Rieske protein

Oxidation of this UQHg occurs in two steps. First, an electron from UQHg is transferred to the Rieske protein and then to cytochrome C. This releases two to the cytosol and leaves UQ , a semiquinone anion form of UQ, at... 

Why has nature chosen this rather convoluted path for electrons in Complex 111 First of all. Complex 111 takes up two protons on the matrix side of the inner membrane and releases four protons on the cytoplasmic side for each pair of electrons that passes through the Q cycle. The apparent imbalance of two protons in ior four protons out is offset by proton translocations in Complex rV, the cytochrome oxidase complex. The other significant feature of this mechanism is that it offers a convenient way for a two-electron carrier, UQHg, to interact with the bj and bfj hemes, the Rieske protein Fe-S cluster, and cytochrome C, all of which are one-electron carriers. 

IC. Bacterial Rieske proteins that are not from bci or b f complexes or of unknown origin [9]... 

Rieske proteins are constituents of the be complexes that are hydro-quinone-oxidizing multisubunit membrane proteins. All be complexes, that is, bci complexes in mitochondria and bacteria, b f complexes in chloroplasts, and corresponding complexes in menaquinone-oxidizing bacteria, contain three subunits cytochrome b (cytochrome 6e in b f complexes), cytochrome Ci (cytochrome f in b(,f complexes), and the Rieske iron sulfur protein. Cytochrome 6 is a membrane protein, whereas the Rieske protein, cytochrome Ci, and cytochrome f consist of water-soluble catalytic domains that are bound to cytochrome b through a membrane anchor. In Rieske proteins, the membrane anchor can be identified as an N-terminal hydrophobic sequence (13). 

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

The bacterial Rieske proteins contain 3—20 extra residues in the catalytic domain these insertions occur in the helix—loop structure and in the loop /35-/S6 (see Section III,B). The insertion of a single residue is observed in some bacterial sequences between the flexible linker and f3 strand 1 as well as in the Pro loop. Twenty-eight residues are fully conserved between 11 mitochondrial and 6 bacterial sequences 22 of these conserved residues are located in the cluster binding subdomain. 

Figure 1 shows the phylogenetic relationship of the mitochondrial and bacterial Rieske proteins. Plant mitochondrial Rieske proteins form a separate cluster, whereas bacterial Rieske proteins are more closely related to Rieske proteins from fungi or mammals, although the subunit composition and organization of the bci complex is compa-... 

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). 

The water-soluble fragment of the Rieske protein from bovine heart bci complex (ISF) was crystallized by Link et al. (30) and the structure was solved at 1.5 A resolution by Iwata et al. (9) (PDB file IRIE). 

The water soluble fragment of the Rieske protein from spinach... 

The structure of the full-length Rieske protein has been determined by X-ray crystallography of the whole bci complex, but at lower resolution (3.0 A), by Zhang et al. (41) and by Iwata et al. (42). 

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 the Rieske proteins from bci or b f complexes, a third loop ( Pro loop, part of (38-/39 containing the highly conserved sequence Gly-Pro-Ala-Pro) covers the cluster from the other side. Mutations in the Pro loop have shown that this loop is critical for cluster stability... 

Comparison of Distances and Angles in the [2Fe-2S] Cluster in Rieske Proteins AND IN Proteins Containing 4-Cysteine Coordinated [2Fe-2S] Clusters... 

Comparative Aspects of Rieske Proteins Similarity and Diversity... 

Rieske proteins from complexes contain an insertion of 11 residues between strands (31 and (32 compared to mitochondrial Rieske proteins as well as an extension of 16 residues at the C-terminus. In the N-terminal part, there is an additional short (3 strand (/31 ), as well as a short helix that has no counterpart in the mitochondrial Rieske protein or in NDO. The N-terminus of (3 strand /31 is hydrogen-bonded to strand (310 in (3 sheet 1 as it is in the other Rieske proteins, but the end of strand (31 connects to strand (32 in (3 sheet 2 thus, the sandwich structure of the (3 sheets 1 and 2 is perturbed and a barrellike structure is formed. The core of this barrel is less hydrophobic than the core of the sandwich formed by /3 sheets 1 and 2 in the ISF. 

Fig. 5. Structure-based alignment of the sequences of the water-soluble Rieske fragment from bovine heart bci complex (ISF), the water-soluble Rieske fragment from spinach b f complex (RFS), and of the Rieske domain of naphthalene dioxygenase (NDO) and of the metal binding loops of rubredoxin (RXN) and transcriptional factor TFIIS (TFI). The numbering of the j3 strands is the same for the ISF and RFS. The metal binding ligands are highlighted the asterisks indicate those residues that are fully conserved between the three Rieske proteins.

The C-terminal extension (residues 163-179) of the Rieske proteins of the bef complex appears to have the same role in stabilizing the open side of the cluster binding subdomain as the helix-loop insertion in the mitochondrial Rieske protein. The C-terminal part of RFS contains a fully conserved arginine (Arg 170 in spinach) this arginine... 

Therefore, although the function of the helix-loop insertion in mitochondrial Rieske proteins appears to be the same as that of the C-terminal extension in chloroplast Rieske proteins, both structures show no structural similarity or sequence homology. 

In bovine heart mitochondrial bc complex, the trans conformation is observed for Pro 175 of the Rieske protein in the P6s22 crystal form, whereas the cis conformation is observed in the P6s crystal form 42) (see Section III,B,5). 

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