Rieske ferredoxins

There are several major types of iron-sulfur proteins, i.e. the rubredoxin (Rb), ferredoxin, Rieske protein, and the high-potential iron-protein (HiPIP). 

Sequences of proteins containing Rieske-type clusters have been deduced from the complete operons of several dioxygenases these dioxygenases require electrons from NAD(P)H to convert aromatic compounds to cis-arene diols. The water-soluble dioxygenase systems consist of a reductase and a terminal dioxygenase many dioxygenases also contain a [2Fe-2S] ferredoxin (20). The terminal oxygenases contain a Rieske-type cluster and the ferredoxins may contain either a Rieske-type or a 4-cysteine coordinated [2Fe-2S] cluster. 

Only a few residues show more than 75% sequence identity, including four glycine residues, a proline residue at the beginning of the Pro loop, and a phenylalanine residue in a position corresponding to the conserved residue Tyr 165 of the bovine heart Rieske protein. However, structure prediction and sequence comparison with Rieske proteins from bci complexes suggests that the fold will be very similar in all Rieske-type ferredoxins, as in the other Rieske or Rieske-type proteins (see Section III,B,1). 

When the distantly related sequences are excluded from the alignment, a total of 22 residues are found to be conserved between 28 sequences 14 of 42 residues around the cluster binding loops are conserved. Therefore, it can be concluded that Rieske domains of the a subunits of dioxygenases show a higher degree of conservation than Rieske-type ferredoxins. 

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. 

Extinction Coefficients and Ellipticities of the Rieske Protein from Bovine Heart bc Complex (ISF) and of the Rieske-Type Ferredoxin from Benzene Dioxygenase (FiIbed)... 

Fig. 12. EPR spectra of the Rieske fragment from the 6ci complex of Paracoccus denitrificans (ISFpd, top) and of the Rieske-type ferredoxin from benzene dioxygenase (FdBED, bottom). EPR conditions were as follows (ISF/FdBEn) microwave frequency, 9.021 GHz modulation amplitude, 1 mT/0.9 mT microwave power, 1 mW/9 mW temperature, 15 K/30 K.

NMR spectra have been reported for the Rieske-type ferredoxins from Xanthobacter strain Py2 (88) and of toluene 4-monooxygenase from Pseudomonas mendocina (T4MOC) (88a) as well as for the water-soluble Rieske fragment from the bci complex of Paracoccus deni-trificans (ISFpd) (89). The spectra of these proteins are similar, which is consistent with the close structural relationship between the three proteins. In the reduced (paramagnetic) state, all three proteins show several hyperfine-shifted resonances between +83 and -16 ppm at 400 MHz or between 110 and +25 ppm at 300 MHz (Table X). 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

Thermodynamic Parameters of the Rieske Fragments from the be. Complexes of Bovine Heart (ISFb) (92) and Paracoccus dentrificans (ISFpd) (140) and of the Rieske-Type Ferredoxin FROM Benzene Dioxygenase (FdBED) (55)... 

While the redox potentials of Rieske clusters are above -1-100 mV at pH 7, values between 100 and 150 mV have been determined for the redox potentials of Rieske-type clusters (Table XI). Several 4-cysteine coordinated [2Fe-2S] clusters have redox potentials similar to those of Rieske-type clusters, for example, the [2Fe-2S] clusters of the dioxygenase reductases [compilation in (104)]-, therefore, the redox potential is not useful for distinguishing between Rieske-type and ferredoxin-type clusters. 

Although the redox potential of Rieske-type clusters is approximately 400 mV lower than that of Rieske clusters, it is 300 mV more positive than the redox potential of plant-type ferredoxins (approximately -400 mV). Multiple factors have been considered to be essential for the redox potential of iron sulfur proteins ... 

The structure of phthalate dioxygenase reductase that transfers electrons directly from NADPH to phthalate dioxygenase has been determined by X-ray crystallography (119). In class II or class III dioxygenases, the ferredoxin obligately transfers electrons from the reductase to the terminal dioxygenase (64a) it can be either a Rieske-type ferredoxin or a ferredoxin containing a 4-cysteine coordinated [2Fe-2S] cluster. 

NDO can be classified as class III dioxygenase the electron transfer chain involves a Rieske-type ferredoxin. Electrons enter NDO through the Rieske-type cluster of the dioxygenase. Kauppi et al. (11) have suggested that the binding site of NDO for the ferredoxin involves the 6 strands 10 and 12 of the Rieske domain as well as residues from the catalytic domain that form a depression in the protein surface close to Cys 101, which is a ligand of the Rieske cluster. In Rieske proteins from be complexes, access to this side of the cluster is blocked by an acidic surface residue (Asp 152 in the ISF, Glu 120 in RFS). 

Rieske centers distinguish themselves from common [2Fe-2S] and [4Fe-4S] clusters (a) by an unusual EPR spectrum characterized by a lowgav-value of 1.91 as compared to 1.96 for most 2Fe2S and 4Fe4S ferredoxins (102) and (b) by an electrochemical potential that is al-... 

Burkholderia cepacia strain 2CBS is able to degrade ort/jo-halogenated benzoates by dioxygenation to catechol with the elimination of halide and decarboxylation. The enzyme contains a ferredoxin-and a Rieske-type [2Fe-2S] center. These could be distinguished on the basis of their EPR spectra, and the results were compared with those for other [2Fe-2S] clusters (Riedel et al. 1995). 

---