Substrate water exchange in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae |
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Authors: | Hå kan Nilsson,Tomasz Krupnik,Joanna Kargul,Johannes Messinger |
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Affiliation: | 1. Department of Chemistry, Kemiskt Biologiskt Centrum (KBC), Umeå University, S-901 87 Umeå, Sweden;2. Department of Plant Molecular Physiology, Faculty of Biology, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland |
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Abstract: | The binding affinity of the two substrate–water molecules to the water-oxidizing Mn4CaO5 catalyst in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae was studied in the S2 and S3 states by the exchange of bound 16O-substrate against 18O-labeled water. The rate of this exchange was detected via the membrane-inlet mass spectrometric analysis of flash-induced oxygen evolution. For both redox states a fast and slow phase of water-exchange was resolved at the mixed labeled m/z 34 mass peak: kf = 52 ± 8 s− 1 and ks = 1.9 ± 0.3 s− 1 in the S2 state, and kf = 42 ± 2 s− 1 and kslow = 1.2 ± 0.3 s− 1 in S3, respectively. Overall these exchange rates are similar to those observed previously with preparations of other organisms. The most remarkable finding is a significantly slower exchange at the fast substrate–water site in the S2 state, which confirms beyond doubt that both substrate–water molecules are already bound in the S2 state. This leads to a very small change of the affinity for both the fast and the slowly exchanging substrates during the S2 → S3 transition. Implications for recent models for water-oxidation are briefly discussed. |
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Keywords: | C. merolae, Cyanidioschyzon merolae PSII, photosystem II PSIIcc, photosystem II core complexes OEC, oxygen evolving complex S states (Si), oxidation states of the OEC Wf, fast exchanging substrate&ndash water Ws, slowly exchanging substrate&ndash water ks, rate constant of slow water exchange kf, rate constant of fast water exchange TR-MIMS, time resolved membrane inlet mass spectrometry FIOPs, flash induced oxygen evolution pattern |
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