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to the haem, as suggested by the structural features already discussed. This
oxy-ferrous trHbO then reacts with the displaced NO to give the above-
mentioned ferric peroxynitrite-bound intermediate, before the ferric species
alone is formed (
Ascenzi, Bolognesi, & Visca, 2007
). The
k
off
for NO was
found to be 1.3
10
4
s
1
at pH 7, and this is thought to be the rate-limiting
step in the proposed reaction mechanism (
Ascenzi, Bolognesi, et al., 2007
).
3.3. Type III trHbs: trHbP
So far, no member of the group III trHb (trHbP) from Mycobacteria has
been isolated and characterised. The occurrence of trHbP is limited to only
a few species of Mycobacteria where it is present along with its other molec-
ular relatives, trHbN and trHbO, and may therefore have distinct cellular
function(s). Inspection of the available Mycobacterial genome data indicated
the presence of trHbP in 8 out of 22 Mycobacterial species (
Table 5.1
). The
identity of trHbP in Mycobacteria was checked on the basis of their homol-
ogy with other type III trHbs (
Fig. 5.3
), in particular, the presence of specific
structural features, such as the absence of the Gly-Gly motif present in group
I and group II trHbs, conservation of residues at key structural positions
(B10, E7, F8, etc.) and similarities in the residues building the proximal
and distal haem pockets. Sequence alignment of Mycobacterial trHbP from
various Mycobacterial species is presented in
Fig. 5.3
. At present, only the
X-ray structure of trHbP of
C. jejuni
has been unravelled (
Nardini et al.,
2006
). Contrary to what has been observed in group I and II trHbs, the func-
tional residues of classical globins, for example, PheCD1 and E7His, are
invariant among this group and no distinct protein matrix tunnels are
formed. It has been proposed that E7His might operate as a gating residue
for ligand entry to the haem distal site, very similar to classical Hbs. All these
functional residues are conserved in Mycobacterial trHbP. In the absence of
any information on the biochemical and structural properties of Mycobac-
terial group III trHbs, the specific
in vivo
function of trHbP in the cellular
metabolism of Mycobacteria remains an open issue.
3.4. Genetic regulation of the trHbs
TrHbs were first identified using a bioinformatics approach, where they
were found in many genomes of the Mycobacteria (
Wittenberg et al.,
2002
). The gene encoding for trHbN is designated
glbN
(Rv1542c) and
for trHbO,
glbO
(Rv2470). Bioinformatic probing of the regions up- and
down-stream of both
glbN
and
glbO
genes did not provide any clues towards
