The bound iron–sulfur clusters of Type-I homodimeric reaction centers

The hallmark of a Type-I photosynthetic reaction center (RC) is the presence of three [4Fe–4S] 2+/1+ clusters, named F X , F A , and F B that act as terminal electron acceptors. Their function is to increase the distance, and hence the lifetime, of the initial charge-separated state so that diffusion-mediated processes, such as the reduction of ferredoxin, can occur. Type-I homodimeric RCs, such as those found in heliobacteria, green-sulfur bacteria, and Candidatus Chloracidobacterium thermophilum, are less well understood than Photosystem I, the prototypical Type-I heterodimeric RC found in cyanobacteria and plants. Here, we review recent progress that has been made in elucidating the spectroscopic and biochemical properties of the bound Fe/S clusters and their cognate proteins in homodimeric Type-I RCs. In Heliobacterium modesticaldum , the identification and characterization of two loosely bound polypeptides, PshBI and PshBII that harbor the F A and F B clusters threatens to break the long-accepted assumption that Type-I RCs harbor one tightly bound F A /F B -containing protein. Additionally, the detection of the F X cluster in S = 1/2 and S = 3/2 ground spin states has resolved the long-standing issue of its missing EPR spectrum. In Chlorobaculum tepidum , the focus is on the biochemical properties of the unusual extrinsic Fe/S protein, PscB, which is readily dissociable from the RC core. The C-terminal domain of PscB is constructed as a bacterial ferredoxin, harboring the F A and F B clusters, but the N-terminal domain contains a number of PxxP motifs and is rich in Lys, Pro, and Ala residues, features characteristic of proteins that interact with SH3 domains . Little is known about Candidatus Chloracidobacterium thermophilum except that the photosynthetic RC is predicted to be a Type-I homodimer with an F X -binding site. These findings are placed in a context that promises to unify the acceptor side of homodimeric Type-I RCs in prokaryotic phototrophs.