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

Abstract

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, 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 acceptors 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 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), HgCl 2-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 ∼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.