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Figure 4.6 The structure of Campylobacter-truncated globin, Ctb. (A) The figure depicts
the 2-over-2 a-helical fold of Ctb ( Nardini et al., 2006 ) (PDB id¼2IG3). (B) The haem-
binding cleft of Ctb, highlighting the hydrogen-bonding network in the distal pocket.
The E7His residue was crystallised in 'closed' and 'open' conformations.
C-terminal to the E-helix, which were considered to be essential for the
2-over-2-fold. However, these were absent in Ctb, further distinguishing
Ctb from other classes of truncated globin.
Figure 4.6 B depicts the distal- and proximal-binding pockets of Ctb with
a cyanide ion bound in the active site. Whereas class I and class II-truncated
globins have been shown to possess cavity/tunnel systems for ligand migra-
tion to/from the distal pocket, these features were absent in the Ctb struc-
ture. However, the crystal structure revealed the E7His residue in two
alternative conformations, 'open' and 'closed' ( Fig. 4.6 B), which implicated
E7His in a potential gating mechanism for ligand entry/exit, as is the case for
Mb ( Nardini et al., 2006 ). The distal pocket also contains conserved
B10Tyr, G8Trp and E15Trp residues, all participating in a hydrogen-
bonding network with an active site water molecule ( Fig. 4.6 B). The prox-
imal pocket of Ctb is reminiscent of that of Mb ( Fig. 4.3 B). As for Mb, the
F8His residue that provides axial coordination to the haem iron does not
participate in hydrogen bonding to other amino acid side chains, although
like Mb does hydrogen bond to the main chain carbonyl of an adjacent res-
idue (Pro68 for Ctb). The F8His is shielded from the solvent by the F7Lys
residue, which is electrostatically coupled to the haem propionates, whereas
class I- and class II-truncated globins have solvent-accessible F8His residues.
Whereas the proximal histidyl coordination of the Ctb haem resembles
that of Mb ( Fig. 4.3 B), the polar distal pocket of Ctb resembles that of CcP
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