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

Alteration of phenylalanine F669(PsaB) slows electron transfer in the PsaA branch of Photosystem I. Art van der Est^*,1, Yuri Kandrashkin¹, John Golbeck², ¹ Dept. of Chemistry, Brock University, St Catharines, Ontario, Canada² Dept. of Biochemistry and Molecular Biology, University Park, PA, USA

ABSTRACT- The current debate concerning the directionality of electron transfer in PS I was initiated by the observation that mutations in the PsaA branch affect the slow phase of electron transfer from A₁ to F_X while the corresponding mutations in the PsaB branch caused changes in the fast phase. Here, we present transient EPR data for two mutations, which do not appear to follow this trend. Phenylalanines F689(PsaA) and F669(PsaB) are in close proximity to the respective quinones in the two branches. The plane of the phenylalanine side group is roughly coplanar with the quinone ring on the opposite face to the -stacked trytophans W687(PsaA) and W677(PsaB). We have investigated tyrosine mutants of the two phenylalanines using transient EPR and find that the slow phase of electron transfer is altered in the PsaB mutant while the PsaA mutation has little or no effect. A global fit of the complete time/field datasets, shows that the contribution of the fast component of electron transfer to the spin polarized signals is very small and yields P₇₀₀⁺A₁^- lifetimes of 280 ns for the wild type, 240 ns for F689Y_PsaA and 350 ns for F669Y_PsaB. Starting from the premise that the slow phase is associated with the PsaA branch, these results suggest that the PsaB mutation causes a remote effect on either Q_KA or F_X. The more likely of these two scenarios is that F_X is affected by the mutation.

KEY WORDS: electron transfer, directionality, spin polarization, photosystem I