Proton release from water oxidation by photosystem II: similar stoichiometries are stabilized in thylakoids and PSII core particles by glycerol.

During the four-stepped catalytic cycle of water oxidation by photosystem II (PSII) molecular oxygen is released in only one of the four reaction steps whereas the release of four protons is distributed over all steps. In principle, the pattern of proton production could be taken as indicative of the partial reactions with bound water. In thylakoids the extent and rate of proton release varies as function of the redox transition and of the pH without concomitant variations of the redox pattern. The variation has allowed to discriminate between deprotonation events of peripheral amino acids (Bohr effects) as opposed to the chemical deprotonation of a particular redox cofactor, and of water. In contrast, in thylakoids grown under intermittent light, as well as in PSII core particles the pattern of proton release is flat and independent of the pH. This has been attributed to the lack in these materials of the chlorophyll a,b-binding (CAB) proteins. We now found that a thylakoid-like, oscillatory pattern of proton release was restored simply by the addition of glycerol which modifies the protein-protein interaction. Being a further proof for the electrostatic origin of the greater portion of proton release, this effect will serve as an important tool in further studies of water oxidation.