Recruitment of a Foreign Quinone into the A1 Site of Photosystem I

• Published in: The Journal of biological chemistry Vol. 275; no. 12; pp. 8531 - 8539
• Main Authors: Zybailov, Boris, van der Est, Art, Zech, Stephan G., Teutloff, Christian, Johnson, T.Wade, Shen, Gaozhong, Bittl, Robert, Stehlik, Dietmar, Chitnis, Parag R., Golbeck, John H.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 24-03-2000; American Society for Biochemistry and Molecular Biology
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.275.12.8531

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance studies of the photosystem (PS) I quinone acceptor, A1, in phylloquinone biosynthetic pathway mutants are described. Room temperature continuous wave EPR measurements at X-band of whole cells of menA andmenB interruption mutants show a transient reduction and oxidation of an organic radical with a g-value and anisotropy characteristic of a quinone. In PS I complexes, the continuous wave EPR spectrum of the photoaccumulated Q−radical, measured at Q-band, and the electron spin-polarized transient EPR spectra of the radical pair P700+ Q−, measured at X-, Q-, and W-bands, show three prominent features: (i) Q− has a larger g-anisotropy than native phylloquinone, (ii) Q− does not display the prominent methyl hyperfine couplings attributed to the 2-methyl group of phylloquinone, and (iii) the orientation of Q− in the A1 site as derived from the spin polarization is that of native phylloquinone in the wild type. Electron spin echo modulation experiments on P700+ Q− show that the dipolar coupling in the radical pair is the same as in native PS I,i.e. the distance between P700+ and Q− (25.3 ± 0.3 Å) is the same as between P700+ and A1− in the wild type. Pulsed electron-nuclear double resonance studies show two sets of resolved spectral features with nearly axially symmetric hyperfine couplings. They are tentatively assigned to the two methyl groups of the recruited plastoquinone-9, and their difference indicates a strong inequivalence among the two groups when in the A1site. These results show that Q (i) functions in accepting an electron from A0− and in passing the electron forward to the iron-sulfur clusters, (ii) occupies the A1 site with an orientation similar to that of phylloquinone in the wild type, and (iii) has spectroscopic properties consistent with its identity as plastoquinone-9.