Identification of F_X in the Heliobacterial Reaction Center as a [4Fe-4S] Cluster with an S = ³/₂ Ground Spin State

Abstract

Type I homodimeric reaction centers, particularly the class present in heliobacteria, are not well understood. Even though the primary amino acid sequence of PshA in Heliobacillus mobilis has been shown to contain an F(X) binding site, a functional Fe-S cluster has not been detected by EPR spectroscopy. Recently, we reported that PshB, which contains F(A)- and F(B)-like Fe-S clusters, could be removed from the Heliobacterium modesticaldum reaction center (HbRC), resulting in 15 ms lifetime charge recombination between P798(+) and an unidentified electron acceptor [Heinnickel, M., Shen, G., Agalarov, R., and Golbeck, J. H. (2005) Biochemistry 44, 9950-9960]. We report here that when a HbRC core is incubated with sodium dithionite in the presence of light, the 15 ms charge recombination is replaced with a kinetic transient in the sub-microsecond time domain, consistent with the reduction of this electron acceptor. Concomitantly, a broad and intense EPR signal arises around g = 5 along with a minor set of resonances around g = 2 similar to the spectrum of the [4Fe-4S](+) cluster in the Fe protein of Azotobacter vinelandii nitrogenase, which exists in two conformations having S = (3)/(2) and S = (1)/(2) ground spin states. The Mössbauer spectrum in the as-isolated HbRC core shows that all of the Fe is present in the form of a [4Fe-4S](2+) cluster. After reduction with sodium dithionite in the presence of light, approximately 65% of the Fe appears in the form of a [4Fe-4S](+) cluster; the remainder is in the [4Fe-4S](2+) state. Analysis of the non-heme iron content of HbRC cores indicates an antenna size of 21.6 +/- 1.1 BChl g molecules/P798. The evidence indicates that the HbRC contains a [4Fe-4S] cluster identified as F(X) that is coordinated between the PshA homodimer; in contrast to F(X) in other type I reaction centers, this [4Fe-4S] cluster exhibits an S = (3)/(2) ground spin state.

A Bridging [4Fe-4S] Cluster and Nucleotide Binding Are Essential for Function of the Cfd1-Nbp35 Complex as a Scaffold in Iron-Sulfur Protein Maturation

“…In some proteins such as benzoyl-CoA reductase (46), iron-only hydrogenase maturation protein HydF (47), and the transcriptional regulator SufR (48), the bridging nature has been postulated or seems likely. For crystallographically characterized proteins (49 -51) high spin states of a bridging [4Fe-4S] cluster occur in the nitrogenase iron protein (38,40), the activator of 2-hydroxyglutaryl-CoA dehydratase (39), and the F X cluster of the photosynthetic reaction center (42). Cfd1-Nbp35 now join the increasing inventory of [4Fe-4S] proteins with a bridging Fe-S cluster and a high spin state in the reduced form.…”

Insights into heliobacterial photosynthesis and physiology from the genome of Heliobacterium modesticaldum

“…6 from Sattley et al 2008). Although much work over the past 25 years has contributed to our current understanding of heliobacterial electron transfer (Nuijs et al 1985;Smit et al 1987;Vos et al 1989;Fischer 1990;Trost et al 1992;Kleinherenbrink et al 1994;Lin et al 1995;Nitschke et al 1995;Kramer et al 1997;Oh-oka et al 2002;Heinnickel et al 2006, some important questions remain. Until recently, the nature and identity of the complex that transfers electrons from NADH to the menaquinone pool was unknown.…”

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

“…Weak resonances were also observed at g = 2. 04 temperature dependence as well as the similarity to the g values of reduced F A and F B , these latter signals were originally assigned to a small amount of retained PshBI/ PshBII (Heinnickel et al 2006). At the same time, Miyamoto and co-workers reported a light-minus-dark difference spectrum at very low temperatures with g z = 2.040, g y = 1.911, and g x = 1.896 in a sample that contained pre-reduced F A and F B .…”

“…The PshA homodimer binds 22-35 bacteriochlorophyll g (BChl g) molecules that function as antenna pigments (Nuijs et al 1985;Trost and Blankenship 1989;Kobayashi et al 1991;Heinnickel et al 2006), as well as many of the cofactors associated with charge separation: the primary donor, which is a special pair of BChl g 0 molecules called P798 due to its bleaching maximum; the primary acceptor, which is a hydroxylated chlorophyll a (Chl a) molecule ; and the interpolypeptide [4Fe-4S] cluster, F X .…”