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 hydrogen bond with the C(4) 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 similar to 100 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 similar to 200 K.