References
Crystal structure of the membrane-exposed domain from
a respiratory quinol oxidase with an engineered dinuclear copper centre.
Wilmanns, M., Lappalainen, P., Kelly, M., Sauer-Eriksson, E. & Saraste,
M. (1995) Proc.Natl. Acad. Sci. USA 92, 11995-11999.
Evolution of cytochrome oxidase. Saraste, M., Castresana, J., Higgins, D.,
Lübben, M. & Wilmanns, M. (1996) in "Origin and Evolution
of Biological Energy Conversion", Ed. H. Baltscheffsky, Verlag Chemie,
New York, in press.
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Summary (04/96)
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Cytochrome oxidase is an integral membrane protein that catalyses the terminal
reaction in aerobic respiration, reduction of oxygen to water which is coupled
to the generation of a proton gradient across the membrane (Babcock &
Wikström, 1992). The cytochrome oxidase superfamily has two major branches;
cytochrome bo quinol oxidases (QO) use ubiquinol and cytochrome c
cytochrome oxidases (CO) use cytochrome c as primary electron donor (Garcia-Horsman
et al., 1994; Saraste et al., 1996). Most of the bacterial cytochrome oxidases
consist of three subunits. Subunit I is localised within the membrane and
contains the active centre. Subunit II is attached to the periplasmic side
of the membrane and is anchored by two N-terminal helices into the membrane.
This subunit functions as primary electron acceptor. Subunit II of CO contains
a dinuclear copper centre, referred to CuA. In contrast, no copper site
is found in subunit II of QO. The CuA binding site in CO contains two copper
ions in a mixed valence [Cu(1.5)-Cu(1.5)]S=1/2 configuration (Kroneck et
al., 1988; Antholine et al., 1992). The CuA binding site has been reengineered
in subunit II of QO from Escherichia coli by mutating five residues
that were identified as ligands of the two coppers (Van der Oost et al.,
1992). The functional role of subunit III, located within the membrane adjacent
to subunit I, remains unclear.
The crystal structure of the periplasmic fragment (205 residues) of subunit
II of QO from Escherichia coli, referred to CyoA, has been determined
at 2.5 Å resolution by multiple isomorphous replacement methods (Wilmanns
et al., 1995). Furthermore, the 3D structure of a shorter fragment (183
residues) with the re-engineered CuA binding site, referred to purple-CyoA
because of its visible spectrum (Kelly et al., 1993), was solved at 2.3
Å resolution using the wt-structure as template (Wilmanns et al.,
1995). At the same time the crystal structures of the entire cytochrome
oxidases from Paracoccus denitrificans with 3 subunits (Iwata et
al., 1995) and from bovine heart with 13 subunits (Tsukihara et al., 1995)
were solved at 3.0 and 2.8 Å resolution, respectively. The fold of
the soluble fragment of subunit II seems to be very similar in all three
structures. A systematic comparison, however, has not yet been possible
since the co-ordinates of the other cytochrome oxidase structures are not
yet available.
CyoA is folded as antiparallel b-sandwich with 11 b-strands followed by
three C-terminal a-helices (Figure 3). The distance between the two copper
ions in the CuA binding site is about 2.5 Å. Two cysteines function
as bridging ligands of the copper ions, and two histidines are terminal
ligands of each of the two copper ions. The overall arrangement of the CuA
centre is highly symmetrical, which is consistent with various spectroscopic,
mutagenesis and theoretical data on the CuA binding site (Malmström
& Aasa, 1993; Kelly et al., 1993; Farrar et al., 1995). The ligand arrangement
is that of the proposed "bridging model" (Farrar et al., 1995;
Henkel et al., 1995) as opposed to the "terminal model" (Blackburn
et al., 1994). The structure of purple-CyoA with reduced CuA binding site
shows a virtually identical arrangement of the two coppers (Djinovic, Wilmanns
& Saraste, unpublished results). However, the shortest distance between
each copper and the imidazole ring of each histidine ligand has increased
from about 1.9 A to 2.1 Å due to increased ion radius of Cu+1.
A shorter version of the b-sandwich fold of the subunit II of quinol oxidase
was found in a variety of blue copper proteins that contain mononuclear
copper binding sites (Adman, 1991). Comparisons of the overall folds and
the copper binding sites have suggested common evolutionary ancestry (Adman,
1995; Saraste et al., 1996; Wilmanns & Saraste, unpublished results).
Furthermore, several members of the dinitrification pathway (nitrite reductase,
nitric oxide reductase and nitrous oxide reductase) seem to be related to
components of cytochrome oxidase. Interpretation of parts of these data
by evolutionary tree analysis has led to the "early respiration hypothesis"
(Castresana et al., 1994; Castresana & Saraste, 1995). This hypothesis
suggests that aerobic respiration preceeded photosynthesis during evolution.
There is evidence from EPR spectroscopy that nitrous oxide reductase shares
the same dinuclear copper centre as that identified in subunit II of cytochrome
oxidase (Scott et al., 1989; Farrar et al., 1991; Antholine et al., 1992).
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