Proton coupling, purple bacteria

Okamura, M. Y., and Feher, G., 1995, Proton-coupled electron transfer reactions of Qg in reaction centers from purple bacteria. In Anoxygenic Photosynthetic Bacteria, (R. E. Blankenship, M. T. Madigan, and C. E. Bauer, eds.) pp. 5779594, Kluwer Academic Publishers, DordrechL The Netherlands. 

Lancaster CRD, ErmlerU and Michel H (1995) The structures of photosynthetic reaction centers from purple bacteria as revealed by X-ray crystallography. In Blankenship RE, Madigan MT and Bauer CE (eds) Anoxygenic Photosynthetic Bacteria, pp 503-526. Kluwer Academic Publishers, Dordrecht Lancaster CRD, Michel H, Honig B and Gunner MR (1996) Calculated coupling of electron and proton transfer in the photosyntheticreaction centre of Rhodopseudomonas viridis. Biophys 1 70 2469-2492... 

It performs water oxidation, resulting in Oj evolution and in proton release. The redox equilibrium 2H2O —> 4H+ -I- 4e + Oj has an Em of -1-0.82 V at pH 7. The reaction is pulled by the oxidized primary donor P (named P-680 in Photosystem II). P-680 is a special pair of chlorophyll a molecules, but these molecules are not as close as in purple bacteria, and they are surrounded by two other chlorophyll a, so that P-680 can also be described as a special tetramer. The P-680/P-680 redox couple must have an Em (perhaps around -1-1.2 V) much more positive than -1-0.82 V, in order to oxidize water irreversibly. P-680 is directly reduced by a tyrosine residue named TyrZ it is Tyr 161 of the polypeptide Dl. Oxidized TyrZ then oxidizes a cluster of four manganese atoms, which is the catalytic site for water oxidation. A histidine residue is probably involved in the process, which also requires Ca and Ck ions. The details of the structure and mechanism of this ensemble are still under intense investigation. "... 

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