Predicting Photosynthesis

Hesketh, J. D.; Woolley, Joseph T.; Peters, D. B.

doi:10.1016/b978-0-12-294302-7.50019-7

Cited by 3 publications

(3 citation statements)

References 93 publications

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“…Recent binding studies (Stemler and Murphy, 1983) have shown that bicarbonate binds to photosys-tem I1 with a K d of 70 to 100 p M in the absence of formate. This is far above physiological concentrations of less than 5 p M (Hesketh et al, 1982) when photosynthesis is taking place in a leaf. By this direct measurement of bicarbonate binding it was also shown that formate is a competitive inhibitor of bicarbonate binding.…”

Section: Discussionmentioning

confidence: 76%

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

Jursinic

Stemler

1986

Photochem & Photobiology

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The binding parameters of bicarbonate to the thylakoid membrane at different formate concentrations have been established [Stemler and Murphy (1983) Photochem. Phorobiol. 38, 701-7071. Based on these parameters, predictions could be made concerning the effects of bicarbonate and formate on photosynthetic electron flow. In this work these effects of various concentrations of bicarbonate and formate are measured and compared to predictions from the binding study. Electron flow is measured between QA and QB (the primary and secondary quinone acceptors) and QB and the plastoquinone pool. Also, these same concentration effects are determined for silicomolybdate supported oxygen evolution. It is found that the results of the bicarbonate binding study are in good agreement with the concentration dependence determined for the quinone reactions, as well as the silicomolybdate reaction. The bicarbonate concentrations required for half-maximal effects are approximately 100 p M , 300 pM and 1.3 mM in the presence of 0,20 mM and 100 mM formate, respectively. It is concluded that a hierarchy of possible electron flow rates exist. The slowest rates occur when formate is bound. A substantially higher rate occurs when neither formate nor bicarbonate (< 2 pM) are present, but only chloride is present. The highest rates of electron flow occur when bicarbonate is bound. The QA-QB -+ Q A QB-, QA-QB*-PQ -+ Q A QB-PQZ-, and the silicomolybdate reactions all have the same concentration dependence on formate and bicarbonate. From this it is concluded that a single binding site for formate and bicarbonate affect all of these reactions. The possibility that multiple sites exist with approximately equal affinities for bicarbonate cannot be excluded.

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Section: Discussionmentioning

confidence: 76%

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

Jursinic

Stemler

1986

Photochem & Photobiology

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“…With the measurement by several laboratories of a dissociation constant for the HCO; * PS II complex of 80 #M (Stemler and Murphy 1983, Snel and van Rensen 1984, Jursinic and Stemler 1986, it has been suggested that the stimulatory effect of HCO3 is no more than a simple reversal of an inhibitory effect of HCO~- (Stemler and Murphy 1983). However, the argument was based on the observation that the intracellular [CO2] is around 5 #M in photosynthesizing chloroplasts (Hesketh et al 1982). It is now established that HCO3, not CO2, is the activating species .…”

Section: The Role Of Hcoj-mentioning

confidence: 99%

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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“…This claim stem~ from an estimated in vivo CO 2 concentration of < 5 ~M [32] which, it has been suggested, would result in the HC03 binding site being unoccupied.…”

Section: Mechanism Of Bicarbonate Action and Possible Physiological Rmentioning

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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Predicting Photosynthesis

Cited by 3 publications

References 93 publications

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

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

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

Electron transfer through photosystem II acceptors: Interaction with anions

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