FX, FA, and FB Iron–Sulfur Clusters in Type I Photosynthetic Reaction Centers

Abstract

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 (FX) as an intermediate electron acceptor and two [4Fe–4S] iron–sulfur clusters (FA and FB) as the terminal electron acceptors. This article focuses on type I reaction centers with special attention paid to the spectroscopic properties of FX, FA, and FB, and to the biochemical properties of the polypeptides that bind these iron–sulfur clusters.