Oct 30, 2007

Access channels and methanol binding site to the CaMn4 cluster in Photosystem II based on solvent accessibility simulations, with implications for substrate water access

Biochimica Et Biophysica Acta
Felix M Ho, Stenbjörn Styring


Given the tightly packed environment of Photosystem II (PSII), channels are expected to exist within the protein to allow the movement of small molecules to and from the oxygen evolving centre. In this report, we calculate solvent contact surfaces from the PSII crystal structures to identify such access channels for methanol and water molecules. In a previous study of the effects of methanol on the EPR split S1-, S3-, and S0-signals [Su et al. (2006) Biochemistry 45, 7617-7627], we proposed that methanol binds to one and the same Mn ion in all S-states. We find here that while channels of methanol dimensions were able to make contact with the CaMn4 cluster, only 3Mn and 4Mn were accessible to methanol. Combining this observation with spectroscopic data in the literature, we propose that 3Mn is the ion to which methanol binds. Furthermore, by calculating solvent contact surfaces for water, we found analogous and more extensive water accessible channels within PSII. On the basis of their structure, orientation, and electrostatic properties, we propose functional assignments of these channels as passages for substrate water access to the CaMn4 cluster, and for the exit of O2 and H+ that are released during water oxidation. Finally...Continue Reading

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