Iron-sulfur clusters in type I reaction centers

Type I reaction centers (RCs) are multisubunit chlorophyll-protein complexe s that function in photosynthetic organisms to convert photons to Gibbs fre e energy. The unique feature of Type I RCs is the presence of iron-sulfur c lusters as electron transfer cofactors. Photosystem I (PS I) of oxygenic ph ototrophs is the best-studied Type I RC. It is comprised of an interpolypep tide [4Fe-4S] cluster, F-X, that bridges the PsaA and PsaB subunits, and tw o terminal [4Fe-4S] clusters, F-A and F-B, that are bound to the PsaC subun it. In this review, we provide an update on the structure and function of t he bound iron-sulfur clusters in Type I RCs. The first new development in t his area is the identification of F-A as the cluster proximal to F-X and th e resolution of the electron transfer sequence as F-X --> F-A --> F-B --> s oluble ferredoxin. The second new development is the determination of the t hree-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 surv ey of the EPR properties and spectra of the iron-sulfur clusters in Type I RCs of cyanobacteria, green sulfur bacteria, and heliobacteria, and we summ arize new information about the kinetics of back-reactions involving the ir on-sulfur clusters. (C) 2001 Published by Elsevier Science B.V.