F_X, F_A, and F_B Iron-Sulfur Clusters in Type I Photosynthetic Reaction Centers

Photosynthetic reaction centers are membrane-bound pigment-protein complexes that use light to catalyze a transmembrane electron transfer against a steep thermodynamic gradient. They are found in plants and photosynthetic bacteria, and have evolved into two types. Type II reaction centers employ a mobile quinone as the terminal electron acceptor. Type I reaction centers employ an interpolypeptide [4Fe-4S] cluster (F_X) as an intermediate electron acceptor and two [4Fe-4S] iron-sulfur clusters (F_A and F_B) as the terminal electron acceptors. This article focuses on type I reaction centers with special attention paid to the spectroscopic properties of F_X, F_A, and F_B, and to the biochemical properties of the polypeptides that bind these iron-sulfur clusters.