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

Abstract

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 (-) remaining bound to the system (Blubaugh 1987).