Biochimica et Biophysica Acta 2014-08-01

Substrate water exchange in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae.

Håkan Nilsson, Tomasz Krupnik, Joanna Kargul, Johannes Messinger

Index: Biochim. Biophys. Acta 1837(8) , 1257-62, (2014)

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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 ± 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.Copyright © 2014 The Authors. Published by Elsevier B.V. All rights reserved.