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 NNM 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. (C) 2001 Published by Elsevier Science B.V.