Mechanisms for proton release during water oxidation in the S2 to S3 and S3 to S4 transitions in photosystem II

The new high-resolution X-ray structure of photosystem II has allowed more detailed studies than before of water oxidation at the oxygen evolving complex (OEC). In the present study the two final S-transitions of water oxidation are studied. The electron coupled proton transfers are followed from the center of the OEC to Asp61, which is considered as the start of the transfer chain through the protein to the lumenal side. It is found that the proton transfers occur in multiple steps. Structures of intermediates and energy diagrams are derived and compared to experimental observations. Since the new experimental structure of the OEC is very similar to the one suggested earlier by density functional calculations, the O-O bond formation step remains essentially the same as the one suggested five years ago. An interesting new result is that the barrier for proton transfer within the OEC actually competes with the O-O bond formation step of being rate-limiting.