Rieske cluster

From an analysis of the EPR spectra, Blumberg and Peisach (4) suggested that the coordination environment of Rieske clusters must include one or more atoms which are less electron donating than sulfur. This was significantly substantiated by studies of the Rieske... 

In addition to the four ligands of the Rieske cluster, three residues are fully conserved in all Rieske proteins ... 

State of the Rieske cluster reduced reduced oxidized... 

The Rieske Cluster Binding Subdomain and the Rieske Cluster... 

In the Rieske proteins from bci or b f complexes, loops (34-/35 and (36-/37 both contain an additional cysteine residue (Cys 144 and Cys 160 in the ISF and Cys 112 and Cys 127 in RFS) these cysteines form a disulfide bridge connecting the two loops (Fig. 3b). These cysteines are not present in the sequences of Rieske-type proteins, that is, in neither NDO nor Rieske-type ferredoxins. In Rieske proteins, the disulfide bridge appears to be important for the stabilization of the fold around the cluster as the two loops are not shielded by other parts of the protein in NDO, the Rieske cluster is stabilized without a disulfide bridge since it is completely buried by surrounding a and (3 subunits. 

In NDO, the corresponding loop is much longer and does not interact with the environment of the Rieske cluster, hut is involved in subunit interactions with the catalytic domain in a neighboring subunit (11). 

A ci positional state where the exposed NeH group of His 161 which is a ligand of the Rieske cluster forms a hydrogen bond with a propionate group of heme Ci (42)... 

An intermediate state where the environment of the Rieske cluster does not interact with any other subunit of the bci complex... 

Fig. 8. (a) Structure of the full-length Rieske protein from bovine heart mitochondrial bci complex. The catalytic domain is connected to the transmembrane helix by a flexible linker, (b) Superposition of the three positional states of the catalytic domain of the Rieske protein observed in different crystal forms. The ci state is shown in white, the intermediate state in gray, and the b state in black. Cytochrome b consists of eight transmembrane helices and contains two heme centers, heme and Sh-Cytochrome c i has a water-soluble catalytic domain containing heme c i and is anchored by a C-terminal transmembrane helix. The heme groups are shown as wireframes, the iron atoms as well as the Rieske cluster in the three states as space-filling representations. 

Early mutational studies of the Rieske protein from 6ci complexes have been performed with the intention of identifying the ligands of the Rieske cluster. These studies have shown that the four conserved cysteine residues as well as the two conserved histidine residues are essential for the insertion of the [2Fe-2S] cluster (44, 45). Small amounts of a Rieske cluster with altered properties were obtained in Rhodobacter capsulatus when the second cysteine in the cluster binding loop II (Cys 155, corresponding to Cys 160 in the bovine ISF) was replaced by serine (45). The fact that all four cysteine residues are essential in Rieske clusters from be complexes, but that only two cysteines are conserved in Rieske-type clusters, led to the suggestion that the Rieske protein may contain a disulfide bridge the disulfide bridge was finally shown to exist in the X-ray structure (9). 

The importance of hydrogen bonds for the redox potential of the Rieske cluster has been demonstrated by site-directed mutagenesis of... 

When the fully conserved residue Thr 140, which is packed against the Pro loop, was substituted by Gly, His, or Arg in Rhodobacter capsulatus, the midpoint potential of the Rieske cluster was decreased by 50-100 mV, the cluster interacted with the quinone pool and the bci complex had 10-24% residual activity but the Rieske cluster was rapidly destroyed upon exposure to oxygen (49). In contrast, the residual activity was <5%, the cluster showed no interaction with the quinone pool, and the interaction with the inhibitor stigmatellin... 

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

The coordination of the Rieske cluster in the a subunit of benzene dioxygenase has been studied by site-directed mutagenesis. The replacement of His 98 or His 119 (corresponding to His 83/104 in NDO) by Cys resulted in a protein that was unable to coordinate a normal Rieske-type cluster (53). In the mutant His 98 Cys, a novel EPR spectrum with av = 1-94 was detected that is intermediate between... 

In its reduced state, the paramagnetic Rieske cluster shows a temperature-dependent MOD spectrum composed of numerous positive and negative C-terms that originate from the 8 = 1 ground state. The MOD spectra lack the Fe" 8 charge transfer bands that are observed as intense negative bands between 300 and 350 nm and a posi-... 

In summary, the Mossbauer data presented by Fee et al. (5) gave the first conclusive evidence that Rieske clusters contain noncysteine ligands bound to the Fe" site of a localized mixed valence cluster. In addition, strong similarities with [2Fe-2S] clusters in bacterial dioxy-... 

Assignments for [2Fe-2S]-CyS4 clusters were taken from Han et al. (128), those for oxidized Rieske clusters from Kuila et al. (67). Symmetry labels refer to idealized cluster cores. 

Upon excitation of the reduced Rieske cluster above 500 nm, in-... 

Comparison of the Distances within the Rieske Cluster Obtained from EXAFS Measurements (72) and from X-Ray Crystallography... 

Fig. 14. Plot of the g values g,g ) and of the average g value g vs rhombicity (UJ of (a) wild type (open symbol) and variant forms (closed symbols) of the Rieske protein in yeast bci complex where the residues Ser 183 and Tyr 185 forming hydrogen bonds into the cluster have been replaced by site-directed mutagenesis [Denke et al. (35) Merbitz-Zahradnik, T. Link, T. A., manuscript in preparation] and of (b) the Rieske cluster in membranes of Rhodobacter capsulatus in different redox states of the quinone pool and with inhibitors added [data from Ding et al. (79)]. The solid lines represent linear fits to the data points the dashed lines reproduce the fits to the g values of all Rieske and Rieske-type proteins shown in Fig. 13.

Fig. 15. EPR spectra of the Rieske cluster in membranes of Paracoccus denitrificans in different redox states of the quinone pool and with inhibitors added. Q x, ascorbate reduced Qred) reduced with trimethylhydroquinone dissolved in dimethyl sulfoxide +EtOH, reduced with trimethylhydroquinone dissolved in 90% ethanol +Myxo, ascorbate reduced with myxothiazol added + Stigma, ascorbate reduced with stigmatellin added. Only the gy and signals are shown. The dotted line has been drawn at...

---