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et al., 2000; Falzone et al., 2002; Hoy et al., 2004; Scott et al., 2002, 2010;
Trent et al., 2004; Vu, Nothnagel, Vuletich, Falzone, & Lecomte, 2004
).
The hexacoordinate
Synechocystis
sp. 2/2HbN shows also a unique haem-
protein covalent interaction between HisH16 and the 2-vinyl group of the
haem. This post-translational modification prevents haem loss and has the
potential to modulate the reactivity of the haem group (
Couture et al.,
2000; Falzone et al., 2002; Hoy et al., 2004; Scott et al., 2002, 2010
). In par-
ticular, for
Synechocystis
6803 2/2HbN, it has been shown that the post-
translational modification has little effect on the protein structure, perturbing
the backbone dynamics only modestly, and that the specificity and rate of the
cross-linking reaction depended critically on the nature of the sixth ligand to
the haem iron (
Nothnagel, Love, & Lecomte, 2009; Nothnagel,
Preimesberger, et al., 2011; Pond, Majumdar, & Lecomte, 2012
).
Although the endogenous haem hexacoordination is not a prominent
trend for 2/2Hbs, it has been also observed, under specific conditions, in
other 2/2Hbs. For instance, the
C. eugametos
2/2HbN may also display a
six-coordinate haem-Fe atom in its ferric state, while the ferrous derivative
displays a five-coordinate high spin haem-Fe atom at neutral pH, and a six-
coordinate low spin species at alkaline pH where the sixth ligand to the
haem-Fe atom is held to be either TyrB10 or to be stabilized by this residue
(
Couture, Das, et al., 1999; Couture & Guertin, 1996
). Haem hexa-
coordination has also been observed in the ferrous derivative of group II
M. leprae
2/2HbO at neutral pH (
Visca, Fabozzi, Petrucca, et al., 2002
).
In group II 2/2HbO, specific residue substitutions characterize the distal
site environment relative to group I 2/2HbN. In this group, TyrB10 is
strictly conserved, and so is TrpG8. The five available structures
(
Bonamore et al., 2005; Giangiacomo et al., 2005; Ilari et al., 2007;
Milani et al., 2003; Pesce et al., 2011
) show these residues to be located
at the haem distal site, but not necessarily involved in direct ligand binding.
Several group II 2/2HbOs display a polar residue, His or Tyr, at the topo-
logical position CD1, which in globins was thought to harbour a strictly
conserved Phe (
Kapp et al., 1995
) whose role was to fasten the haem in
its binding site (
Ptitsyn & Ting, 1999
).
In
M. tuberculosis
2/2HbO, TyrCD1 is the residue responsible for hydro-
gen bonding to the diatomic ligand (
Milani et al., 2003
). Further ligand-
stabilizing interactions are provided by TrpG8, whose indole NE1 atom
is hydrogen bonded to the haem-bound ligand and to TyrCD1 OH
(
Boechi et al., 2008; Guallar, Lu, Borrelli, Egawa, & Yeh, 2009; Milani
