PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

A new model for electron transfer in PSI: equilibrium between F_X and both phylloquinones. Stefano Santabarbara^*,1, Peter Heathcote¹, Micheal Evans², ¹ School of Biological Sciences, Queen Mary University of London, London, United Kingdom² Department of Biology, University College London, United kingdom

ABSTRACT- Electron transfer reactions in the Photosystem I reaction centre have been investigated by means of the Marcus theory for electron transfer tunnelling. A new kinetic model for these electron transfer reactions is proposed which takes into consideration the thermodynamic equilibrium between the iron-sulphur centre FX and the phylloquinone bound to either the PsaA (A1A) or the PsaB (A1B) subunit of the reaction centre. The equilibrium between the iron-sulphur centres FA and FB is also taken into consideration. The experimentally determined decay lifetimes in the range of sub-picosecond to the microsecond can be satisfactorily simulated, taking into consideration the edge-to-edge distances between redox cofactors and driving forces reported in the literature. The only exception to this general behaviour is the case of phylloquinone (A1) reoxidation. In order to describe the reported rates of the biphasic decay of about 20 and 200 ns associated with this electron transfer step, the redox potentials of the quinones are estimated to be almost isoenergetic with that of the iron sulphur centre FX. In particular a driving force in the range of 5 to 15 meV is estimated for these reactions, being slightly exergonic in case of the A1B quinone and slightly endergonic, in case of the A1A quinone. The simulation presented in this analysis not only describes the kinetic data obtained for the wild type samples at room temperature, and is consistent with estimates of activation energy by the analysis of temperature dependence, but can also explain the effect of the mutations around the PsaB quinone binding pocket. A model of the whole energetics of the system is derived which suggest the only substantially irreversible electron transfer reactions are the reoxidation of A0 on both electron transfer branches and the reduction of FA by FX.

KEY WORDS: Electron Transfer, Tunneling , Photosystem I, Phylloquinones