Bicarbonate, not CO2, is the species required for the stimulation of Photosystem II electron transport

Abstract

Evidence is presented that the bicarbonate ion (HCOf), not CO 2, H2CO 3 or CO~-, is the species that stimulates electron transport in Photosystem II from spinach (Spinacia oleracea). Advantage was taken of the pH dependence of the ratio of HCO 3-to CO 2 at equilibrium in order to vary effectively the concentration of one species while holding the other constant. The Hill reaction was stimulated in direct proportion with the equilibrium HCO 3-concentration, but it was independent of the equilibrium CO 2 concentration. The other two carbonic species, H2CO 3 and CO 2-, are also shown to have no direct involvement. It is suggested that HCO 3-is the species which binds to the effector site.Bicarbonate appears to be an allosteric activator of the photosynthetic reduction of plastoquinone (PQ) in plant thylakoids. Warburg and Krippahl [1] demonstrated that the Hill reaction is impaired when CO 2 is removed from thylakoid membranes. It was later shown that this impairment is on the reducing side of Photosystem II (PS II; for a review, see Ref. 2). A number of anions, particularly formate and acetate, have been shown to interact with the binding of bicarbonate (HCO 3) suggesting a more general anion binding site (see e.g., Refs. 3 and 4). However, only HCO 3 has been shown to exert a stimulatory effect on PS II. Although a partial inhibition of the quinone reactions has been observed in CO2-depleted thylakoids * To whom correspondence and reprint requests should be addressed. Abbreviations: Chl, chlorophyll; [CO2], the CO 2 concentration; DCIP, 2,6-dichlorophenolindophenol; [HCO3], the HCO 3 concentration; PQ, plastoquinone; PS II, Photosystem II; QA and QB, first and second quinone acceptors of PS II, respectively. in the absence of other anions [5], the full inhibitory effect seems to require their presence.In the presence of formate, electron transfer from the secondary quinone Q s to the PQ pool is blocked by HCO 3 depletion [6][7][8], and electron transfer from the primary quinone QA tO QB is slowed down [7,9]. Herbicides acting at the Qs site bind less tightly when HCO 3 is removed, supporting the conclusion that the site of HCO Z action is at the quinone level [10][11][12]. It is now generally accepted that Qs is a bound PQ which, when fully reduced, exchanges for an oxidized PQ from the PQ pool [13,14]. Competitive binding between herbicides and quinones supports this view [15][16][17][18]. Our current hypothesis for the HCO 3 requirement, as noted earlier, is that HCO 3 acts as an allosteric activator for the reduction of bound PQ, and induces a conformational change which permits the efficient exchange between the bound PQH 2 and an oxidized PQ. Bicarbonate may also be involved in the protonation of PQH 2 [19,20]. The exact mechanism of action, however, is unknown. Before a reasonable mechanistic model 0005-2728/86/$03.50

Bicarbonate is an essential constituent of the water-oxidizing complex of photosystem II

“…The same plot for O2 evolution in untreated (Mn-containing) BBY membrane fragments in bicarbonate-depleted medium is given on A and B by Ⅵ (left scales). A component with Kd lower than 10 M (we could not determine its definitive value) is designated as Kd x in C. The equilibrium bicarbonate concentration after the addition of an aliquote of NaHCO3 and 3-min incubation at 20ЊC in the dark in a tightly closed cell was calculated using the equation for the equilibrium between carbonic species in aquous solution at a given pH (27).…”

Role of bicarbonate at the acceptor side of Photosystem II

“…Because he found that dependence on bicarbonate was correlated with the presence of other small anions during the depletion period, Good concluded that bicarbonate is the important species, not CO 2 . This conclusion was later confirmed by Blubaugh and Govindjee (1986) by taking advantage of the pH dependence of the equilibrium Figure 1. Experiments with grana suspended in 0.1% KCl (3 ml vessel).…”

Adventures with cyanobacteria: a personal perspective

“…Data on higher plant thylakoids suggest that the diffusing species is CO 2 and the binding species is ${\text{HCO}}_3^{-}/{\text{CO}}_3^{2-}$ (Sarojini and Govindjee, 1981; Blubaugh and Govindjee, 1986). Using thylakoids, the HC effect has been located on the electron-acceptor side, close to the herbicide-binding side, particularly on the electron transport and protonation reactions at the secondary quinone Q B and near the non-heme iron (Govindjee et al, 1976, 1984, 1991a, 1992; Jursinic et al, 1976; Khanna et al, 1980, 1981; Van Rensen and Vermaas, 1981; Eaton-Rye and Govindjee, 1988a,b).…”