Biology Reference
In-Depth Information
Figure 5.1 Structure-based sequence alignment of group I (trHbN) haemoglobins of
Mycobacteria.
The crystal structure of oxygenated Mtb trHbN was solved in 2001; it
contains a 2-over-2
a
-helical fold which retains the main protein regions
(B, E, G and H) for stabilisation of the trHb fold (
Milani et al., 2001
). As
in other trHbs, these exist as two anti-parallel helix pairs (B/E and G/H)
(
Pesce et al., 2000
). The protein is homodimeric and contains a short
a
-helix, termed the pre-A region, at the N-terminal end; deletion of the
A-helix is also seen in a number of other trHbs, along with the removal
of the traditional F-helix (
Pesce et al., 2000
). Features that contribute to
the stabilisation of the globin fold include the C-helix that supports
PheCD1, and the E-helix that supports LeuE7 and PheE14. The haem iron
coordinating residue, a proximal unstrained HisF8 (
Couture et al., 1999
), is
supported by the pre-F region, a 10-residue extension on the haem proximal
side that is followed by the F-helix (
Milani et al., 2001
); TyrB10 is involved
in ligand stability via hydrogen bonds (
Couture et al., 1999
). Analysis of
trHb amino acid sequences from
P. caudatum
and
C. eugametos
by
Pesce
et al. (2000)
showed that important structural features on the haem distal site
include TyrB10, GlnE7, PheE14 and the residue located at E11. The trHbs
from these species do not, however, feature the pre-A region, nor are they
homodimeric (
Milani et al., 2001
).
The X-ray structure provides clues as to how small gaseous ligands such
as NO and O
2
can gain access to the buried haem group. Initial work
showed a unique tunnel system with two main branches leading from the
haem group to distinct areas (one between the AB and GH hinge regions
and the other between the G and H helices) on the surface of the protein
(
Milani et al., 2001
). Inside the distal pocket, the two tunnels intersect at
the haem-bound O
2
molecule near the two important residues TyrB10
and GlnE11; it is this intersection that suggests the tunnels function as a
hydrophobic pathway through the protein for O
2
and maybe NO, and per-
haps for escape of nitrate. The long tunnel is 20
˚
long and has the biggest
