|Composite Phylogeny of the Superfamily|
|Genomic Survey of cbb3-Type Oxidases|
|Characterisation of the cbb3-Type Oxidase from Rubrivivax gelatinosus|
|Characterisation of qNOR from Sulfurihydrogenibium azorenze|
|Binding of NO to HCOs|
|aa3:c552-Interaction in Paracoccus denitrificans|
|The figure to the left shows the composite phylogeny of the catalytic subunits of all hitherto known classes of O2- and NO-reductases. As discussed in the corresponding article (Ducluzeau et al. 2009), we interpret the tree topology as indicating qNO-reductase (i.e. quinol-oxidising NO reductase) to represent the ancestral enzyme present prior to the divergence of Archaea and Bacteria. In our scenario, several duplicate versions of this enzyme existed in the common ancestor of Archaea and Bacteria, some of which evolved independently into oxygen reductases after photosynthetic O2-production has set in by positioning electron-donating tyrosine residues in appropriate positions close to the binuclear centre (Figure to the right). For details and underpinnings of this scenario, please see Ducluzeau et al. 2009.|
These results imply that the enzyme NO-reductase in fact was the ancestor of all extant O2-reductases.
In order to be energetically sensible, an ancestral NO-reductase requires sufficient amounts of its substrate,
nitric oxide, to be present in the Archaean environment. Our colleague Michael J. Russell made us aware
of a possible paleogeochemical source for the mass production of nitrogen oxides, via the reaction of
atmospheric CO2 and N2 to NO. The detailed chemical reaction schemes are described in
Ducluzeau et al. 2009. According to our scenario, the denitrification pathway or
segments thereof would have been producing energy via Mitchellian chemiosmosis in the early Archaean (see
Figure to the right). One enzyme from this chain, i.e. NO-reductase, would then have (several times
independently) evolved into the extant forms of O2 reductases.
The conclusions drawn in this work are the result of an extensive interdisciplinary interaction of bioenergetics (BIP9) and palaeogeochemistry (Michael J. Russell, JPL).
The distribution of gene clusters related to cbb3-type oxidases within published genomes was analysed.
Members of this class of HCO were found in almost all phyla of the Bacteria (A in figure to the right).
No archaeal representatives were detected. The gene cluster turned out to be strongly heterogeneous and
only the diade of genes coding for the catalytic subunit, ccoN and the monoheme cytochrome subunit,
ccoO, were found to be ubiquitous and in a conserved gene order. This suggests the CcoO and CcoN
subunits as the functional core of the enzyme to which additional subunits have been added in different
phyla of the bacterial tree (see figure to the right, B), very much like the situation observed for the
Rieske/cytb complexes. For details of the obtained results and conclusions, please see
Ducluzeau et al. 2008.
An Excel-file detailing the list of entry numbers for all proteins analysed in this study can be downloaded here