Correlation Between the Binding of Formate and Decreased Rates of Charge Transfer Through the Photosystem Ii Quinones

Jursinic, Paul A.; Stemler, Alan

doi:10.1111/j.1751-1097.1986.tb09515.x

Cited by 20 publications

(21 citation statements)

References 29 publications

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“…3A). This enhanced inhibition also agrees with the literature (25,26). In contrast, formate has a significantly smaller effect on the E m of Q A =Q −• A (Fig.…”

Section: Q −•supporting

confidence: 81%

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Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

Brinkert

Causmaecker

Krieger‐Liszkay

et al. 2016

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

120

173

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The midpoint potential (E m ) of Q A =Q −• A , the one-electron acceptor quinone of Photosystem II (PSII), provides the thermodynamic reference for calibrating PSII bioenergetics. Uncertainty exists in the literature, with two values differing by ∼80 mV. Here, we have resolved this discrepancy by using spectroelectrochemistry on plant PSII-enriched membranes. Removal of bicarbonate (HCO 3 − ) shifts the E m from ∼−145 mV to −70 mV. The higher values reported earlier are attributed to the loss of HCO 3 − during the titrations (pH 6.5, stirred under argon gassing). P hotosystem II (PSII), the water/plastoquinone photooxidoreductase, is at the heart of the major energy cycle that powers the biosphere. Chlorophyll-based photochemistry drives charge separation followed by electron transfer reactions that result in the reduction of quinone on one side of the thylakoid membrane ( Fig. 1) and the oxidation of water on the other. The photochemistry is intrinsically a one-photon/one-electron process, but the quinone reduction and water oxidation are two-and fourelectron processes, respectively. As a result, both processes involve the accumulation of intermediates that are stabilized by the protein and by coupling to protonation reactions (1-3).The electron acceptor side of PSII contains a nonheme ferrous iron (Fe 2+ ) that is flanked by the two quinones, Q A and Q B (Fig. 1) (reviewed in refs. 3 and 4). The Fe 2+ is coordinated by four histidine residues, two from D1 and two from D2, and a bicarbonate ion (HCO 3 − ) that provides a bidentate ligand (3-5). The HCO 3 − is thought to play a role in the Q B protonation pathway (reviewed in refs. 3-5). Recent EPR (6) and computation chemistry studies (7) based on the highest-resolution X-ray crystallographic model (8) implicated HCO 3 − in the second of two protonation steps that are associated with Q B reduction. Measurements of the dissociation constant of HCO 3 − (K d = 40-80 μM), compared with estimates of the HCO 3 − concentration in the stroma (reviewed in ref. 9), led to the assumption that HCO 3 − remains bound under physiological conditions (10). Recently, however, it was reported that the HCO 3 − bound to PSII in Chlamydomonas can be displaced by acetate when present in the culture medium (11).The redox potential of Q A has been the subject of research for decades (12). The current detailed thermodynamic picture of PSII redox chemistry is based on estimates of energy differences between the electron transfer components, but these estimates A , differing by ∼150 mV, depending on the nature of the PSII (12-14). The fully active enzyme showed an E m value that was ∼150 mV lower than that in PSII lacking the Mn 4 CaO 5 cluster. This shift was, in fact, due to the binding (or absence thereof) of the Ca 2+ ion involved in water splitting, but because the Mn cluster provides the Ca binding site, the potential is indirectly determined by the presence of Mn cluster (12)(13)(14). Given the high valence state on the Mn cluster even in the lowest redox state of the water splitti...

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“…3A). This enhanced inhibition also agrees with the literature (25,26). In contrast, formate has a significantly smaller effect on the E m of Q A =Q −• A (Fig.…”

Section: Q −•supporting

confidence: 81%

“…5). Reversible changes in the kinetics were seen, closely matching those reported earlier (25,26). When HCO 3 − depletion was accompanied by the addition of formate as a replacement ion, more marked changes were seen in the kinetics of Q −• A oxidation as measured by the fluorescence decay seen after a flash (Fig.…”

Section: Q −•supporting

confidence: 71%

Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

Brinkert

Causmaecker

Krieger‐Liszkay

et al. 2016

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

120

173

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“…Clearly, in these experiments, added HCOT became bound to empty sites. The sites involved showed a binding constant of about 80 jM C02/ HCO in the absence of formate (8). Curiously, in this case, the binding of HCOi led to an inhibition, not a stimulation, of electron flow rates.…”

Section: Resultsmentioning

confidence: 88%

“…Yet it is unnecessary to add supplementary HCOT to obtain high rates of electron transport activity except in the presence of inhibitory anions. (b) Untreated chloroplasts, showing normal rates of electron flow, are inhibited by exogenous HCO when silicomolybdate is used as an electron acceptor (2,8). If HCO3 was already present, to give normal rates of electron flow, exogenous HCO should have no effect.…”

Section: Resultsmentioning

confidence: 99%

Absence of a Formate-Induced Release of Bicarbonate from Photosystem II

Stemler¹

1989

Plant Physiol.

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Formate has been proposed to inhibit electron flow in photosystem 11 by replacing endogenous bound bicarbonate on the reaction center complex. A mass spectrometer was used to measure directly the CO2/HCO3 released when maize thylakoids, showing normal rates of electron flow, were treated with formate. Although the formate inhibited electron flow by 95%, no release (displacement) of CO2/HCO-was detected. This is consistent with the concept that membrane-bound HCO-is not a requirement for normal rates of electron flow through photosystem 11. Moreover, formate and other monovalent anions do not inhibit electron flow by removing bound HCO3 but by binding to empty sites. The "bicarbonate effect" is a reversal, by high concentrations of exogenous bicarbonate, of anion inhibition of photosystem I. MATERIALS AND METHODS Maize (Zea mays) plants were grown in a greenhouse, and broken chloroplasts were isolated as previously described (16). The grinding medium contained 0.05 M sodium phosphate (pH 7.0), 0.4 M sucrose, 5 mM MgCl2. After the final centrifugation, the pellet was resuspended in a small amount of grinding medium and frozen at -80°C. Just before the experiments began, the thylakoids were thawed and washed once in a fivefold excess of reaction mixture that contained 0.05 M sodium phosphate (pH 6.0) and 0.05 M NaCl, and then collected by centrifugation. The pellet was resuspended in a small amount of reaction mixture, kept on ice, and used in either the Hill reaction measurements or mass spectrometry experiments.Oxygen evolution supported by added electron acceptors was monitored with a Rank-Brothers, Clark-type electrode system. The light source was a Sylvania 500-W projector lamp, and the light was focused with a slide projector. Samples were illuminated with saturating white light of approximately 500 W/m2. The temperature was 25°C.CO2 in the chloroplast samples was measured with a VG Trio-2 mass spectrometer. A Plexiglas reaction chamber was separated from the vacuum line leading to the instrument by means of a General Electric MEM-213 membrane. The sample was stirred constantly during the measurements with a magnetic flea. The temperature was kept constant at 25°C. RESULTSControl chloroplasts illuminated at pH 6.0 in the presence ofelectron acceptors showed normal rates ofoxygen evolution (Table I). Chloroplasts given 10 mm NaHCO-3 in addition showed essentially the same rate as the controls. In view of the alternatives discussed in the Introduction, this means that either the controls already had a full complement of membrane-bound HCO-, or

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“…Jursinic and Stemler [42] have -also demSD~trated, using identical experimental conditions as employed for the H]4CO~-blndlng constant determination, an 80 ~M K m (concentration required to re~tore half-maxlmal activity) for QA reoxidation as monitored by the decay of variable Chl a fluorescence following a single actinic flash.…”

Section: The Binding Constant For Bicarbonate At the Anion Sitementioning

confidence: 99%

Electron transfer through photosystem II acceptors: Interaction with anions

Govindjee

Eaton‐Rye

1986

Photosynth Res

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We present an overview of anionic interactions with the oxidationreduction reactions of photosystem II (PSII) acceptors.In section I, a framework is lald for the electron ~cceptor side of PSII:the overview begins with a current scheme of the electron transport pathway and of the localization of components in the thylakoid membrane, which is followed by a brief description of the electron acceptor Q or QA and the various heterogeneities associated with it. In section 2, we review briefly the nature of the active species of the bicarbonate (HC03) effect, the location of the site of action of HCO], and its Pelatlonship to interactions with other anions.In section 3,-we review data on the anion effects on the reoxldation of QA and on the various reactions involved in the twoelectron gate mechanism of PSII,~and prOvldela:hypothesis as to the action of HCO 3 on the protonation reactions.New data ~btalned by one of us (G) in collaboration with J.J.S. van Rensen, J.F.H Sne! and W. Tonk for HCO3-depleted thylakolds, demonstrating the abolition iof the binary oscill~-tlons contained within the periodicity of 4 observed for proton release, are also reviewed.In section 4, we comment on t~e measured binding constant of HCO3 at the anion binding site. And, in section 5, we review our current concept of the mechanism of the HCO3 effect on the electron accepter side of PSII, and_comment on the possible physiological roles for HCO 3.Measurements of HCOg reverslble anionle inhlbltlon in intact cells of ~ green alga Soenedesmus-are also reviewed.

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Correlation Between the Binding of Formate and Decreased Rates of Charge Transfer Through the Photosystem Ii Quinones

Cited by 20 publications

References 29 publications

Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

Absence of a Formate-Induced Release of Bicarbonate from Photosystem II

Electron transfer through photosystem II acceptors: Interaction with anions

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