Biology Reference
In-Depth Information
Ph
-2/2HbO shows structural differences with respect to other TrHbsII. In
particular, the insertion of three residues in the CD loop (
Howes et al., 2011
;
Fig. 8.4
) confers higher flexibility that may facilitate its action at low temper-
ature, providing greater freedom for the correct positioning of ligand(s)
(
Feller & Gerday, 2003; Siddiqui &Cavicchioli, 2006
). In contrast to TrHbsI,
the E7 and E11 positions are occupied by non-polar residues, Ile and Phe,
respectively, precluding haem-bound ligand stabilisation. On the proximal
side, HisF8, conserved in all members of the globin superfamily (
Howes
et al., 2011
), is coordinated to the haem-Fe atom (
Table 8.2
and
Fig. 8.4
).
Ph
-2/2HbO shows an unusual extension of 15 residues at the
N-terminus (pre-helix A), similar to
M. tuberculosis
HbN and to many
slow-growing species of
Mycobacterium
, such as
M. bovis
,
M. avium
,
M. microti
,
M. marinum
(
Lama et al., 2009
) and
Shewanella oneidensis
(
Vuletich & Lecomte, 2006
). The pre-A motif of
M. tuberculosis
HbN does
not significantly contribute to the structural integrity of the protein, pro-
truding out of the compact globin fold (
Milani et al., 2001
). However,
the deletion of this motif reduces the ability of
M. tuberculosis
to scavenge
NO (
Lama et al., 2009
). Unlike in
M. tuberculosis
HbN, the deletion of
the N-terminal extension of
Ph
-2/2HbO does not seem to reduce the
NO scavenging activity (
Coppola et al., 2013
) (see
Section 6.2.4
).
6.2.2 Hexacoordination
Ph
-2/2HbOdisplays hexacoordination of the ferric and ferrous haem-Fe atom
(
Giordano et al., 2011; Howes et al., 2011
). Hydrostatic pressure enhances
hexacoordination in both oxidation states of the haem-Fe atom, as previously
shown in other haemproteins (
Hamdane et al., 2005
), indicating that a flexible
protein allows structural changes (
Russo et al., 2013
).
Binding of O
2
to Mb and Hb occurs on the distal side of the
pentacoordinated haem-Fe atom, where O
2
establishes a sixth coordination
bond to the Fe atom, whereas the fifth coordination position is occupied by
invariant HisF8 (
Fig. 8.5
). The haem-Fe-bound O
2
is generally stabilised by
interaction(s) with distal residues. The main O
2
stabilising interaction is
usually provided by an H bond donated by HisE7 (
Fig. 8.5
).
In hexacoordinated globins, where, in the absence of external ligands,
the sixth position is taken by an internal residue, exogenous ligands (e.g.
O
2
, CO and NO) compete with the internal ligand to bind Fe, this behav-
iour being the basis of the control of Fe reactivity (
Smagghe, Trent, &
Hargrove, 2008; Trent, Hvitved, & Hargrove, 2001
).
Haem-Fe hexacoordination is widespread in globins, having been found
in unicellular eukaryotes (
Wittenberg et al., 2002
), plants (
Watts et al.,
