2. The initial phase of H+ uptake, analyzed by a flash-yield technique, exhibits linear kinetics (0–3 s) with no sign of transient phenomena such as the very rapid initial uptake (“pH gush”) encountered in the overall Hill reaction with methylviologen. Thus the initial rate of H+ uptake obtained by the flash-yield method is in good agreement with the initial rate estimated from a pH change tracing obtained under continuous illumination.
3. Dibromothymoquinone reduction, observed as O2 evolution by a similar flash-yield technique, is also linear for at least the first 5 s, the rate of O2 evolution agreeing well with the steady-state rate observed under continuous illumination.
4. Such measurements of the initial rates of O2 evolution and H+ uptake yield an H+/e− ratio close to 0.5 for the Photosystem II partial reaction regardless of pH from 6 to 8. (Parallel experiments for the methylviologen Hill reaction yield an H+/e− ratio of 1.7 at pH 7.6.)
5. When dibromothymoquinone is being reduced, concurrent phosphorylation (or arsenylation) markedly lowers the extent of H+ uptake (by 40–60%). These data, unlike earlier data obtained using the overall Hill reaction, lend themselves to an unequivocal interpretation since phosphorylation does not alter the rate of electron transport in the Photosystem II partial reaction. ADP, Pi and hexokinase, when added individually, have no effect on proton uptake in this system.
6. The involvement of a proton uptake reaction with an H+/e− ratio of 0.5 in the Photosystem II partial reaction H2O → Photosystem II → dibromothymoquinone strongly suggests that at least 50% of the protons produced by the oxidation of water are released to the inside of the thylakoid, thereby leading to an internal acidification. It is pointed out that the observed efficiencies for ATP formation (P/e2) and proton uptake (H+/e−) associated with Coupling Site II can be most easily explained by the chemiosmotic hypothesis of energy coupling. 相似文献