Biology Reference
In-Depth Information
Sequence alignments in Gram-negative and Gram-positive bacteria and
unicellular eukaryotes indicate that the FHbs family is a very homogeneous
group of proteins that share highly conserved active sites in both domains.
Amino acid residues building up the haem domain and the flavin-binding
region are widely conserved, and the architecture of FHb domains is closely
similar to those of globins and flavodoxin-reductase proteins. This finding sug-
gests that the haem domain displays globin-like functional properties and that
the flavin moiety acts as an electron-transfer module fromNADH to the haem.
However, sequence alignments on separate domains strongly diverge towards
the homologous proteins, suggesting that the rise of FHbs comes from fusion of
a protoglobin ancestor and a flavin-binding domain (
Bonamore & Boffi, 2008
).
Details of the structure, function and reaction mechanism of purified
native or recombinant FHbs from bacteria and yeast are available (
Lewis,
Corker, Gollan, & Poole, 2008
). FHb from
Escherichia coli
(Hmp) is the best
characterised member of the family. Hmp is distributed into both the cyto-
plasmic and periplasmic space, although the biochemically active protein is
exclusively found in the cytoplasmic fraction (
Vasudevan, Tang, Dixon, &
Poole, 1995
). It is subject to complex control (reviewed by
Spiro, 2007;
Poole, 2008
), being dramatically up-regulated in response to NO and
nitrosating agents (
Membrillo-Hern´ndez et al., 1999; Membrillo-
Hern
´
ndez, Coopamah, Channa, Hughes, & Poole, 1998; Poole et al.,
1996
). In particular, the
hmp
gene is predominantly regulated at the tran-
scriptional level by NO-sensitive transcription factors, especially NsrR
and Fnr (
Spiro, 2007
). Remarkably, the fine-tuning of Hmp synthesis
appears critical for
E. coli
survival. In fact, the constitutive expression of
Hmp in the absence of NO generates oxidative stress because of partial
O
2
oxidation by the haem to superoxide and peroxide anion; accumulation
of O
2
radicals has been stressed both in kinetic studies on the purified protein
(
Orii, Ioannidis, & Poole, 1992; Poole, Rogers, D'mello, Hughes, & Orii,
1997; Wu, Corker, Orii, & Poole, 2004
) and by detecting the
superoxide-generating activity of Hmp
in vivo
(
Membrillo-Hern´ndez,
Ioannidis, & Poole, 1996
). Similar results have also been obtained in
Salmonella enterica
where the constitutive expression of Hmp makes cells
hypersensitive to paraquat and H
2
O
2
(
Gilberthorpe, Lee, Stevanin,
Read, & Poole, 2007
), as well as to peroxynitrite (ONOO
)(
McLean,
Bowman, & Poole, 2010
).
FHbs display a pivotal role in NO detoxification (
Poole & Hughes,
2000
). NO, involved in many beneficial and/or dangerous physiological
and pathological processes, is a signalling and defence molecule of great
