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.