| Root |
Created: 3 March 1997
| Centre | Haem type | Haem iron coordination | Axial iron ligands | Formal metal oxidation/spin states | |
|---|---|---|---|---|---|
| a |  |
 |
 His;
N |
FeII (S=0); FeIII (S=1/2) |
|
| a3 |  |
His
|
FeII (S=2); FeIII (S=5/2)| | Spincoupled to CuB |
||
|
His;
O2, OH¯, NO, CO, CN¯ or other ligand |
FeII (S=0); FeIII (S=1/2)| | Spincoupled to CuB |
|||
|
His;
O |
FeIV (S=1) | |||
| Copper centre | Copper ligands | ||||
| CuA |  |
CuICuII (S=1/2); CuIICuII (S=0) |
|||
 His;
S |
His;
O |
||||
 Cys | |||||
| CuB |  |
His;
2 × N |
CuI (S=0); CuII (S=1/2)¶ ¶ Spincoupled to a3 |
||
| Haem a3-CuB centre | Metal ligands | ||||
| a3-CuB |  |
a3II (S=2),
CuBI (S=0);
a3III (S=1/2),
a3III (S=5/2),
* S´ = Sa3 - SCuB
| |||
| His
|
His;
2×N |
||||
| Magnesium centre | Magnesium ligands | ||||
| Mg |  |
Asp;
O
N H2O |
|||
Haem/copper terminal oxidases catalyse the fourelectron reduction of
dioxygen to water, coupled to generation of a proton electrochemical gradient
across the membranes in which they are embedded
[1]:
Mitochondrial COX has been isolated in a number of spectroscopically distinct
species [4]:
3D structures of COXs from
Paracoccus denitrificans [6]
and bovine mitochondria [7]
have been determined. Bacterial COX consists of four subunits whereas the
mitochondrial enzyme is a dimer, each monomer being composed of 13 subunits.
Figure 1OCC shows the catalytic core of
mitochondrial COX.
Subunit I contains 12 closely packed transmembrane helices. Three semicircular
arrangements of four helices each form a `whirl' with a quasithreefold
symmetry axis [7].
Two of the three semicircles envelope haems a and
a3, which are perpendicular to the membrane plane
(see Figure 1OCC e, f);
the interplanar angles between the two haem groups are 108°
in P. denitrificans COX and 104° in the bovine enzyme.
Both the N and Ctermini of subunit I are located on the `inner'
side.
Subunits II and III interact with the transmembrane domain of subunit I without
contact with each other (see Figure 1OCC a).
Subunit II contains two transmembrane helices, closely associated with the
transmembrane domain of subunit I, and a large polar domain at the
`outer' side of membrane. The polar domain has a tenstrand
ßbarrel structure that binds the CuA centre.
Both the N and Ctermini of subunit II are on the `outer' side.
Subunit III contains seven transmembrane helices organised in two bundles, viz.
(I, II) and (III to VII), which form a Vshaped cleft.
Helices I and III form contacts with subunit I.
The Nterminus of subunit III is on the `inner' side.
The two copper atoms of the CuA centre are coordinated by two His,
one Met, a backbone carbonyl oxygen of Glu, and two bridging Cys residues.
On the interface between subunits I and II, the Mg2+binding
site is located, whose function remains unknown
[1].
Magnesium is coordinated by His, carboxylates of Asp and Glu
residues (Glu residue is shared between CuA and Mg centres)
and water.
Lowspin haem a is coordinated by two axial His ligands.
Highspin haem a3 has one axial His ligand; the
copper atom in the CuB centre is ligated by three His residues.
The haem a3-CuB centre in both the reduced and
oxidised states is EPR silent. Highspin haem
a3III (S=5/2) is
antiferromagnetically coupled to CuBII
(S=1/2), yielding an S=2 binuclear centre.
Both haem a3 and CuB have vacant coordination
sites suitable for substrate binding and possibly are coupled by a bridging
ligand in the oxidised COX [8].
The proposed catalytic cycle for COX is shown below
[1].
Dred and Dox represent the reduced and oxidised forms of
the electron donor; H+in and H+out
represent protons which are pumped from the `inner' side of the membrane
(cytosol in bacteria or matrix in mitochondria) toward the `outer' side
(periplasmic space in bacteria or cytosol in mitochondria).
Cytochrome c oxidase (COX; EC
1.9.3.1)
is a haem/copper terminal oxidase which uses cytochrome c (cyt c)
as electron donor (see list of reviews on
structure and function of COX).
The enzyme contains two iron centres, haem a and haem
a3 (also referred to as cytochromes a and
a3), and two copper centres,
CuA and CuB.
The number of subunits in COX varies from three in some bacteria to 13 in
mammalian mitochondria.
The sequences of subunits I, II and III are well conserved throughout the
haem/copper terminal oxidase superfamily
[2].
Subunit I of COXs contains the lowspin haem a and the
oxygenbinding site, composed of the highspin haem
a3 and CuB.
Haem a3 can bind a variety of ligands (NO, CO, CN¯).
Subunit II of COXs contains the binuclear CuA centre, which receives
the electrons from cyt c and transfers them to haem a and finally
to the oxygenbinding site [3]:
4Dox + 2H2O + 4H+out(1)
Subunit III contains no metal centres but, together with subunits I and II,
forms the catalytic core of COX.
Cytochrome c oxidase
cyt c
CuA
haem a
haem a3-CuB
O2
(2)
However, this classification ignores the possibility that CuA
is a mixedvalence [Cu1.5+, Cu1.5+] centre
[5].
Species Metal oxidation states
Absorption spectral features
Fully oxidised `fast'
max = 424 nm
Fully oxidised `slow'
max = 417 nm
Reduced (fully reduced)
max = 443 nm
Mixedvalence CO
Mixedvalence CN
Mixedvalence formate
The enzyme in the resting state (fully oxidised; compound O) is reduced
by cyt c yielding
a3II-CuBI form R
(fully reduced). Dioxygen (O2) reacts with R to form the
shortlived ferrousoxy intermediate (compound A).
A is converted into the bridging peroxide
(compound P), characterised by an absorption difference feature at
607 nm. Oneelectron reduction results in a ferryl intermediate
(compound F), characterised by an absorption difference feature
at 580 nm. F undergoes further oneelectron reduction to form
oxidised compound O. The two latter successive electron
transfers are coupled to proton translocation across the membrane.
Interestingly, the haem a3 is pentacoordinate
in the crystal structure of `fully oxidised' COX
[7] and no bridging ligand between
haem a3 and CuB has been found.
Determination of the structure of intermediates in detail is necessary to
understand the mechanism of action of COX.
| ENZYME | LIGAND | BRENDA | Official name | Alternative names |
|---|---|---|---|---|
| 1.9.3.1 | 1.9.3.1 | 1.9.3.1 | Cytochromec oxidase | Cytochrome oxidase; cytochrome a3; cytochrome aa3; (mitochondrial electron transport) Complex IV |
| PRINTS ID | PRINTS AC | PROSITE/BLOCKS ID | PROSITE AC | BLOCKS AC |
|---|---|---|---|---|
| COX1 | PS00077 | BL00077 | ||
| COX2 | PS00078 | BL00078 | ||
| COX5B | PS00848 | BL00848 | ||
| COX10_CTAB_CYOE | PS00943 | BL00943 |
| Protein Superfamily | Protein Homology Domain | Pfam | LPFC 3D alignment |
|---|---|---|---|
|
00147;
COX chain I
00148; Acanthamoeba COX chain I/II | 00291; COX chain I | ||
|
00149;
COX chain II
00148; Acanthamoeba COX chain I/II | 00292; COX chain II | ||
| 00150; COX chain III | |||
| 00151; COX chain IV | |||
|
00153;
yeast COX chain V
00154; mammalian COX chain Va 00155; mammalian COX chain Vb 03631; Dictyostelium COX chain V | |||
|
00156;
mammalian COX chain VIa
00157; mammalian COX chain VIb 00158; mammalian COX chain VIc 00159; Dictyostelium COX chain VI | |||
|
00160;
yeast COX chain VII
00161; yeast COX chain VIIa 03830; mammalian COX chain VIIa 03832; COX chain VIIb 00162; COX chain VIIc | |||
| 00163; COX chain VIII |
| PDB | scop | BSM | RELI Base | Header | 
¹ |
|---|---|---|---|---|---|
| 1ar1 | 1ar1 | 1ar1 | 1ar1 | Cytochrome c oxidase (complex with an antibody Fv fragment, lauryl dimethylamine oxide, Ca2+ and Mg2+); Paracoccus denitrificans | MS6PT3 |
| 1occ | 1occ | 1occ | 1occ | Cytochrome c oxidase (fully oxidised) (dimer) (complex with Mg2+ and Zn2+); bovine | MS7MB2 |
| 2occ | 2occ | Cytochrome c oxidase (fully oxidised) (dimer) (complex with peroxide, Mg2+, Na+ and Zn2+); bovine |
¹ Macromolecular Structures abstract.
Full text is available to BioMedNet
Members
References
| Bibliography on structural studies of cytochrome c oxidase |
|
| Reviews on cytochrome c oxidase |
| Cytochrome Oxidase Home Page |
Glu