluorescence-quencher or primary-electron-acceptor molecule of photosystem II. These findings are consistent with the current proposal that a membrane-bound plastoquinone functions as the primary acceptor of photosystem II.

Inhibitors in the functional dissection of the photosynthetic electron transport system

The significance of inhibitors and artificial electron acceptor and donor systems as experimental tools for studying the photosynthetic system is described by reviewing early classical articles. The historical development in unravelling the role and sequence of electron carriers and energy conserving sites in the electron transport chain is acknowledged. Emphasis is given to inhibitors of the acceptor side of photosystem II and of the plastoquinol oxidation site in the cytochrome b6/f complex. Their role in regulatory processes under redox control is introduced.

Redox study of electron donation to P-680 in Photosystem II

Flash-induced absorption changes at 820 nm were studied as a function of redox potential in Tris-extracted Photosystem II oxygen-evolving particles and Triton subchloroplast fraction II particles. The rereduction kinetics of P-680+ in both preparations showed biphasic recovery phases with half-times of 42 and 625 microseconds at pH 4.5. The magnitude of the 42 microseconds phase of P-680+ rereduction was strongly dependent on the redox potential of the medium. This absorption transient, attributed to electron donation from D1 (the secondary electron donor in oxygen-inhibited chloroplasts), titrated as a single redox component with a midpoint potential of +240 +/- 35 mV. The experimentally determined midpoint potential was found to be independent of pH over the tested range 4.5-6.0. In contrast, the magnitude of the 625 microseconds phase of P-680+ rereduction was independent of redox potential between +350 and +100 mV. These results are interpreted in terms of a model in which an alternate electron donor with Em approximately equal to 240 mV, termed D0, serves as a rapid donor (t 1/2 less than or equal to 2 microseconds) to P-680+ in Tris-extracted and Triton-treated Photosystem-II preparations. According to this model, the slower electron donor, D1, is functional only when D0 becomes oxidized.

Characterization of photosystem ii electron acceptors in Phormidium laminosum

Chlorophyll a fluorescence has been used to monitor the redox state of the primary electron acceptor of photosystem II (PS II) in the blue-green alga Phormidium laminosum during equilibrium titrations. The shape of induction curves measured in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) have been analyzed. The induction curves were very similar in unfractionated thylakoid membranes and PS II particles. In both, the fast (alpha) phase was sigmoidal, and was followed by a slow (beta) exponential tail. Thus, the structural organization and complexity of the particles (J. M. Bowes and P. Horton, Biochim, Biophys. Acta 680, 127-133 (1982), as indicated by the occurrence of energy transfer between alpha centers and presence of beta centers, must preexist in the membranes. Redox titration of the initial level of fluorescence indicated the presence of a single quencher QH in the unfractionated thylakoids, midpoint potential: Em7.0 approximately -35 mV (n = 1). Thus, the occurrence of a single acceptor is characteristic for P. laminosum and the absence of a low potential acceptor in PS II particles (J.M. Bowes, P. Horton, and D.S. Bendall, FEBS Lett. 135, 261-264 (1981] was not the result of its removal during their preparation. The midpoint potential of Q varied by -60 mV/pH unit in PS II particles and membrane fragments, with a pK at pH greater than 8.5 (particles) and at pH 7.5 (fragments). In PS II particles, DCMU raised the pK by approximately 0.5 pH units. It is argued that the pH dependence of Q is conferred by protonation of a protein which accompanies its reduction rather than protonation of the semiquinone Q X itself.

Characterization of two quenchers of chlorophyll fluorescence with different midpoint oxidation-reduction potentials in chloroplasts

The properties of two redox quenchers of chlorophyll fluorescence in chloroplasts at room temperature have been investigated. (1) Redox titration of the fluorescence yield reveals two n = 1 components with Em7.8 at--45 and --247 mV, accounting for approx. 70 and 30% of the total yield, respectively. (2) Neutral red, a redox mediator often used at redox potentials below --300 mV, preferentially quenches the fluorescence controlled by the --247 mV component. Titrations using neutral red artifactually create an n = 2 quenching component with Em7.8 = --375 mV. (3) Analysis of fluorescence induction curves recorded at different redox potentials indicates that both the --45 and --247 mV components can be photochemically reduced. The reduction of the --247 mV component corresponds to a fast phase of the induction curve whilst the slower reduction of the 45 mV component accounts for the tail phase. (4) The excitation spectra for the fluorescence controlled by the two quenchers show small differences in the ratio of chlorophyll a and b. (5) Whereas the --247 mV component readily shows a 60 mV per pH unit dependency on solution pH, the ability of the --45 mV component to respond to pH change is restricted. (6) Triton Photosystem II particles contain both quenchers but the --247 mV component accounts for approx. 70% of the fluorescence and the high component has an Em7.8 of +48 mV. The relative merits of sequential and parallel models in explaining the presence of the two quenchers are considered.

The role of plastoquinone and beta-carotene in the primary reaction of plant photosystem II

Extraction of Triton Photosystem II chloroplast fragments with 0.2% methanol in hexane for 3 h results in the removal of 90 to 95% of the plastoquinone in the original preparation. The extracted fragments (chlorophyll:plastoquinone ratio, 900: 1) showed no P-680 photooxidation at 15 K after a single laser flash. The extracted fragments also showed no light-induced C-550 absorbance change at 77 K. Reconstitution of the primary reaction of Photosystem II, as evidenced by restoration of low-temperature photooxidation of P-680, could be obtained by the addition of plastoquinone A but not by the addition of beta-carotene. The addition of beta-carotene plus plastoquinone A restored the C-550 absorbance change. These results indicate that plastoquinone functions as the primary electron acceptor of Photosystem II and that beta-carotene does not play a direct role in the primary photochemistry but is required for the C-550 absorbance change.