title: Modeling of the P700+ charge recombination kinetics with phylloquinone and plastoquinone-9 in the A1 site of photosystem I.
abstract: Light activation of photosystem I (PS I) induces electron transfer from the excited primary electron donor P700 (a special pair of chlorophyll a/a' molecules) to three iron-sulfur clusters, F(X), F(A), and F(B) via acceptors A(0) (a monomeric chlorophyll a) and A(1) (phylloquinone). PS I complexes isolated from menA and menB mutants contain plastoquinone-9 rather than phylloquinone in the A(1) site and show altered rates of forward electron transfer from A to [F(A)/F(B)] and altered rates of back electron transfer from [F(A)/F(B)](-) to P700+ (Semenov, A. Y., et al., J. Biol. Chem. 275:23429-23438, 2000). To identify the modified electron transfer steps, we studied the kinetics of flash-induced P700+ reduction in PS I that contains either an intact set or a subset of iron-sulfur clusters F(X), F(A), and F(B) and with the A(1) binding site occupied by phylloquinone or plastoquinone-9. A modeling of the forward and backward electron transfer kinetics in P700-F(A)/F(B) complexes, P700-F(X) cores, and P700-A(1) cores shows that the replacement of phylloquinone by plastoquinone-9 induces a decrease in the free energy gap between A(1) and F(A)/F(B) from approximately -205 mV in wild-type PS I to approximately -70 mV in menA PS I. The +135 mV increase in the midpoint potential of A(1) explains the acceleration in the rate of P700+ dark reduction in menA PS I, and the resulting uphill electron transfer from A(1) to F(X) in menA PS I explains the absence of a contribution from F to the reduction of P700+. This fully quantitative description of PS I relates electron transfer rates, equilibrium constants, and redox potentials, and can be used to predict changes in these parameters upon substitution of electron transfer cofactors.
authors.affiliation: Department of Biochemistry, University of Illinois at Urbana-Chamnpaign, 156 Davenport Hall, 607 Mathews Avenue, Urbana, IL 61801-3838, USA. vshinkar@uiuc.edu
authors.forename: Vladimir P
authors.initials: VP
authors.lastname: Shinkarev
authors.forename: Boris
authors.initials: B
authors.lastname: Zybailov
authors.forename: Ilya R
authors.initials: IR
authors.lastname: Vassiliev
authors.forename: John H
authors.initials: JH
authors.lastname: Golbeck
mesh_terms: D002352:Carrier Proteins; D002734:Chlorophyll / Q000737:chemistry*; D003198:Computer Simulation; D000458:Cyanobacteria / Q000145:classification / Q000378:metabolism; D003598:Cytoskeletal Proteins; D003623:Dark Adaptation; D004579:Electron Transport / Q000528:radiation effects; D004735:Energy Transfer / Q000528:radiation effects; D007700:Kinetics; D008027:Light; D046911:Macromolecular Substances; D008840:Microfilament Proteins; D008954:Models, Biological; D008956:Models, Chemical; D010084:Oxidation-Reduction / Q000528:radiation effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000528:radiation effects*; D010971:Plastoquinone / Q000737:chemistry; D011809:Quinones / Q000737:chemistry*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012016:Reference Values; D013057:Spectrum Analysis / Q000379:methods; D055672:Static Electricity; D010837:Vitamin K 1 / Q000737:chemistry
chemical_list: D002352:Carrier Proteins; D003598:Cytoskeletal Proteins; C102176:Enah protein, mouse; D046911:Macromolecular Substances; D008840:Microfilament Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones; D011994:Recombinant Proteins; D002734:Chlorophyll; C001785:chlorophyll P 700; D010837:Vitamin K 1; D010971:Plastoquinone