A kinetic assessment of the sequence of electron transfer from FX to FA and further to FB in photosystem I:: The value of the equilibrium constant between FX and FA

The x-ray structure analysis of photosystem I (PS I) crystals at 4-Angstrom resolution (Schubert et al., 1997, J. Mol. Biol. 272:741-769) has revealed the distances between the three iron-sulfur clusters, labeled F-x, F-1, and F-2, 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 accepters labeled F-1 and F-2 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 duster 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 PS I 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 F-1 and F-2 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 similar to 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.