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et al., 2002, 2010; Trent et al., 2004; Vu, Vuletich, Kuriakose, Falzone, &
Lecomte, 2004; Vuletich, Falzone, & Lecomte, 2006
). The function of
Syn-
ecocistis
2/2HbNs and their relationship to the metabolism of dioxygen,
nitric oxide, or various reactive nitrogen and oxygen species are still largely
unknown, although comparison to similar 2/2 haemoglobins suggests that
reversible dioxygen binding is not its main activity. Recently,
in vitro
and
in vivo
experiments on cyanobacterium
Synechococcus
sp. strain PCC 7002
showed that its transcription profiles indicate that the protein is not strongly
regulated under any of a large number of growth conditions and that the
gene is probably constitutively expressed. High levels of nitrate, used as
the sole source of nitrogen, and exposure to nitric oxide were tolerated
better by the wild-type strain than by a 2/2HbN null mutant, whereas
overproduction of protein in the null mutant background restored the
wild-type growth. The cellular contents of reactive oxygen/nitrogen species
were elevated in the null mutant under all conditions and were highest under
NO challenge or in the presence of high nitrate concentrations. A
peroxidase assay showed that purified 2/2HbN does not possess significant
hydrogen peroxidase activity. Taken together, all these evidences suggested
for 2/2HbN from cyanobacterium
Synechococcus
sp. strain PCC 7002 a
protection role from reactive nitrogen species which cells could encounter
naturally during growth on nitrate or under denitrifying conditions (
Scott
et al., 2010
).
The physiological role of
M. tuberculosis
2/2HbO has been primarily
related to O
2
metabolism. 2/2HbO was hypothesized to be endowed with
O
2
uptake or delivery properties during mycobacterial hypoxia and latency
(
Liu, He, & Chang, 2004; Pathania, Navani, Gardner, et al., 2002
). This
hypothesis is in apparent contrast with the low O
2
association and dissoci-
ation rates reported for 2/2HbO (
Ouellet et al., 2003
), and with its consti-
tutive expression under aerobic conditions during the whole growth cycle of
M. bovis
(
Mukai, Savard, Ouellet, Guertin, & Yeh, 2002; Pathania, Navani,
Rajamohan, et al., 2002
). 2/2HbO (II)-O
2
could still be able to sustain bac-
terial aerobic respiration by scavenging NO or other reactive species that
would block the respiratory chain. In this context, the high stability of
2/2HbO(II)-O
2
would secure the reaction with NO even at very low
O
2
tensions, as those that may exist in infected or necrotic tissue (
Fabozzi
et al., 2006
). Interestingly,
M. leprae
2/2HbO has been proposed to be
involved in both H
2
O
2
and NO scavenging, protecting from nitrosative
and oxidative stress, and sustaining mycobacterial respiration (
Ascenzi, De
Marinis, Coletta, & Visca, 2008
). Under anaerobic and highly oxidative
