Biology Reference
In-Depth Information
(
Vuletich & Lecomte, 2006
). Group I and group II can be further separated
into two and four subgroups, respectively, whereas group III displays a
high level of overall sequence conservation. Despite the conserved small
size of 2/2Hbs, sequence identity among proteins from the different
groups is low (
20% overall identity), but may be higher than 80% within
a given group. Phylogenetic analyses further suggest an evolutionary
scenario where group II
HbO
gene is the ancestral gene, and group I and
group III genes are the result of duplication and transfer events
(
Vuletich & Lecomte, 2006
).
In some cases, 2/2Hbs frommore than one group can coexist in the same
organism, indicating a diversification of their functions. In general, members
of the 2/2Hb family are monomeric or dimeric proteins characterized by
medium to very high oxygen affinities, with cases of ligand-binding coop-
erativity (
Couture, Yeh, et al., 1999
). Some of the organisms hosting
2/2Hbs are aggressive pathogenic bacteria; others perform photosynthesis,
fix nitrogen, or may display distinctive metabolic capabilities. Although very
little is known about their role
in vivo
, possible functions of 2/2Hbs that are
consistent with the observed biophysical properties include long-term
ligand or substrate storage, nitric oxide (NO) detoxification, O
2
/NO
sensing, redox reactions, and O
2
delivery under hypoxic conditions
(
Nardini et al., 2007; Vuletich & Lecomte, 2006; Wittenberg et al., 2002
).
So far, a number of three-dimensional structures belonging to all three
groups have been solved by X-ray crystallography and NMR methods, thus
providing a clear picture of the structural features specific for each group: six
structures from group I 2/2HbNs (from
Chlamydomonas eugametos
(
Pesce
et al., 2000
),
Paramecium caudatum
(
Pesce et al., 2000
),
Mycobacterium tubercu-
losis
(
Milani et al., 2001
),
Synechocystis
sp. (
Synechocystis
6803:
Falzone, Vu,
Scott, & Lecomte, 2002; Hoy, Kundu, Trent, Ramaswamy, & Hargrove,
2004; Trent, Kundu, Hoy, & Hargrove, 2004
;
Synechococcus
7002:
Scott
et al., 2010
), and
Tetrahymena pyriformis
(
Igarashi, Kobayashi, & Matsuoka,
2011
)), five structures of group II 2/2HbOs (from
Mycobacterium tuberculosis
(
Milani et al., 2003
),
Bacillus subtilis
(
Giangiacomo, Ilari, Boffi, Morea, &
Chiancone, 2005
),
Thermobifida fusca
(
Bonamore et al., 2005
)
Geobacillus
stearothermophilus
(
Ilari et al., 2007
), and
Agrobacterium tumefaciens
(
Pesce
et al., 2011
)), and one structure from group III 2/2HbPs (from
Campylobacter jejuni
(
Nardini et al., 2006
)). Additionally, the NMR method
was applied to characterize the haem ligand binding site of 2/2HbP
from
Helicobacter hepaticus
in solution (
Nothnagel, Winer, Vuletich,
Pond, & Lecomte, 2011
).
