Alteration of the H-bond to the A (1A) phylloquinone in Photosystem I: influence on the kinetics and energetics of electron transfer

Srinivasan, N.; Santabarbara, S.; Rappaport, F.; Carbonera, D.; Redding, K.; van der Est, A.; Golbeck, J.H.

Journal of Physical Chemistry. B 115(8): 1751-1759


ISSN/ISBN: 1520-5207
PMID: 21299208
DOI: 10.1021/jp109531b
Accession: 051441905

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In Photosystem I, the backbone nitrogen of Leu722(PsaA) forms a hydro-gen 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 ∼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.