title: Substrate water exchange in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae.
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 ± 8s⁻¹ and ks=1.9 ± 0.3s⁻¹ in the S₂ state, and kf=42 ± 2s⁻¹ and kslow=1.2 ± 0.3s⁻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.
authors.affiliation: Department of Chemistry, Kemiskt Biologiskt Centrum (KBC), Umeå University, S-901 87 Umeå, Sweden.
authors.forename: Håkan
authors.initials: H
authors.lastname: Nilsson
authors.affiliation: Department of Plant Molecular Physiology, Faculty of Biology, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland.
authors.forename: Tomasz
authors.initials: T
authors.lastname: Krupnik
authors.affiliation: Department of Plant Molecular Physiology, Faculty of Biology, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland.
authors.forename: Joanna
authors.initials: J
authors.lastname: Kargul
authors.affiliation: Department of Chemistry, Kemiskt Biologiskt Centrum (KBC), Umeå University, S-901 87 Umeå, Sweden. Electronic address: Johannes.Messinger@umu.se.
authors.forename: Johannes
authors.initials: J
authors.lastname: Messinger
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D007700:Kinetics; D008345:Manganese / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000235:genetics; D000461:Rhodophyta / Q000737:chemistry / Q000235:genetics / Q000254:growth & development*; D013379:Substrate Specificity; D014867:Water / Q000737:chemistry*
chemical_list: D045332:Photosystem II Protein Complex; D014867:Water; D008345:Manganese; D010100:Oxygen
keywords: Cyanidioschyzon merolae; Membrane-inlet mass spectrometry; Oxygen evolution; Photosystem II; Substrate–water exchange; Water oxidation