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In-Depth Information
Frey & Kallio, 2003, Frey, Shepherd, Jokipii-Lukkari, Haggman, & Kallio,
2011; Vinogradov et al., 2005, 2013
).
4.3. Truncated haemoglobins
Members of the T family are found in eubacteria, cyanobacteria, protozoa
and plants but not in animals (
Milani et al., 2005; Wittenberg, Bolognesi,
Wittenberg, & Guertin, 2002
). The N-termini of these globins are
20-40-residue shorter; these globins display the 2/2
a
-helical sandwich fold
(composed of helices B, E, G and H), which has been recognised as a subset
of the classical 3/3
a
-helical sandwich (
Milani et al., 2005; Wittenberg et al.,
2002
). The 2/2
a
-helical sandwich fold results in four
a
-helices (B/E and
G/H) arranged in antiparallel pairs connected by an extended polypeptide
loop that replaces the
a
-helix F (
Fig. 8.2
).
It is noteworthy that the haem-proximal helix F is replaced by a poly-
peptide segment (
Milani et al., 2001; Pesce et al., 2000
). The residues com-
prising the F loop affect the orientation of the proximal HisF8 modulating
the O
2
-binding properties (
Milani et al., 2005; Pathania, Navani, Gardner,
Gardner, & Dikshit, 2002
). Helix E is very close to the haem distal site;
therefore, residues at positions B10, CD1, E7, E11, E15 and/or G8
(
Table 8.2
) modulate ligand binding (
Milani et al., 2005
).
On the basis of phylogenetic analysis, the T family can be further divided
into three distinct sub-families: TrHbI (or N), TrHbII (or O) and TrHbIII
(or P); specific structural features that depend on residues of the distal haem
pocket distinguish each group (
Table 8.2
). In group I, the hydrogen bond
network stabilising the haem-bound ligand involves the B10, E7 and E11
residues. Strongly conserved Tyr B10 plays a key role in ligand stabilisation
through OH pointing directly to the heam-bound ligand. Normally, com-
plete stabilisation by the H-bond network is provided by Glu located at E7
or E11, or at both positions. In group II, TrpG8 is fully conserved, contrib-
uting to ligand stabilisation by the H bond linking the indole nitrogen atom
and the ligand.
Further, Tyr at CD1 in some TrHbIIs drastically modifies the interaction
network (
Pesce et al., 2000
). In group III, for example,
C. jejuni
TrHbIII
(Ctb), the hydrogen bond network stabilising the haem-bound ligand
involves TyrB10 and TrpG8 (
Nardini et al., 2006
). Interestingly, the affinity
of cyanide for the ferrous derivative of
C. jejuni
Ctb is higher than that
reported for any known (in)vertebrate ferrous globin and is reminiscent
