Absence of a substrate water ‘flip’ during the S2 → S3 transition of the oxygen-evolving complex in photosystem II
(English)Manuscript (preprint) (Other academic)
After a sequential storage of four oxidizing equivalents created by light-induced charge separations in the reaction center of PSII the oxygen-evolving complex (OEC) in photosystem II (PSII) catalyzes the fast oxidation of two bound substrate water molecules into molecular oxygen and protons. The oxidation states of the OEC are known as the S0, S1, S2, S3, and S4 states and involve MnIII to MnIV oxidation state changes of the Mn4CaO5 cluster. The binding of the two substrate water molecules to the manganese cluster in the S0 to S3 states is reversible and their exchange with 18O-labelled bulk water can be observed by time-resolved H218O/H216O-exchange membrane inlet mass spectrometry. One fast and one slowly exchanging substrate water were identified in both the S2 and S3 states, indicating that the two substrates are ligated in different ways. The easy interconversion of two structural forms of the Mn4CaO5 cluster in the S2 state and the possibility that only one of these forms can be oxidized to the S3 state open up the question whether the identities of the fast (Wf) and slow (Ws) exchanging substrate waters are identical in both states or if they reverse. In this study, we measured the substrate water exchange rates in the S2 and S3 states of Cyanidioschyzon merolae thylakoids. We then probed the possible interchange of the fast (Wf) and slow (Ws) substrate water molecules by inducing the S2 → S3 transition after the completion of the fast water exchange (Wf) in the S2 state. The results proved that Wf (S2) is identical to Wf (S3) and that Ws (S2) = Ws (S3). Consequences for the mechanism of water oxidation in PSII are discussed.
Photosystem II (PSII), thylakoids, substrate water, Cyanidioschyzon merolae, MIMS, FIOP
IdentifiersURN: urn:nbn:se:umu:diva-111861OAI: oai:DiVA.org:umu-111861DiVA: diva2:873745