Biology Reference
In-Depth Information
the entire A-helix and CD-D region and a shortening of the F-helix. In
addition, there are only a small number of conserved residues as shown
by the alignments in
Fig. 5.1
; these include TyrB10 and PheE15 and will
be discussed later.
TrHbs from the three groups have varying functions not restricted to O
2
carriage, and as we will see in this section, all three are represented in the
Mycobacteria (
Ascenzi, Bolognesi, Milani, Guertin, & Visca, 2007
).
Table 5.1
shows the Mycobacterial species which contain either one or
more trHbs. While a few species, such as
Mycobacterium avium,
have genes
encoding for all three trHbs others, such as
M. leprae
, have only one
(
Fabozzi, Ascenzi, Renzi, & Visca, 2006
). The authors suggest that
M. leprae
has lost trHbN and can survive with only trHbO, which has con-
currently acquired the ability to detoxify NO (
Fabozzi et al., 2006
), and that
this probably reflects the reductive evolution of its genome (
Cole et al.,
2001
). Some of the truncated globins are similar to each other, such as
trHbN from
M. tuberculosis
,
M. avium
and
Mycobacterium smegmatis
, where
high-sequence identity points towards similar functions in the cell
(
Pathania, Navani, Gardner, Gardner, & Dikshit, 2002
). Interestingly, in
the Mycobacteria, sequence identity between different trHbN proteins is
less—between 21% and 32% (
Couture et al., 1999
)—than that between
trHbO proteins, and most trHbs from other bacterial species share similarity
with trHbO rather than trHbN, which is more related to globins from algae,
protozoa and cyanobacteria (
Pathania, Navani, Rajamohan, & Dikshit,
2002
). Currently, there is no published data on the function of trHbP,
and its existence has only been garnered from genome sequence data. There-
fore, this section of the review will concentrate on the trHbs, N and O.
3.1. Group I trHbs: trHbN
3.1.1 Structural features and dynamics
TrHbs in the Mycobacteria have conserved structural features that maintain
the globin fold. The first paper regarding
M. tuberculosis
trHbN (Mtb trHbN)
was published in 1999. Alignment with other Hbs showed the conservation
of the proximal HisF8 and the distal PheCD1 (
Couture et al., 1999
). Clues as
to the function of important residues can be found from these alignments.
For example, in vertebrate Hbs, the E7 position distal to the haem is always
occupied by His or Gln, residues which contribute to the stabilisation of
bound O
2
via hydrogen bonds; in Mtb trHbN, the residue at E7 is Leu,
not capable of hydrogen bonds; therefore this position inMtb trHbN cannot
be involved in stabilising O
2
to the haem.
