Giuffrè
A, Stubauer G, Sarti P, Brunori M, Zumft WG, Buse G, Soulimane T
The heme-copper oxidases of Thermus
thermophilus catalyze the reduction of nitric oxide: evolutionary
implications
Proc Natl Acad Sci USA
96:14718-14723
(1999)
We show that the heme-copper
terminal
oxidases of Thermus thermophilus (called ba3
and caa3 ) are able to catalyze the
reduction
of nitric oxide (NO) to nitrous oxide (N2O)
under
reducing anaerobic conditions. The rate of NO consumption and N2O
production were found to be linearly dependent on enzyme concentration,
and activity was abolished by enzyme denaturation. Thus, contrary to
the
eukaryotic enzyme, both T. thermophilus oxidases display a NO
reductase
activity (3.0 ± 0.7 mol NO/mol ba3 × min and
32 ± 8 mol NO/mol caa3
×
min at [NO] 50 µM and 20°C) that, though considerably
lower
than that of bona fide NO reductases (300-4,500 mol NO/mol
enzyme
× min), is definitely significant. We also show that for ba3
oxidase, NO reduction is associated to oxidation of cytochrome b
at a rate compatible with turnover, suggesting a mechanism consistent
with
the stoichiometry of the overall reaction. We propose that the NO
reductase
activity of T. thermophilus oxidases may depend on a peculiar CuB+
coordination, which may be revealed by the forthcoming
three-dimensional
structure. These findings support the hypothesis of a common phylogeny
of aerobic respiration and bacterial denitrification, which was
proposed
on the basis of structural similarities between the Pseudomonas
stutzeri
NO reductase and the cbb3 terminal
oxidases.
Our findings represent functional evidence in support of this
hypothesis.