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Alteration of the H-Bond to the A1A Phylloquinone in Photosystem I: Influence on the Kinetics and Energetics of Electron Transfer

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journal contribution
posted on 2011-03-03, 00:00 authored by Nithya Srinivasan, Stefano Santabarbara, Fabrice Rappaport, Donatella Carbonera, Kevin Redding, Art van der Est, John H. Golbeck
In Photosystem I, the backbone nitrogen of Leu722PsaA forms a hydro-gen bond with the C4 carbonyl oxygen of phylloquinone in the A1A site. A previous low-temperature EPR study indicated that substitution of Leu722PsaA 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 A1A to FX in the L722WPsaA variant is faster by a factor of 3. This change is attributed to a lowered midpoint potential of A1A/A1A, resulting in a larger Gibbs free energy change between A1A/A1A and FX/FX. An activation energy of 180 ± 10 meV is determined for the A1A-to-FX forward electron transfer step in the L722WPsaA 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.