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

Abstract

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.

Structural Analysis of the Homodimeric Reaction Center Complex from the Photosynthetic Green Sulfur Bacterium Chlorobaculum tepidum

“…Furthermore, on the basis of the hydropathy plot, the region containing residues 525–536, which represents the F X binding motif FPCxGPxxGGTC, should also be on the cytoplasmic side of the membrane. 39 In analogy with Photosystem I, it is anticipated that F X , F A , and F B are all on the cytoplamic side to facilitate electron transfer. Several monolinks were found on the PscD protein, such as 40 K, 46 K, 107 K, and 111 K, some of which were linked to FMO as mentioned above.…”

Calculating standard reduction potentials of [4Fe–4S] proteins

“…[1] However, proteins contain only a limited number of different types of redox sites to cover this range, so electron transfer pathways rely on the protein environment to tune the reduction potential to the appropriate value. For instance, multiple [4Fe–4S] redox sites are used in the electron transport chains in mitochondrial respiratory complex I[2] and chloroplastic photosystem I[3]; the former has six [4Fe–4S] redox sites with reduction potentials spanning a range between −0.3 and −0.1 V[4] and the latter has three [4Fe–4S] redox sites with reduction potentials spanning a range between −0.55 and −0.59 V.[5, 6]…”

15N Photo-CIDNP MAS NMR To Reveal Functional Heterogeneity in Electron Donor of Different Plant Organisms

“…1). Furthermore, PSI contains three iron-sulphur [4Fe-3S] clusters as terminal intrinsic electron acceptors [8]. The primary electron donor P700 is a heterodimer consisting of one chlorophyll a (Chl a, P B ) and one Chl a 0 (P A ) which is the 13 2 -epimer of Chl a.…”