Alteration of the H-Bond to the A1A phylloquinone in photosystem I: Influence on the kinetics and energetics of electron transfer

Nithya Srinivasan, Stefano Santabarbara, Fabrice Rappaport, Donatella Carbonera, Kevin Redding, Art Van Der Est, John H. Golbeck

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Abstract

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.

Original languageEnglish
Pages (from-to)1751-1759
Number of pages9
JournalJournal of Physical Chemistry B
Volume115
Issue number8
DOIs
Publication statusPublished - Mar 3 2011

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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