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

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.