Figure 1.

(a) Scheme of cofactor arrangement in PS I. P₇₀₀ is a pair of chlorophylls that serves as the primary electron donor. The accessory (“acc”) and A₀ chlorophylls are both single chlorophyll molecules; A₀ is known to serve as an intermediate in electron transfer, but the role of the accessory chlorophyll in this transfer is still not clear (1, 2). A₁ is a phylloquinone. F_X, F_A, and F_B are 4Fe-4S clusters. Trp represents the tryptophan near the phylloquinone that has been mutated in this study. (b–d) Different models to explain the biphasic kinetics of A reoxidation. (b) Only one branch is active. A rapid equilibrium is established between A₁ and F_X, which is depleted by forward electron transfer to F_A/F_B. (c) Only one branch is active, but the phylloquinone can exist in one of two possible conformations, which may be in equilibrium (slow on the time scale of electron transfer). (d) Both branches are active, but the two phylloquinones are in slightly different environments, giving rise to different kinetics of electron transfer to F_X.