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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• 作者：Nithya Srinivasan ; Stefano Santabarbara ; Fabrice Rappaport ; Donatella Carbonera ; Kevin Redding ; Art van der Est ; John H. Golbeck
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：March 3, 2011
• 年：2011
• 卷：115
• 期：8
• 页码：1751-1759
• 全文大小：948K
• 年卷期：v.115,no.8(March 3, 2011)
• ISSN：1520-5207

文摘

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^{鈭?/sup> 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鈭?/sup>, resulting in a larger Gibbs free energy change between A_1A/A_1A鈭?/sup> and F_X/F_X鈭?/sup>. An activation energy of 180 卤 10 meV is determined for the A_1A鈭?/sup>-to-F_X forward electron transfer step in the L722W_PsaA variant compared with 220 卤 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.}