1. 1. The reduction of lipophilic (Class III) oxidants such as oxidized p-phenylenediamine consists of two components. One component requires both Photosystem II and Photosystem I and includes both sites of energy coupling associated with noncyclic electron transport. The second component requires only Photosystem II and includes only the site of energy coupling located before plastoquinone (Site II). When oxidized p-phenylenediamine is being reduced by both pathways, the overall rate of electron transport is stimulated by the addition of ADP plus phosphate or the uncoupler methylamine. However, if the Photosystem I component of oxidized p-phenylenediamine reduction is eliminated by a low concentration of the plastoquinone-antagonist dibromothymoquinone, the stimulation of electron transport by ADP plus phosphate or methylamine is also abolished, although the remaining Photosystem II-dependent electron transport remains firmly coupled to phosphorylation (via coupling Site II). These results indicate that coupling Site II, unlike the well-known rate-limiting coupling site between plastoquinone and cytochrome f (Site I), does not exert any control over the rate of associated electron transport. 2. 2. When substituted p-benzoquinones (e.g. 2,5-dimethyl-p-benzoquinone) or quinonediimides (e.g. p-phenylenediimine) are used as Class III acceptors in conjunction with dibromothymoquinone, a small but significant stimultation of electron transport by ADP plus phosphate is observed. However, it can be shown that this stimulation does not arise from coupling Site II but rather is due to a low rate of electron flux through coupling Site I even in the presence of dibromothymoquinone. Apparently the p-benzoquinones can catalyze an electron "bypass" around the dibromothymoquinone-induced block at plastoquinone, possibly by substituting partially for the natural electron carrier. If this bypass electron flow is blocked at plastocyanin by KCN treatment, the stimulation of electron transport by ADP plus phosphate is eliminated, although a high rate of phosphorylation (from Site II only) remains. 3. 3. These results provide strong evidence that a profound difference exists between the two sites of energy coupling associated with non-cyclic electron transport in isolated chloroplasts. That is, the rate of electron flow through coupling Site I, which is the rate-determining step in the Hill reaction, is strictly regulated by phosphorylating conditions, whereas the rate of electron flux through coupling Site II is independent of phosphorylating conditions. 4. 4. A model is presented which accounts for the lack of control over electron transport exhibited by coupling Site II. It is postulated that Site II is coupled to an essentially irreversible electron transport step, so that conditions which affect the phosphorylation reaction would have no effect on the rate of electron transport through the coupling site. Two essentially irreversible reactions, closely associated with Photosystem II-the water-splitting reaction and the System II photoact itself-are discussed as possible locations for coupling Site II.
ASJC Scopus subject areas
- Cell Biology