4 years ago

Structural Factors That Alter the Redox Potential of Quinones in Cyanobacterial and Plant Photosystem I

Structural Factors That Alter the Redox Potential of Quinones in Cyanobacterial and Plant Photosystem I
Hiroshi Ishikita, Keisuke Kawashima
Using the cyanobacterial and plant photosystem I (PSI) crystal structures and by considering the protonation states of all titratable residues, redox potentials (Em) of the two phylloquinones—A1A and A1B—were calculated. The calculated Em values were Em(A1A) = −773 mV and Em(A1B) = −818 mV for the plant PSI structure and Em(A1A) = −612 mV and Em(A1B) = −719 mV for the cyanobacterial PSI structure. Our analysis of the PSI crystal structures suggested that the side-chain orientations of Lys-B542 and Gln-B678 in the cyanobacterial crystal structure differ from these side-chain orientations in the plant crystal structure. Quantum mechanical/molecular mechanical calculations indicated that the geometry of the cyanobacterial PSI crystal structure was best described as the conformation where Asp-B575 is protonated and A1A is reduced to A1A•–, which might represent the high-potential A1A form (Rutherford, A. W., Osyczka, A., Rappaport, F. (2012) FEBS Lett.586, 603−616). Reorienting the Lys-B542 and Gln-B678 side-chains and rearranging the H-bond pattern of the water cluster near Asp-B575 lowered the Em to Em(A1A) = −718 mV and Em(A1B) = −795 mV. It seems possible that PSI has two conformations: the high-potential A1A form and the low-potential A1A form.

Publisher URL: http://dx.doi.org/10.1021/acs.biochem.7b00082

DOI: 10.1021/acs.biochem.7b00082

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