Fe4S4 clusters

The Fe proteins are homodimers containing a single Fe4S4 cluster. Site-directed mutagenesis experiments showed that the cluster was probably held between the two subunits by ligation to two of the invariant cysteine residues from each subunit (16). This observation was confirmed later by X-ray crystallography (1) of the Fe protein... 

Fig. 2. The structure of the Fe protein (Av2) from Azotobacter vinelandii, after Geor-giadis et al. (1). The dimeric polypeptide is depicted by a ribbon diagram and the Fe4S4 cluster and ADP by space-filling models (MOLSCRIPT (196)). The Fe4S4 cluster is at the top of the molecule, bound equally to the two identical subunits, Emd the ADP molecule spans the interface between the subunits with MoO apparently binding in place of the terminal phosphate of ATP.

Yet further oxidation removes at least one more electron from each P cluster with an +90 mV to yield a protein oxidized by a total of at least eight electrons and with EPR signals from mixed spin states of S = I and S = I (42, 47). The combined integrations of these signals demonstrated that their intensity was equivalent to that of the FeMoco EPR signals in the same preparations. This provided the first evidence (47) that MoFe proteins contained equivalent numbers of FeMoco centers and P clusters and that P clusters contained 8 Fe atoms. Previously it had been considered that the P clusters were fully reduced Fe4S4 clusters and thus that there were two P clusters for every FeMoco center per molecule. 

Fig. 6. Equilibrium between two electron distributions in oxidized [Fe4S4] + clusters (68).

CODH and ACS shown to occur at discrete [Ni-X-Fe4S4] clusters, called Cluster C and Cluster A, respectively. ... 

Methyl coenzyme M reductase plays a key role in the production of methane in archaea. It catalyzes the reduction of methyl-coenzyme M with coenzyme B to produce methane and the heterodisulfide (Figure 3.35). The enzyme is an a2P2Y2 hexamer, embedded between two molecules of the nickel-porphinoid F jg and the reaction sequence has been delineated (Ermler et al. 1997). The heterodisulfide is reduced to the sulfides HS-CoB and HS-CoM by a reductase that has been characterized in Methanosarcina thermoph-ila, and involves low-potential hemes, [Fe4S4] clusters, and a membrane-bound metha-nophenazine that contains an isoprenoid chain linked by an ether bond to phenazine (Murakami et al. 2001). 

Boyington JC, VN Gladyshev, SV Khangulov, TC Stadtman, PD Sun (1997) Crystal structure of formate dehydrogenase H catalysis involving Mo, molybdopterin, selenocysteine, and an Fe4S4 cluster. Science 275 1305-1308. 

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