Biology Reference
In-Depth Information
proteins for single-domain globins has been speculated upon for many years.
The use of electrons from the respiratory chain for the reduction of Cgb has
arisen as a possibility. Indeed, the ability of Vgb and other bacterial
haemoglobins to associate with the cytoplasmic membrane (
Park, Kim,
Howard, Stark, &Webster, 2002
) and their involvement in oxygen transfer
(
Dikshit et al., 1992
) support this idea.
FHbs circumvent the need for a separate redox partner by encoding a
reductase domain. The reduction of the ferric haem (Fe(III)) takes place via
electron transfer from the reductase domain (or FNR, ferredoxin-NADP
reductase-like domain) to the N-terminal haem domain in an NAD(P)H-
dependent reaction via a non-covalently bound FAD (
Gardner, Costantino,
et al., 1998; Hausladen et al., 1998; Hernandez-Urzua et al., 2003
). Since there
are important differences between Cgb and FHbs, it has been suggested that
Cgb may not interact with a protein homologous to the reductase domain
of FHbs. Indeed, the residue Lys-84 conserved in the globin domain of FHbs
is responsible for the formation of a salt bridge between the domains (
Ermler,
Siddiqui, Cramm, & Friedrich, 1995
), yet is absent in Cgb.
A lactate dehydrogenase enzyme, encoded by a gene (
cj1585c
) adjacent
to
cgb
, has been suggested as a candidate for a redox partner for Cgb (
Thomas
et al., 2010
). The spectroscopic characteristics of this protein make it a good
candidate for the Cgb electron donor, and
cj1585c
is also upregulated by NO
(
Avila-Ramirez et al., 2013
). However, the upregulation of
cj1585c
in
response to NO occurs only in oxygen-limited conditions, where
cgb
induc-
tion is diminished (
Avila-Ramirez et al., 2013
) (see
Section 7
). Hence, the
involvement of Cj1585c in an oxygen-dependent detoxification mechanism
seems rather unlikely.
5.3. Structural characterisation
At least three classes of bacterial globin are recognised, namely, the FHbs, the
single-domain 'myoglobin-like' globins and the truncated globins: for a
recent review, see
Poole and Shepherd (2010)
. FHbs are distinguished by
the presence of an N-terminal globin domain (a 3-over-3
-helical fold sim-
ilar toMb) with an additional C-terminal domain with binding sites for FAD
and NAD(P)H. Single-domain globins also exhibit a 3-over-3
a
-helical fold
similar to Mb but have no separate C-terminal domain. All globin subunits
consist of 6-8
a
-helical segments that fold around a haem group, which is
coordinated to a histidine residue via a central iron atom. Indeed, the struc-
ture of Cgb conforms to this general globin fold (
Fig. 4.1
).
a
