PARENT SESSION

Symposium S4Ab Type II reaction centres: acceptor side
Tuesday August 31st, 2004 2:40 PM-4:40 PM Room 510B
Chair: Marilyn Gunner
Co-Chair: Bill Rutherford

Electron transfer coupled protein relaxation in reaction centers of RB. sphaeroides. Maria Kozlova^*,1, Dmitry Cherepanov¹, Laura Baciou², Pierre Sebban², Armen Mulkidjanian¹, ¹ Universität Osnabrück, Abt. Biophysik/FB Biologie, Osnabrück, Germany² Laboratoire de Chimie Physique, Bat. 350, University Paris-IX, Orsay, France

ABSTRACT- Proteins can control the low-exothermic electron transfer via charge redistribution and/or conformational changes [1]. We studied how the replacement of Pro-209 of the L-subunit by Tyr or Glu [2,3] in the Rb. sphaeroides reaction center (RC) affected protein relaxation accompanying the flash-induced reduction of the secondary quinone Qb. In native membrane vesicles, we traced electrometrically proton transfer to Qb upon both semiquinone and quinol formation (in response to the first and second flash, respectively). In the mutants, activation energy of the slower component of proton transfer was lower (∼30 kJ/mol) than in the wild type (∼60 kJ/mol), both after the first and the second flash. Together with increased inhibitor accessibility of quinone-binding sites in the mutants, this evidence suggested that the quinone pocket in the mutant RCs was less tight than in the wild type RCs. The steered MD simulations revealed that in the case of the mutant RC a smaller force was needed to cause a flip motion of the Qb ring between its two functional positions in the pocket, as compared to the wild type. The energy difference was in quantitative agreement with the experimental data. This evidence indicated that the high activation barrier upon the electron/proton transfer at the Qb site might be due to the protein conformational change enabling the twisting movement of the Qb ring between its two functional positions, as suggested previously [1, 4-6]. We conclude that Pro-209 essentially contributes to the tightness of the quinone-binding pocket. References: 1. Cherepanov, D. A., Krishtalik, L. I., and Mulkidjanian, A. Y. Biophys. J. 2001, 80, 1033-1049. 2. Kuglstatter A, Ermler U, Michel H, Baciou L, Fritzsch G. Biochemistry 2001 40, 4253-4260 3. Tandori, J., Maroti, P., Alexov, E., Sebban, P., Baciou, L. Proc. Natl. Acad. Sci. USA 2002, 99 6702-6706. 4. O.A. Gopta, D.A. Bloch, D.A. Cherepanov, A.Y. Mulkidjanian, FEBS Lett. 1997, 412, 490-494 5. Mulkidjanian, A. Y. FEBS Lett. 1999, 463, 199-204. 6. Cherepanov, D. A., Bibikov, S. I., Bibikova, M. V., Bloch, D. A., Drachev, L. A., Gopta, O. A., Oesterhelt, D., Semenov, A. Y., and Mulkidjanian, A. Y. Biochim. Biophys. Acta 2000, 1459, 10-34.

KEY WORDS: Qb site, molecular dynamics, reaction center, proton transfer