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suggests that these proteins fulfil important physiological roles, perhaps asso-
ciated to the peculiar features of the Antarctic habitat (
Giordano et al., 2007
).
Inactivation of the
PSHAa0030
gene renders the mutant bacterial strain sen-
sitive to high O
2
levels, hydrogen peroxide, and nitrosating agents (
Parrilli
et al., 2010
). Furthermore, when the
PSHAa0030
gene was cloned and
over-expressed in a flavohaemoglobin-deficient mutant of
E. coli
, unable
to metabolize NO, and the resulting strain was analyzed for its growth prop-
erties and oxygen uptake in the presence of NO, it was shown that
P. haloplanktis
2/2HbO indeed protects growth and cellular respiration of
the heterologous host from the toxic effects of NO donors. Moreover,
the ferric form of
P. haloplanktis
2/2HbO was shown to catalyze per-
oxynitrite isomerization
in vitro
, confirming its potential role in scavenging
reactive nitrogen species (
Coppola et al., 2013
).
Recently, group II 2/2HbOs have been also implicated in metabolic
pathways involving physiologically relevant sulphur compounds.
B. subtilis
and
T. fusca
group II 2/2HbOs have been shown to bind sulphide
with an affinity constant in the sub-micromolar range such that they are par-
tially saturated with sulphide when recombinantly expressed in
E. coli
(
Nicoletti et al., 2010
). Thus, these proteins have been proposed to play a
direct role as sulphide scavenger under high oxygen growth conditions,
due to the oxygen-dependent down-regulation of the competing cysteine
synthase B (which is instead a very effective sulphide scavenger under
low oxygen conditions). Also, a highly oxidative environment would favour
the oxidation of the 2/2Hb haem-Fe atom, thus allowing prompt formation
of the high affinity ferric sulphide adduct (
Nicoletti et al., 2010
). Interest-
ingly, it has recently been demonstrated that the gene encoding
B. subtilis
2/2HbO (as well as most 2/2Hbs from bacilli and staphylococci) is con-
tained within a thiol redox pathway that is implicated in the bacterial
response to the thiol oxidative stress (
Larsson, Rogstam, & von
Wachenfeldt, 2007
). In this framework, it has been proposed that
B. subtilis
2/2HbO could participate (directly or indirectly) in the complex
redox pathway of sulphur metabolism in
Bacillus
sp. (
Nicoletti et al., 2010
).
Among group III 2/2HbPs, the protein from
Campylobacter jejuni
is
the most characterized from the structural and biochemical view points
(
Bolli et al., 2008; Nardini et al., 2006
), together with the more recently
reported 2/2HbP from
Helicobacter hepaticus
(
Nothnagel, Winer, et al.,
2011
).
C. jejuni
, one of the most important etiological agents of bacterial gas-
troenteritis worldwide, hosts two Hbs: a single domain globin named Cgb,
and a group III 2/2HbP named Ctb. Although both globins are up-regulated
