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

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 Sy and S3 states by the exchange of bound 160-substrate against 180-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 raiz 34 mass peak: kf = 52 8 s(-1) and ks = 1.9 0.3 s(-1) in the Sy state, and kf = 42 2 s(-1) and km., = 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 Sy state, which confirms beyond doubt that both substrate-water molecules are already bound in the Sy state. This leads to a very small change of the affinity for both the fast and the slowly exchanging substrates during the Sy S3 transition. Implications for recent models for water-oxidation are briefly discussed. (C) 2014 The Authors. Published by Elsevier B.V.