Biology Reference
In-Depth Information
NOC-18, spermine NONOate, nitrate and nitrite (
Elvers et al., 2005;
Pittman et al., 2007
).
Little work has been performed on the reaction intermediates of the
Cgb-mediated detoxification of NO. However, a close homolog in
E. coli
, the FHb Hmp, is an extensively studied microbial NO detoxification
mechanism converting NO and O
2
to the harmless nitrate ion via a
dioxygenase (NO dioxygenase, NOD) (
Gardner et al., 2006, 2000;
Gardner, Gardner, Martin, & Salzman, 1998
) or denitrosylase reaction
(
Hausladen et al., 1998, 2001
). Biophysical studies of the Cgb haem pocket
support the conversion of NO to nitrate (
Lu, Mukai, et al., 2007
) via the
following general reaction:
NO
3
Þ
If Cgb catalyses the conversion of NO to nitrate, there are two condi-
tions that must be fulfilled
in vivo
; oxygen availability and the existence of an
efficient reductase system for the regeneration of the ferrous haem cofactor
(Fe(II)) following oxidation by NO. The correlation between oxygen avail-
ability and resistance to nitrosative stress has indeed been demonstrated in
Campylobacter
, and resistance to NO and GSNO under microaerobic or
oxygen-limited conditions shows important differences. For instance, cul-
tures pre-treated with GSNO at higher rates of oxygen diffusion show that
respiration and growth are better protected from NO inhibition, and con-
sumption of NO is also more efficient (
Avila-Ramirez et al., 2013; Monk
et al., 2008
). Since the expression of Cgb occurs maximally in the presence
of oxygen, albeit microaerobic conditions (
Avila-Ramirez et al., 2013;
Elvers et al., 2005, 2004; Monk et al., 2008; Wainwright et al., 2005
), an
O
2
-dependent NO detoxification mechanism for Cgb seems plausible.
However, nitrate production by Cgb has not yet been demonstrated either
in vitro
or
in vivo
. Indeed, overexpression of Cgb in
E. coli
failed to increase
the NO consumption activity measured in soluble cell extracts, and there
were no differences in nitrate production compared to the control (
Frey
et al., 2002
), although it is possible that the presence of the FHb Hmp in
the system could be masking the effect of Cgb.
CgbFe II
ðÞþ
O
2
þ
NO
!
CgbFe III
ðÞþ
ð
4
:
1
5.2. Cgb reduction: The redox partner conundrum
The conversion of NO and O
2
to NO
3
by single-domain globin proteins
involves the oxidation of the haem cofactor, which must be reduced for sub-
sequent enzymatic turnovers. Hence, the existence of partner reductase