A kinetic assessment of the sequence of electron transfer from F(X) to F(A) and further to F(B) in photosystem I: The value of the equilibrium constant between F(X) and F(A)

The x-ray structure analysis of photosystem I (PS I) crystals at 4-Å resolution (Schubert et al., 1997, J. Mol. Biol. 272:741-769) has revealed the distances between the three iron-sulfur clusters, labeled F(X), F₁, and F₂, which function on the acceptor side of PS I. There is a general consensus concerning the assignment of the F(X) cluster, which is bound to the PsaA and PsaB polypeptides that constitute the PS I core heterodimer. However, the correspondence between the acceptors labeled F₁ and F₂ on the electron density map and the F(A) and F(B) clusters defined by electron paramagnetic resonance (EPR) spectroscopy remains controversial. Two recent studies (Diaz-Quintana et al., 1998, Biochemistry. 37:3429-3439; Vassiliev et al., 1998, Biophys. J. 74:2029-2035) provided evidence that F(A) is the cluster proximal to F(X) and F(B) is the cluster that donates electrons to ferredoxin. In this work, we provide a kinetic argument to support this assignment by estimating the rates of electron transfer between the iron- sulfur clusters F(X), F(A), and F(B). The experimentally determined kinetics of P700⁺ dark relaxation in PS I complexes (both F(A) and F(B) are present), HgCl₂-treated PSI complexes (devoid of F(B)), and P700-F(X) cores (devoid of both F(A) and F(B)) from Synechococcus sp. PCC 6301 are compared with the expected dependencies on the rate of electron transfer, based on the x-ray distances between the cofactors. The analysis, which takes into consideration the asymmetrical position of iron-sulfur clusters F₁ and F₂ relative to F(X), supports the F(X) → F(A) → F(B) → Fd sequence of electron transfer on the acceptor side of PS I. Based on this sequence of electron transfer and on the observed kinetics of P700⁺ reduction and F(X) oxidation, we estimate the equilibrium constant of electron transfer between F(X) and F(A) at room temperature to be ~47. The value of this equilibrium constant is discussed in the context of the midpoint potentials of F(X) and F(A), as determined by low-temperature EPR spectroscopy.