Biology Reference
In-Depth Information
that these Hbs may be playing diverse functions in the cellular metabolism of
Mycobacteria. TrHbs and flavoHbs from some of the severe human pathogens such
as Mycobacterium tuberculosis and Mycobacterium leprae have been studied recently
and their roles in effective detoxification of reactive nitrogen and oxygen species, elec-
tron cycling, modulation of redox state of the cell and facilitation of aerobic respiration
have been proposed. This multiplicity in the function of Hbs may aid these pathogens to
cope with various environmental stresses and survive during their intracellular regime.
This chapter provides recent updates on genomic, structural and functional aspects of
Mycobacterial Hbs to address their role in Mycobacteria.
ABBREVIATIONS
flavoHb flavohaemoglobin
Mb
Mycobacterium bovis
Ms
Mycobacterium smegmatis
Mtb
Mycobacterium tuberculosis
MtbFHb
Mycobacterium tuberculosis
flavohaemoglobin
NO nitric oxide
NOD nitric oxide dioxygenation
trHb truncated haemoglobin
1. INTRODUCTION: DISCOVERY OF HAEMOGLOBINS
IN MYCOBACTERIA
The genus
Mycobacteria
consists of a group of acid-fast, Gram-positive
bacteria, some of which are severe human pathogens, while others are either
opportunistic pathogens or environmental saprophytes (
Grange, 1996
).
Members of this group have been categorised into three distinct classes,
on the basis of their growth characteristics: the fast growers, the slow growers
and those strains or species that have never been cultured in
in vitro
condi-
tions. The O
2
availability in the natural niche of these Mycobacteria ranges
from O
2
-rich to near anaerobic, and although all species are considered to
have an obligate requirement of O
2
for their life cycle (
Berney & Cook,
2010; Park, Myers, & Marzella, 1992
), paradoxically, the majority of them
have the ability to grow and metabolise under hypoxia (
Voskuil, Visconti, &
Schoolnik, 2004
). Some species, such as
Mycobacterium tuberculosis
and
Myco-
bacterium leprae
, are human pathogens that encounter severe hypoxia and
environmental stresses within macrophages and avascular calcified granu-
loma where these bacilli primarily reside (
Ehrt & Schnappinger, 2009;
Rustad, Sherrid, Minch, & Sherman, 2009
); they also get exposed to an
