Iron-sulfur clusters in type I reaction centers

Type I reaction centers (RCs) are multisubunit chlorophyll-protein complexes that function in photosynthetic organisms to convert photons to Gibbs free energy. The unique feature of Type I RCs is the presence of iron-sulfur clusters as electron transfer cofactors. Photosystem I (PS I) of oxygenic phototrophs is the best-studied Type I RC. It is comprised of an interpolypeptide [4Fe-4S] cluster, F_X, that bridges the PsaA and PsaB subunits, and two terminal [4Fe-4S] clusters, F_A and F_B, that are bound to the PsaC subunit. In this review, we provide an update on the structure and function of the bound iron-sulfur clusters in Type I RCs. The first new development in this area is the identification of F_A as the cluster proximal to F_X and the resolution of the electron transfer sequence as F_X → F_A → F_B → soluble ferredoxin. The second new development is the determination of the three-dimensional NMR solution structure of unbound PsaC and localization of the equal- and mixed-valence pairs in F_A^- and F_B^-. We provide a survey of the EPR properties and spectra of the iron-sulfur clusters in Type I RCs of cyanobacteria, green sulfur bacteria, and heliobacteria, and we summarize new information about the kinetics of back-reactions involving the iron-sulfur clusters.