Biology Reference
In-Depth Information
O
2
-rich environment during lung infection (
Balasubramanian, Wiegeshaus,
Taylor, & Smith, 1994
). The remarkable ability of these Mycobacteria to
adapt their aerobic metabolism in response to the changing level of O
2
and environmental stresses depends on systems that can efficiently sense,
procure and conserve O
2
in the cell. For example, haemoglobin (Hb)-like
proteins are generally involved in binding and/or transporting O
2
for energy
generation and management of environmental stresses against reactive oxy-
gen and nitrogen species via the nitric oxide dioxygenation (NOD) reaction
(
Poole & Hughes, 2000; Weber & Vinogradov, 2001; Wittenberg &
Wittenberg, 1990
). Although Hbs in bacteria were discovered in 1986
(
Wakabayashi, Matsubara, &Webster, 1986
), no such O
2
-binding molecule
had been detected in any species of Mycobacteria before the unravelling of
the genome sequence of
M. tuberculosis
in 1998 (
Cole et al., 1998
)
which revealed two genomic loci,
glbN
and
glbO
, that could encode two
distinct Hb-like proteins, later termed truncated haemoglobins (trHbs)
(
Wittenberg, Bolognesi, Wittenberg, & Guertin, 2002
). More recently,
the presence of two-domain haemoglobins (flavoHbs) with novel structural
features has been identified in many Mycobacterial genomes (
Gupta et al.,
2012; Gupta, Pawaria, Lu, Yeh, &Dikshit, 2011
). The diversity in the num-
ber and types of Hb-encoding genes in the genomes of Mycobacteria sug-
gests that they have distinct cellular functions and reflect the difference in the
functions of these O
2
-binding proteins in individual Mycobacterial species.
This chapter deals with recent genomic, biochemical, structural and
functional information on Hb-like proteins and addresses the functional rel-
evance of Hbs in cellular metabolism, stress management and pathogenicity
of Mycobacteria.
2. CO-OCCURRENCE OF MULTIPLE Hbs
IN MYCOBACTERIA
While trHbs were first identified in cyanobacteria with the character-
isation of cyanoglobin in
Nostoc commune
(
Thorsteinsson et al., 1999
), the
distinctive features and structural characteristics of this class of globins were
only unravelled after the discovery of small Hbs in Mycobacteria (
Cole et al.,
1998; Couture et al., 1999
) and the determination of their three-
dimensional structures (
Milani, Pesce, Ouellet, Guertin, & Bolognesi,
2003; Milani et al., 2001
), along with other small Hbs from the ciliated pro-
tozoa
Paramecium caudatum
and the unicellular algae
Chlamydomonas
eugametos
(
Pesce et al., 2000
). Since then, a number of plants, unicellular
