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to adopt multiple orientations. In addition, differences unrelated to the PTM
are observable in the A helix and the EF loop, which sample a large confor-
mational space in solution and are restrained in the solid state.
What happens to the bis-histidine structure once an exogenous ligand
binds?Hargrove and colleagues pursued theX-ray structures of the cyanomet
and azidomet complexes ( Trent et al., 2004 ). The result shows clearly the
resemblance of the bound state to the cyanomet CtrHb structure. The con-
formational change ( Fig. 6.12 ) is of unusual amplitude for a globin as the ter-
tiary structure of these proteins is typically not affected to an appreciable
extent by ligand binding. This raises the possibility of a signalling function
coupled to the presence of exogenous ligand. But it also suggests that rotation
of the E helix and B helix can allow for coordination of the residue at position
E10, as surmised for the Tyr B10 Lys variant of CtrHb.Once again, the capac-
ity of the haem pocket for deformation in TrHbs is confirmed.
X-ray structures were obtained for the H117A variant in the reduced
and oxidized states ( Hoy, Smagghe, Halder, & Hargrove, 2007 ). The
replacement or change in redox state does not appear to have an effect
on the conformation of the protein. Removal of the bulky histidine group,
however, opens a cavity that is filled by water molecules. Overall,
the TrHb1 from the cyanobacterium resembles closely that from the
green alga, despite the low level of sequence identity ( Table 6.3 ). Also of
note is the opening of tunnels and cavities in Synechocystis 6803 GlbN only
upon exogenous ligand binding.
Figure 6.12 Ribbon structure of Synechocystis 6803 GlbN in the bis-histidine state (PDB
ID 1RTX, Hoy, Kundu, Trent, Ramaswamy, & Hargrove, 2004 ) and the cyanomet state
(PDB ID 1S69, Trent, Kundu, Hoy, & Hargrove, 2004 ). The conformational change upon
exogenous ligand binding affects the B and the E helix. His E10 does not interact with
the bound ligand. Note the resemblance of the cyanomet structures of CtrHb ( Fig. 6.2 )
and Synechocystis 6803 GlbN.
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