Biology Reference
In-Depth Information
Figure 8.5 Schematical representation of the iron coordination in hexacoordinated,
pentacoordinated and oxygenated forms. The protein is in hexacoordinated conforma-
tion when, in the absence of external ligands, there is an amino acid residue as internal
ligand. The sixth ligand is usually provided by His. Upon addition of gaseous ligands, for
example, O
2
, there is competition between the external ligand and the sixth ligand, with
replacement of the internal ligand with O
2
.
2001
), invertebrates (
Dewilde et al., 2006
), but only a few cases have been
reported in bacterial TrHbs (
Falzone, Christie Vu, Scott, & Lecomte, 2002;
Razzera et al., 2008; Scott et al., 2002; Vinogradov & Moens, 2008; Visca
et al., 2002
). Hexacoordination has also been found in higher vertebrates, for
example, in ferric
b
-chains of tetrameric Antarctic fish Hbs (
Riccio,
Vitagliano, di Prisco, Zagari, & Mazzarella, 2002; Vergara et al., 2007;
Vergara, Vitagliano, Verde, di Prisco, & Mazzarella, 2008; Vitagliano
et al., 2004, 2008
) and in the ferric and ferrous states of mammalian
(
Pesce et al., 2003; Vallone, Nienhaus, Brunori, & Nienhaus, 2004
) and
Antarctic fish (
Giordano et al., 2012
) Ngbs and Cygbs (
de Sanctis et al.,
2004
; Alessia Riccio et al., unpublished results). The occurrence of ferrous
(haemochrome) and ferric (haemichrome) oxidation states in members of
the Hb superfamily is not uniform, suggesting that the functional roles of
these states are multiple, possibly being a tool for modulating ligand-binding
or redox properties (
Vergara et al., 2008; Vitagliano et al., 2008
). Exchange
between haemichrome and pentacoordinated forms may play a physiolog-
ical role in Antarctic fish due to higher peroxidase activity (
Vergara et al.,
2008; Vitagliano et al., 2008
).
Over the years, haemichromes in tetramers have been considered
precursors of Hb denaturation (
Rifkind, Abugo, Levy, & Heim, 1994
);
however, haemichromes can be obtained under non-denaturing as well as
physiological conditions (
Vergara et al., 2008
). It has also been suggested that
haemichromes can be involved in Hb protection from peroxide attack (
Feng
et al., 2005
), given that the haemichrome species of human
a
-subunits com-
plexed with the
a
-helix-stabilising protein do not exhibit peroxidase activity
(
Feng et al., 2005
).
