PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

Exploring electron-transfer function in photosynthesis by electrostatic-energy computations. Ernst-Walter Knapp^*,1, Hiroshi Ishikita¹, ¹ Free University of Berlin, Berlin, Berlin, Germany

ABSTRACT- Energy conversion and transformation as well as the primary steps of enzymatic reactions generally involve electron transfer (ET) processes. This contribution explores the ET processes in photosynthetic protein membrane complexes of photosystem I (PSI) ¹ and photosystem II (PSII) reaction centers (RC) by calculating the redox potentials of the participating cofactors and compares these data with recent results on bacterial reaction centers (bRC) ^2,3. The computations are based on the most recent PSII structures ^4,5 using the solution of the linearized Poisson-Boltzmann equation. The accuracy and feasibility of this approach was recently demonstrated by calculating the heme redox potentials of the cytochrom c subunit in bRC from Rhodospeudomonas viridis ⁶. The focus of this contribution is on ET processes from the chlorophyll special pair to the quinones of the two branches of cofactors and the ET processes between the quinones and in PSI from the quinones to the iron-sulfur center. The computed energetics of the ET processes in these systems compares favorable with known experimental results on cofactor redox potentials and estimates of ET rates and demonstrates how the protein tunes the redox potentials of its cofactors. The computations explain in particular why the quinone redox potentials in PSI are so high while they are so small in PSII and bRC. They also provide large values of the chlorophyll P680 redox potentials in PSII, which are needed for oxidation of Tyr_Z a prerequisite of water oxidation at the manganese cluster.  1. Ishikita, H. and Knapp, E. W. (2003) J. Biol. Chem. 278, 52002-52011.  2. Ishikita, H. and Knapp, E. W. (2004) J. Am. Chem. Soc. in press.  3. Ishikita, H., Morra, G. and Knapp, E. W. (2003) Biochemistry.42, 3882-3892.  4. Ferreira, K. N., Iverson, T. M., Maghlaoui, K., Barber, J., and Iwata, S. (2004) Science 303, 1831-1838.  5. Biesiadka, J., Loll, B., Kern, J., Irrgang, K.D., Saenger, W., and Zouni, A., in prep. (2004).  6. Voigt, P. and Knapp, E. W. (2003) J. Biol. Chem. 278, 51993-52001.

KEY WORDS: Photosystem II, electron transfer processes, Photosystem I, cofactor redox potentials