Modeling of the P700+ charge recombination kinetics with phylloquinone and plastoquinone-9 in the A1 site of photosystem I.

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, F(X), F(A), and F(B) via acceptors A(0) (a monomeric chlorophyll a) and A(1) (phylloquinone). PS I complexes isolated from menA and menB mutants contain plastoquinone-9 rather than phylloquinone in the A(1) site and show altered rates of forward electron transfer from A to [F(A)/F(B)] and altered rates of back electron transfer from [F(A)/F(B)](-) 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 F(X), F(A), and F(B) and with the A(1) binding site occupied by phylloquinone or plastoquinone-9. A modeling of the forward and backward electron transfer kinetics in P700-F(A)/F(B) complexes, P700-F(X) cores, and P700-A(1) cores shows that the replacement of phylloquinone by plastoquinone-9 induces a decrease in the free energy gap between A(1) and F(A)/F(B) from approximately -205 mV in wild-type PS I to approximately -70 mV in menA PS I. The +135 mV increase in the midpoint potential of A(1) explains the acceleration in the rate of P700+ dark reduction in menA PS I, and the resulting uphill electron transfer from A(1) to F(X) in menA PS I explains the absence of a contribution from F 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.