Light activation of photosystem I (PS I) induces electron transfer from the excited primary electron donor P700 (a special pair of chlorophyll a/a′ molecules) to three iron-sulfur clusters, FX, FA, and FB via acceptors AO (a monomeric chlorophyll a) and A1 (phylloquinone). PS I complexes isolated from menA and menB mutants contain plastoquinone-9 rather than phylloquinone in the A1 site and show altered rates of forward electron transfer from A1- to [FA/FB] and altered rates of back electron transfer from [FA/FB]- to P700+ (Semenov, A. Y., et al., J. Biol. Chem. 275:23429-23438, 2000). To identify the modified electron transfer steps, we studied the kinetics of flash-induced P700+ reduction in PS I that contains either an intact set or a subset of iron-sulfur clusters FX, FA, and FB and with the A1 binding site occupied by phylloquinone or plastoquinone-9. A modeling of the forward and backward electron transfer kinetics in P700-FA/FB complexes, P700-FX cores, and P700-A1 cores shows that the replacement of phylloquinone by plastoquinone-9 induces a decrease in the free energy gap between A1 and FA/FB from ∼-205 mV in wild-type PS I to ∼-70 mV in menA PS I. The +135 mV increase in the midpoint potential of A1 explains the acceleration in the rate of P700+ dark reduction in menA PS I, and the resulting uphill electron transfer from A1 to FX in menA PS I explains the absence of a contribution from FX- to the reduction of P700+. This fully quantitative description of PS I relates electron transfer rates, equilibrium constants, and redox potentials, and can be used to predict changes in these parameters upon substitution of electron transfer cofactors.
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