Interaction of photosystem I-derived protons with the water-splitting enzyme complex. Evidence for localized domains

The induction of millisecond delayed fluorescence mediated by PS I-dependent proton pumping has been used as an indicator of the time course with which those protons equilibrate with sites on the oxygen-evolving enzyme complex (Bowes, J. M., and Crofts, A. R. (1978).Z. Naturforsch. 33C, 271–275). We found that the induction curves were retarded by a reversible exposure of non-energized thylakoids to low concentrations of the uncoupler, desaspidin, at alkaline, but not at neutral, pH. The induction curves were not retarded by increasing the buffering capacity of the thylakoid lumen with Tricine, and were inhibited by the energy transfer inhibitors, dicyclohexylcarbodiimide (DCCD) and triphenyltin chloride (TPT). These data suggest that (i) the catalytic site of the water-splitting complex is located in proton-sequestering membrane domains, rather than at the lumen-exposed inner membrane surface, (ii) protons released during PS I-mediated electron transport might equilibrate with protonatable sites on the oxygen-evolving complex without passing through the lumen, and (iii) those protons may travel over specific conducting pathways which can be blocked by DCCD and TPT.