| Root |
Created: 14 August 1998
| Centre | Em (mV) |
Iron centre structure | Iron ligands | Formal iron oxidation/spin states |
||
|---|---|---|---|---|---|---|
| FeS | +280 |   Cys)2(N His)2 |
[Fe2S2]+ (S=1/2);
[Fe2S2]2+ (S=0) |
|||
| Cys
|
His
|
|||||
| Haem type | Haem iron coordination |
Axial iron ligands | Formal iron oxidation/spin states |
|||
| bH (b560) |
+40 | 
|
 |
 His;
N |
FeII (S=0);
FeIII (S=1/2) |
|
| bL (b566) |
-90 | |||||
| c1 | +290 |  |  |
His;
S |
FeII (S=0);
FeIII (S=1/2) |
|
Cytochrome bc1 complex (ubiquinol:ferricytochrome
c oxidoreductase; EC
1.10.2.2)
is found in mitochondria, photosynthetic bacteria and other
prokaryotes [1,
2].
It is minimally composed of three subunits:
cytochrome b
(cyt b), carrying a low and a highpotential haem group
(bL and bH);
cytochrome c1
(cyt c1) with one covalently bound haem c; and a
highpotential Rieske iron-sulphur protein
(ISP) containing a single [Fe2S2] cluster.
Some bacterial cyt bc1 complexes consist of only those three
redox proteins, whereas the mitochondrial complexes contain up to eight
additional subunits whose functions in the complex remain largely unknown.
In plants, the mitochondrial processing peptidase (MPP) is part of the
cyt bc1 complex [3].
The general function of the complex is electron transfer
between two mobile redox carriers, ubiquinol (QH2) and cytochrome
c (cyt c); electron transfer is coupled with the translocation
of protons across the membrane
[matrix 
intermembrane space
(in mitochondria), cytoplasm periplasm
(in purple bacteria)], thus
generating protonmotive force in the form of an electrochemical proton
potential which can drive ATP synthesis [1].
In its structure and functions, the cyt bc1 complex
bears extensive analogy to the cyt b6f complex
of chloroplasts and cyanobacteria; cyt c1 plays a role
analogous to that of cyt f, in spite of their
different structures [4].
The path of electron transfer from QH2 to cyt c through the
cyt bc1 complex is the protonmotive Qcycle:
The Qcycle model postulates two separate ubiquinone binding sites, called
Qo (quinoloxidising site) and Qi
(quinonereducing site). Qo is located near the positive
side of the membrane (intermembrane/periplasmic side); Qi
is located near the negative side of the membrane (matrix/cytoplasmic side).
The first electron of ubiquinol is transferred via the
`highpotential' chain (ISP and cyt c1) to the soluble
cyt c.
The second electron is sequentially transferred to bL,
bH and to a ubiquinone (Q) or a ubisemiquinone anion
(Q·¯) in the Qi site.
During one complete Q cycle, per one molecule of ubiquinol oxidised to
ubiquinone, two molecules of cyt c are reduced, two protons are
consumed on the negative (n) side of the membrane and four
protons are released on the positive (p) side of the membrane:
The structural states of the ISP in relation to its neighbours in different
crystal structures were categorised using the
[Fe2S2]-haem c1 and
[Fe2S2]-haem bL distances. The observed
"c1", "b" and "int" (intermediate of the
"c1" and "b") states form a basis for a new
`three state' model of the bifurcated reaction
[6].
3D structures of the cyt bc1 complex from
mitochondria have been determined
[5, 6;
2 and references therein].
The dimeric cyt bc1 complex is pearshaped with a
maximal diameter of 130 Å and a height of 155 Å. The
membranespanning region of each cyt bc1 complex monomer
consists of 13 transmembrane helices, eight of which belong to cyt b
[5].
The structures were solved with either inhibitors or substrates bound at the
catalytic sites. The Qosite is
a bifurcated pocket, with domains distal and proximal to haem
bL. Two classes of inhibitors bind differentially in these
two domains. One class comprises
5nundecyl6hydroxy4,7dioxobenzothiazol
(UHDBT),
3nundecyl2hydroxy1,4naphthoquinone
(UHNQ), stigmatellin and funiculosin, which interact with the
[Fe2S2] centre and prevent electron transfer to
cyt c1.
The second class shares a methoxyacrylate (MOA) group
(myxothiazol, MOA-stilbene).
The Qisite binds the inhibitor antimycin A which specifically
blocks electron flow from bH to ubiquinone. There is
crystallographic evidence that a ubiquinone molecule is bound in the native
complex at the Qisite but is displaced by bound antimycin A.
QH2+ + 2 cyt c(Fe3+)
+ 2H+n
Q + 2 cyt c(Fe2+) + 4H+p(1)
| ENZYME | LIGAND | BRENDA | Official name | Alternative names |
|---|---|---|---|---|
| 1.10.2.2 | 1.10.2.2 | 1.10.2.2 | Ubiquinol-cytochrome c reductase | Cytochrome bc1 complex; (mitochondrial electron transport) Complex III |
| PRINTS ID | PRINTS AC | PROSITE/BLOCKS ID | PROSITE AC | BLOCKS AC |
|---|---|---|---|---|
| CYTOCHROME_B_HEME CYTOCHROME_B_QO |
PS00192
PS00193 |
BL00192 | ||
| CYTOCHROMEC1 | PR00603 | CYTOCHROME_C | PS00190 | BL00190 |
| RIESKE | PR00162 | RIESKE_1
RIESKE_2 |
PS00199 PS00200 |
BL00199 |
| Protein Superfamily | Protein Homology Domain | Pfam | LPFC 3D alignment | MolMovDB | ||
|---|---|---|---|---|---|---|
| 00019; cytochrome b |
|
PF00033;
cytochrome_b_N;
PF00032; cytochrome_b_C | ||||
| 00003; cytochrome c1 heme protein | 00189; cytochrome c1 heme protein | |||||
| 00010; Rieske iron-sulfur protein | PF00355; Rieske |
| PDB | scop | BSM | RELI Base | Header | 
¹ |
|---|---|---|---|---|---|
| 1bcc | 1bcc | Cytochrome bc1 complex (complex with ubiquinone10, phosphatidyl ethanolamine and Boctylglucoside); chicken (heart mitochondrial) | |||
| 1be3 | 1be3 | Cytochrome bc1 complex; bovine (heart mitochondrial) | |||
| 1bgy | 1bgy | Cytochrome bc1 complex; bovine (heart mitochondrial) | |||
| 1qcr | 1qcr | Cytochrome bc1 complex (C atoms only); bovine (heart mitochondrial) | |||
| 3bcc | 3bcc | Cytochrome bc1 complex (complex with stigmatellin and antimycin); chicken (heart mitochondrial) |
¹ Macromolecular Structures abstract.
Full text is available to BioMedNet
Members
References
Information resources on cyt bc1 complex
| Bibliography on structural studies of cytochrome bc1 complex |
|
| Reviews on cytochrome bc1 complex |
