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

Low-temperature electron transfer in A₀ binding site point mutants studied using transient EPR and CW-EPR. Ingelög Wyndhamn^*,1, Art van der Est¹, Rachel Cohen², John Golbeck², ¹ Department of Chemistry, Brock University, 500 Glenridge Avenue, St.Catharines, ON, CANADA²

ABSTRACT- At low temperature, two fractions of electron transfer are observed in PS I. One fraction shows cyclic ET to A₁ while trapping of the state P₇₀₀⁺(F_A/F_B)^- occurs in the other fraction. The relative sizes of the two fractions can be monitored using the amplitude of the transient EPR signal of P₇₀₀⁺A₁^- (cyclic electron transfer) and the stable EPR signal of P₇₀₀⁺ (trapping). It has been proposed that these two fractions may be related to electron transfer in the two branches of cofactors. Thus, we have compared their relative amplitudes in PS I particles from Synechocystis sp6803 in which the methionine, forming the axial ligand to A₀ has been changed to asparagine or leucine. The B-branch mutants show very strong spin polarized spectra, while the stable P₇₀₀⁺ signal is slightly weaker than in wild type. The A-branch mutants show the opposite behavior with very weak spin polarized signals and very large stable P₇₀₀⁺ signals. This result can be interpreted as being due to either (i) changes in the efficiency of electron transfer past A₁ and/or F_x or (ii) a change in the efficiency of electron transfer in the two branches. The latter explanation predicts that in wild type PS I, trapping occurs when electron transfer takes place in the B-branch. Consequently, when electron transfer past A₁ is blocked, e.g. in PS I devoid of iron-sulfur clusters, a large fraction of spin polarized EPR spectrum would be due to electron transfer in the B-branch. We have investigated this possiblity by comparing simulated P₇₀₀⁺A₁^- spectra with experimental spectra of PS I from wild type and the rubA- mutant which is unable to assemble F_X, F_A and F_B. All of the parameters used in the simulations are fixed using independent experimental data and show that the experimental spectra are incompatible with radical pairs involving the quinone in PsaB branch. In contrast, excellent agreement is obtained if they are assumed to arise entirely from radical pairs involving the A-branch quinone. Based on this we conclude that in both the wild type and rubA- mutant the electron transfer is very strongly biased towards the PsaA branch and that the changes observed in the A₀ binding site mutants are probably due to alteration of electron transfer on the stromal side of the complex.

KEY WORDS: Cofactor Branches, Photosystem I, Electron Transfer, Electron Paramagentic Resonance