Alteration of the H-Bond to the A_1A Phylloquinone in Photosystem I: Influence on the Kinetics and Energetics of Electron Transfer

In Photosystem I, the backbone nitrogen of Leu722_PsaA forms a hydro-gen bond with the C₄ carbonyl oxygen of phylloquinone in the A_1A site. A previous low-temperature EPR study indicated that substitution of Leu722_PsaA with a bulky Trp residue results in a weakened H-bond. Here, we employ room temperature, time-resolved optical spectroscopy and variable temperature, transient EPR spectroscopy to probe the effect of the altered H-bond on the energetics and kinetics of electron transfer. Relative to the wild type, we find that the rate of electron transfer from A_1A⁻ to F_X in the L722W_PsaA variant is faster by a factor of 3. This change is attributed to a lowered midpoint potential of A_1A/A_1A⁻, resulting in a larger Gibbs free energy change between A_1A/A_1A⁻ and F_X/F_X⁻. An activation energy of 180 ± 10 meV is determined for the A_1A⁻-to-F_X forward electron transfer step in the L722W_PsaA variant compared with 220 ± 10 meV in the wild type. The Arrhenius plot shows a break at ∼200 K, below which the rate becomes nearly independent of temperature. This behavior is described using a quantum mechanical treatment that takes the zero-point energy into account as well as an alternative model that invokes a dynamical transition in the protein at ∼200 K.