Replacement of photosynthetic electron transport inhibitors by silicomolybdate

Böger, Peter

doi:10.1111/j.1399-3054.1982.tb06329.x

Cited by 11 publications

(1 citation statement)

References 18 publications

(20 reference statements)

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“…Similarly, the binding of HCO 3 at site A would be expected to affect diuron binding, even though the sites are spatially separated. The apparent ability of silicomolybdate (SiMo) to reduce the binding affinity of diuron (B6ger 1982(B6ger , Graan 1986) may be due to SiMo removing HCO~-from the Fe 2+ (Site A), rather than due to competition between SiMo and diuron for the same site, as was suggested by Brger (1982). That SiMo does remove bound HCO3 was shown previously by Stemler (1977), who observed that when the diuron was added before the SiMo, about half as much HCO~-was removed.…”

Section: Discussion Of the Modelmentioning

confidence: 97%

The molecular mechanism of the bicarbonate effect at the plastoquinone reductase site of photosynthesis

1988

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It has been known for some time that bicarbonate reverses the inhibition, by formate under HCO3 (-)-depletion conditions, of electron transport in thylakoid membranes. It has been shown that the major effect is on the electron acceptor side of photosystem II, at the site of plastoquinone reduction. After presenting a historical introduction, and a minireview of the bicarbonate effect, we present a hypothesis on how HCO3 (-) functions in vivo as (a) a proton donor to the plastoquinone reductase site in the D1-D2 protein; and (b) a ligand to Fe(2+) in the QA-Fe-QB complex that keeps the D1-D2 proteins in their proper functional conformation. They key points of the hypothesis are: (1) HCO3 (-) forms a salt bridge between Fe(2+) and the D2 protein. The carboxyl group of HCO3 (-) is a bidentate ligand to Fe(2+), while the hydroxyl group H-bonds to a protein residue. (2) A second HCO3 (-) is involved in protonating a histidine near the QB site to stabilize the negative charge on QB. HCO3 (-) provides a rapidly available source of H(+) for this purpose. (3) After donation of a H(+), CO3 (2-) is replaced by another HCO3 (-). The high pKa of CO3 (2-) ensures rapid reprotonation from the bulk phase. (4) An intramembrane pool of HCO3 (-) is in equilibrium with a large number of low affinity sites. This pool is a H(+) buffering domain functionally connecting the external bulk phase with the quinones. The low affinity sites buffer the intrathylakoid [HCO3 (-)] against fluctuations in the intracellular CO2. (5) Low pH and high ionic strength are suggested to disrupt the HCO3 (-) salt bridge between Fe(2+) and D2. The resulting conformational change exposes the intramembrane HCO3 (-) pool and low affinity sites to the bulk phase.Two contrasting hypotheses for the action of formate are: (a) it functions to remove bicarbonate, and the low electron transport left in such samples is due to the left-over (or endogenous) bicarbonate in the system; or (b) bicarbonate is less of an inhibitor and so appears to relieve the inhibition by formate. Hypothesis (a) implies that HCO3 (-) is an essential requirement for electron transport through the plastoquinones (bound plastoquinones QA and QB and the plastoquinone pool) of photosystem II. Hypothesis (b) implies that HCO3 (-) does not play any significant role in vivo. Our conclusion is that hypothesis (a) is correct and HCO3 (-) is an essential requirement for electron transport on the electron acceptor side of PS II. This is based on several observations: (i) since HCO3 (-), not CO2, is the active species involved (Blubaugh and Govindjee 1986), the calculated concentration of this species (220 μM at pH 8, pH of the stroma) is much higher than the calculated dissociation constant (Kd) of 35-60 μM; thus, the likelihood of bound HCO3 (-) in ambient air is high; (ii) studies on HCO3 (-) effect in thylakoid samples with different chlorophyll concentrations suggest that the "left-over" (or "endogenous") electron flow in bicarbonate-depleted chloroplasts is due to "left-over" (or endogenous) HCO3...

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Section: Discussion Of the Modelmentioning

confidence: 97%

The molecular mechanism of the bicarbonate effect at the plastoquinone reductase site of photosynthesis

1988

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The molecular mechanism of the Bicarbonate effect at the Plastoquinone reductase site of Photosynthesis

Blubaugh

1988

Molecular Biology of Photosynthesis

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The donor side of Photosystem II as the copper-inhibitory binding site

et al. 1995

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We have measured, under Cu (II) toxicity conditions, the oxygen-evolving capacity of spinach PS II particles in the Hill reactions H2O→SiMo (in the presence and absence of DCMU) and H2O→PPBQ, as well as the fluorescence induction curve of Tris-washed spinach PS II particles. Cu (II) inhibits both Hill reactions and, in the first case, the DCMU-insensitive H2O → SiMo activity. In addition, the variable fluorescence is lowered by Cu (II). We have interpreted our results in terms of a donor side inhibition close to the reaction center. The same polarographic and fluorescence measurements carried out at different pHs indicate that Cu (II) could bind to amino acid residues that can be protonated and deprotonated. In order to reverse the Cu (II) inhibition by a posterior EDTA treatment, in experiments of preincubation of PS II particles with Cu (II) in light we have demonstrated that light is essential for the damage due to Cu (II) and that this furthermore is irreversible.

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Replacement of photosynthetic electron transport inhibitors by silicomolybdate

Cited by 11 publications

References 18 publications

The molecular mechanism of the bicarbonate effect at the plastoquinone reductase site of photosynthesis

The molecular mechanism of the bicarbonate effect at the plastoquinone reductase site of photosynthesis

The molecular mechanism of the Bicarbonate effect at the Plastoquinone reductase site of Photosynthesis

The donor side of Photosystem II as the copper-inhibitory binding site

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