title: Alteration of the H-bond to the A(1A) phylloquinone in Photosystem I: influence on the kinetics and energetics of electron transfer.
abstract: 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.
authors.affiliation: Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
authors.forename: Nithya
authors.initials: N
authors.lastname: Srinivasan
authors.forename: Stefano
authors.initials: S
authors.lastname: Santabarbara
authors.forename: Fabrice
authors.initials: F
authors.lastname: Rappaport
authors.forename: Donatella
authors.initials: D
authors.lastname: Carbonera
authors.forename: Kevin
authors.initials: K
authors.lastname: Redding
authors.forename: Art
authors.initials: A
authors.lastname: van der Est
authors.forename: John H
authors.initials: JH
authors.lastname: Golbeck
mesh_terms: D019943:Amino Acid Substitution; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D006860:Hydrogen Bonding; D007700:Kinetics; D009154:Mutation; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011789:Quantum Theory; D013696:Temperature; D013816:Thermodynamics; D010837:Vitamin K 1 / Q000737:chemistry*
chemical_list: D045331:Photosystem I Protein Complex; D010837:Vitamin K 1