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.
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