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-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, an d F-2, which function on the acceptor side of PS I. There is a general cons ensus concerning the assignment of the F-x cluster, which is bound to the P saA and PsaB polypeptides that constitute the PS I core heterodimer. Howeve r, the correspondence between the accepters labeled F-1 and F-2 on the elec tron density map and the F-A and F-B clusters defined by electron paramagne tic 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 pr oximal 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 r elaxation in PS I complexes (both F-A and F-B are present), HgCl2-treated P S 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 dependen cies on the rate of electron transfer, based on the x-ray distances between the cofactors. The analysis, which takes into consideration the asymmetric al position of iron-sulfur clusters F-1 and F-2 relative to F-x, supports t he 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 obser ved kinetics of P700(+) reduction and F-x(-) oxidation, we estimate the equ ilibrium constant of electron transfer between F-x and F-A at room temperat ure to be similar to 47. The value of this equilibrium constant is discusse d in the context of the midpoint potentials of F-x and F-A, as determined b y low-temperature EPR spectroscopy.