Path of Electron Transfer in Photosystem 1: Direct Evidence of Forward Electron Transfer from A₁ to Fe-S_X

Pulsed EPR spectroscopy and selective removal of the iron-sulfur centers in photosystem 1 have been used to study forward electron transfer from the secondary electron acceptor A₁. At cryogenic temperatures where forward electron transfer is inhibited, we have observed a g = 2.003 electron spin-echo signal presenting a characteristic phase shift. This out-of-phase signal is attributed to the electron spin-polarized pair P₇₀₀⁺/A₁⁻, it decays with t_1/e = 23 µs, reflecting the recombination reaction. At room temperature the out-of-phase signal is also observed, but it decays with t_1/e = 200 ns in untreated photosystem 1, due to forward electron transfer from A₁⁻ to one of the iron-sulfur centers. This rate is unchanged in Fe-S_A/B-depleted PS1 but is lost when the iron-sulfur center Fe-S_X is removed. In the preparations depleted of all iron-sulfur centers the out-of-phase signal decays with t_1/e = 1.3 µs, reflecting either the back reaction or the decay of polarization. These results demonstrate that the electron transfer pathway in photosystem 1 is P₇₀₀ → A₁ → Fe-S_X → Fe-S_A/B.