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by the transcription factor NssR (which regulates expression of a small regulon
that includes
cgb
and
ctb
) in response to nitrosative stress (
Elvers et al., 2005
),
only Cgb has been proposed to protect the bacterium against nitrosative
stress, likely
via
a NO dioxygenase reaction during the initial stages, followed
by a denitrosylase mechanism upon prolonged exposure to NO (
Elvers, Wu,
Gilberthorpe, Poole, & Park, 2004; Shepherd, Bernhardt, & Poole, 2011
).
The role of
C. jejuni
2/2HbP is unclear: it is not directly involved in NO
detoxification and it displays an extremely high O
2
affinity, making it unlikely
to be anO
2
carrier. Based on the polarity of the haem distal cavity, reminiscent
of that found in cytochrome
c
peroxidase,
C. jejuni
2/2HbP has been proposed
to be involved in (pseudo)enzymatic O
2
chemistry (
Lu et al., 2007; Nardini
et al., 2006; Wainwright, Elvers, Park, & Poole, 2005
). Recently, attempts
to define a function for
C. jejuni
2/2HbP have been pursued by examining
the effects of a
ctb
mutation on the NO transcriptome and
cgb
gene expression
during normoxia and hypoxia. Based on these data, it was proposed that, by
binding NO or O
2
,
C. jejuni
2/2HbP dampens the response to NO under
hypoxic conditions and limits
cgb
expression, perhaps because Cgb function
(i.e. NO detoxification)
requires O
2
-dependent
chemistry
(
Smith,
Shepherd, Monk, Green, & Poole, 2011
).
It is worth noticing also that
C. jejuni
2/2HbP displays the highest affinity
as well as the fastest combination and the slowest dissociation rates for
cyanide binding within the Hb superfamily. Thus, it was suggested that this
2/2HbP may act as a cyanide scavenger facilitating
C. jejuni
survival (
Bolli
et al., 2008
).
6. CONCLUSIONS
2/2Hbs populate a branch of the Hb superfamily tree, which has been
discovered and enriched with data starting in the early 1990s. Our current
knowledge on these 'minimal' globins is rather extensive as far as their pri-
mary structures are concerned (more than 1000 gene sequences have been
identified to date;
Vinogradov et al., 2013
). Such information allowed to
distinguish groups I, II, and III 2/2Hbs and provided insight into their dis-
tribution through the evolutionary
phyla
. Indeed, 2/2Hbs have been mostly
found among protozoa and bacteria, although their presence in plants and in
some lower eukaryotes has also been reported. Despite the relatively con-
tained size of 2/2Hbs in all three groups, several crystal structures have
shown that careful editing of the 'classical' globin (i.e. Mb) 3-on-3
a
-helical
fold, and the introduction of Gly-based motifs (in groups I and II), provides
