Authors: Boris K Semin Lira N Davletshina Kirill N Timofeev Il’ya I Ivanov Andrei B Rubin Michael Seibert
Publish Date: 2013/06/21
Volume: 117, Issue: 1-3, Pages: 385-399
Abstract
Extraction of Ca2+ from the oxygenevolving complex of photosystem II PSII in the absence of a chelator inhibits O2 evolution without significant inhibition of the lightdependent reduction of the exogenous electron acceptor 26dichlorophenolindophenol DCPIP on the reducing side of PSII The phenomenon is known as “the decoupling effect” Semin et al Photosynth Res 98235–249 2008 Extraction of Cl− from Ca2+depleted membranes PSII–Ca suppresses the reduction of DCPIP In the current study we investigated the nature of the oxidized substrate and the nature of the products of the substrate oxidation After elimination of all other possible donors water was identified as the substrate Generation of reactive oxygen species HO H2O2 and O 2 ·− as possible products of water oxidation in PSII–Ca membranes was examined During the investigation of O 2 ·− production in PSII–Ca samples we found that i O 2 ·− is formed on the acceptor side of PSII due to the reduction of O2 ii depletion of Cl− does not inhibit water oxidation but iii Cl− depletion does decrease the efficiency of the reduction of exogenous electron acceptors In the absence of Cl− under aerobic conditions electron transport is diverted from reducing exogenous acceptors to reducing O2 thereby increasing the rate of O 2 ·− generation From these observations we conclude that the product of water oxidation is H2O2 and that Cl− anions are not involved in the oxidation of water to H2O2 in decoupled PSII–Ca membranes These results also indicate that Cl− anions are not directly involved in water oxidation by the Mn cluster in the native PSII membranes but possibly provide access for H2O molecules to the Mn4CaO5 cluster and/or facilitate the release of H+ ions into the lumenal space
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