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• W1500291257 abstract "The goal of this thesis is the characterization of specific protein-cofactor interactions which control transmembrane electron transfer processes in Photosystem I of the cyanobacterial photosynthesis apparatus. Multifrequency Time Resolved Electron Paramagnetic Resonance (TR EPR) spectroscopy is applied to detect the sequential P700A1 and P700FX radical pair states created by light induced charge separation. Systematic variation of functionally relevant properties of protein-cofactor interactions is carried out in two ways: A) substitution of the native phylloquinone in the A1-binding site of Photosystem I complex by suitably modified synthetic or various other quinones. For the extraction/reconstitution procedure two methods were available for comparison: a “harsh” one, extraction of native phylloquinone by organic solvents, and a “soft” one, use of native quinone biosynthetic pathway deletion mutants with a more weakly bound rescue quinone (plastoquinone-9) in the A1-binding site; in each case reconstitution/replacement followed by offering the wanted quinone in excess. B) Selective variation of the protein environment in the cofactor site by site-directed mutation. In Part A the first incorporation concerns 2-methyl-1,4-naphthoquinones selectively labelled with a C isotope at one of the carbon positions (Chapter 5). A highly asymmetric spin density distribution is observed for the functional P700A1 RP state. It provides solid evidence for a highly asymmetric (single-sided) hydrogen bond scheme between the protein environment and the quinone as it was suggested in the X-ray structure model for the ground state. The results on PS I complexes isolated from the menB rubA double mutant show: i) In spite of the absence of the [4Fe-4S] clusters FX, FA, FB and all stromal subunits PsaC, PsaD, PsaE the TR EPR detectable structural and essential protein-cofactor interaction features remain the same as observed for phylloquinone in the wild type. ii) Because plastoquinone-9 is more weakly bound into the A1-binding site of PS I it can be exchanged easily by a number of quinones (Chapter 6). In particular, 9,10-anthraquinone with a more negative redox potential than native phylloquinone was chosen for incorporation into the A1-binding site of Photosystem I both, in the presence and absence of the [4Fe-4S] clusters. Corresponding to the altered energetics the kinetics of consecutive electron transfer processes from A0 to AQ to FX are changed in a complementary way (Chapter 7). The first ET kinetics from A0 to AQ slows down, the second one from AQ to FX speeds up. Moreover, for the first time, the effect of the formal negative charge [Fe4S4(SCH3)4] associated with FX on an electron transfer step from A0 to A1 could be demonstrated. In part B first the influence of the specific aspartate residues on electron transfer from A1 to FX was studied by site-directed mutants in either the environment of the phylloquinone or the first iron-sulphur FX cofactor (Chapter 8). Changing from a negatively charged side chain to a neutral and to a positively charged one results in very small but systematic changes of the electron transfer kinetics from A1 to FX, consistent with the expected redox potential shifts depending whether the aspartate mutation concerns more the vicinity of A1 or FX. Furthermore, the axial methionine ligand to the central Mg atoms of the first acceptor A0 was chosen for mutation, either to asparagine (N) or leucine (L), selectively in the PsaAor PsaB-branch (Chapter 9). In comparison to the wild type both PsaA-branch mutants show a decrease in the electron transfer rate from A0 to A1. These results agree well with those from complementary optical studies. They confirm asymmetric electron transfer, predominantly along the PsaA-branch. Extended conclusions are presented in the final Chapter 10." @default.
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• W1500291257 date "2008-01-01" @default.
• W1500291257 modified "2023-09-24" @default.
• W1500291257 title "Study of Protein-Cofactor Interaction and Electron Transfer Properties in Modified Photosystem I Complexes by Multifrequency Time Resolved Electron Paramagnetic Resonance" @default.
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