Cytochrome c oxidase (EC 1.9.3.1) [1] is an oligomeric integral membrane
protein complexes that catalyze the terminal step in the respiratory chain:
they transfer electrons from cytochrome c or a quinol to oxygen. Some terminal
oxidases generate a transmembrane proton gradient across the plasma membrane
(prokaryotes) or the mitochondrial inner membrane (eukaryotes). The enzyme
complex consists of 3-4 subunits (prokaryotes) up to 13 polypeptides (mammals)
of which only the catalytic subunit (equivalent to mammalian subunit 1 (CO I))
is found in all heme-copper respiratory oxidases. The presence of a bimetallic
center, formed by a high-spin heme (heme a3) and copper B, as well as a
low-spin heme (heme a), both ligated to six conserved histidine residues near
the outer side of four transmembrane spans within CO I is common to all family
members
[1][4][3].
In contrary to eukaryotes the respiratory chain of prokaryotes is branched to
multiple terminal oxidases. The enzyme complexes vary in heme and copper
composition, substrate type and substrate affinity. The different respiratory
oxidases allow the cells to customize their respiratory systems according a
variety of environmental growth conditions [1].
The crystal structure of the whole enzyme complexe have been solved
[5].
Subunit I contains 12 transmembrane helical segments and binds heme a and heme
a3-copper B binuclear centre where molecular oxygen is reduced to water..
Recently also a component of an anaerobic respiratory chain has been found to
contain the copper B binding signature of this family: nitric oxide reductase
(NOR) exists in denitrifying species of Archae and Eubacteria.
Enzymes that belong to this family are:
- Mitochondrial-type cytochrome c oxidase (EC 1.9.3.1) which uses cytochrome
c as electron donor. The electrons are transferred via copper A (Cu(A)) and
heme a to the bimetallic center of CO I that is formed by a penta-
coordinated heme a and copper B (Cu(B)). Subunit 1 contains 12
transmembrane regions. Cu(B) is said to be ligated to three of the
conserved histidine residues within the transmembrane segments 6 and 7.
- Quinol oxidase from prokaryotes that transfers electrons from a quinol to
the binuclear center of polypeptide I. This category of enzymes includes
Escherichia coli cytochrome O terminal oxidase complex which is a component
of the aerobic respiratory chain that predominates when cells are grown at
high aeration.
- FixN, the catalytic subunit of a cytochrome c oxidase expressed in
nitrogen-fixing bacteroids living in root nodules. The high affinity for
oxygen allows oxidative phosphorylation under low oxygen concentrations. A
similar enzyme has been found in other purple bacteria.
- Nitric oxide reductase (EC 1.7.99.7) from Pseudomonas stutzeri. NOR reduces
nitrate to dinitrogen. It is a heterodimer of norC and the catalytic
subunit norB. The latter contains the 6 invariant histidine residues and 12
transmembrane segments
[2].
As a signature pattern we used the copper-binding region. We also developed a
profile that cover the whole subunit I.