title: Introduction of a hydrogen bond between phylloquinone PhQ(A) and a threonine side-chain OH group in photosystem I.
abstract: The phylloquinone acceptor PhQ(A) in photosystem I binds to the protein through a single H-bond to the backbone nitrogen of PsaA-L722. Here, we investigate the effect of this H-bond on the electron transfer (ET) kinetics by substituting threonine for PsaA-L722. Room temperature spin-polarized transient EPR measurements show that in the PsaA-L722T mutant, the rate of PhQ(A)(-) to F(X) ET increases and the hyperfine coupling to the 2-methyl group of PhQ(A) is much larger than in the wild type. Molecular dynamics simulations and ONIOM type electronic structure calculations indicate that it is possible for the OH group of the Thr side chain to form an H-bond to the carbonyl oxygen atom, O(4) of the phylloquinone, and that this results in an increase in the 2-methyl hyperfine couplings as observed in the transient EPR data. The Arrhenius plot of the PhQ(A)(-) to F(X) ET in the PsaA-L722T mutant suggests that the increased rate is probably the result of a slight change in the electronic coupling between PhQ(A)(-) and F(X). The strong deviation from Arrhenius behavior observed at ∼200 K can be reproduced using a semiclassical model, which takes the zero-point energy of the mode coupled to the ET into account. However, since the change in slope of the Arrhenius plot occurs at the protein glass transition temperature, it is argued that it could be the result of a change in the protein relaxation dynamics at this temperature rather than quantum mechanical effects.
authors.affiliation: Department of Chemistry, Brock University, St. Catharines, Ontario, Canada.
authors.forename: Sam
authors.initials: S
authors.lastname: Mula
authors.forename: Michael D
authors.initials: MD
authors.lastname: McConnell
authors.forename: Amy
authors.initials: A
authors.lastname: Ching
authors.forename: Nan
authors.initials: N
authors.lastname: Zhao
authors.forename: Heather L
authors.initials: HL
authors.lastname: Gordon
authors.forename: Gary
authors.initials: G
authors.lastname: Hastings
authors.forename: Kevin E
authors.initials: KE
authors.lastname: Redding
authors.forename: Art
authors.initials: A
authors.lastname: van der Est
mesh_terms: D016825:Chlamydomonas reinhardtii / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D006860:Hydrogen Bonding; D007700:Kinetics; D056004:Molecular Dynamics Simulation; D009154:Mutation; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013912:Threonine / Q000737:chemistry / Q000378:metabolism*; D010837:Vitamin K 1 / Q000737:chemistry / Q000378:metabolism*
chemical_list: D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D013912:Threonine; D010837:Vitamin K 1