Nilsson H. auteur aut IRD Cournac Laurent auteur aut IRD Rappaport F. auteur aut IRD Messinger J. auteur aut IRD Lavergne J. auteur aut IRD text journalArticle eng text/pdf born digital access Photosynthetic water oxidation to molecular oxygen is carried out by photosystem II (PSII) over a reaction cycle involving four photochemical steps that drive the oxygen-evolving complex through five redox states S-i (i = 0, ... , 4). For understanding the catalytic strategy of biological water oxidation it is important to elucidate the energetic landscape of PSII and in particular that of the final S-4 --> S-0 transition. In this short-lived chemical step the four oxidizing equivalents accumulated in the preceding photochemical events are used up to form molecular oxygen, two protons are released and at least one substrate water molecule binds to the Mn4CaO5 cluster. In this study we probed the probability to form S-4 from S-0 and O-2 by incubating YD-less PSII in the S-0 state for 2-3 days in the presence of O-18(2) and (H2O)-O-16. The absence of any measurable O-16,18(2) formation by water-exchange in the S-4 state suggests that the S-4 state is hardly ever populated. On the basis of a detailed analysis we determined that the equilibrium constant K of the S-4 --> S-0 transition is larger than 1.0 x 10(7) so that this step is highly exergonic. We argue that this finding is consistent with current knowledge of the energetics of the S-0 to S-4 reactions, and that the high exergonicity is required for the kinetic efficiency of PSII. specialized Photosystem II Water-oxidizing complex (WOC) Oxygen-evolving complex (OEC) Equilibrium constant for S-4 -> S-0 transition 020 034 1857 1 23-33 2016 0005-2728 https://www.documentation.ird.fr/hor/fdi:010066005 10.1016/j.bbabio.2015.09.011 0005-2728 [F B010066005] https://www.documentation.ird.fr/hor/fdi:010066005 https://www.documentation.ird.fr/intranet/publi/2016/01/010066005.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2016-02-02 2017-08-23 fdi:010066005 fre