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indicates that more than one flavoHb-encoding gene may be present in
many Mycobacterial species (
Table 5.1
). An interesting pattern in the occur-
rence of flavoHbs can be observed in slow- and fast-growing Mycobacteria.
Opportunistic pathogens and fast-growing Mycobacteria carry two
flavoHb-encoding genes, one encoding flavoHb similar to conventional
flavoHbs present in other microbes (Type I), and the second one encoding
an unusual flavoHb, displaying unconventional globin and reductase
domains (Type II). The occurrence of two distinct classes of flavoHbs in
Mycobacteria suggests their distinct cellular functions. Notably, the new
class of Type II flavoHb is present in the majority of Mycobacterial species
and also co-exists with a conventional flavoHb (Type I) in some of the
Mycobacteria. Unlike conventional and Type I flavoHbs, Type II flavoHbs
are not widely distributed among microbial systems and are restricted to
Mycobacterial and certain species of actinomycetes (
Gupta et al., 2011
).
4.1. Structural features
Sequence alignment of Type I and Type II flavoHbs of various Mycobac-
terial species are presented in
Fig. 5.5
, which indicates that structural features
required for adopting a classical 3-over-3 globin fold and the signature
sequences of typical microbial globin fold (e.g. TyrB10, PheCD1, GlnE7
and HisF8) are well preserved in both classes of Mycobacterial flavoHbs.
Type I flavoHbs of Mycobacteria appear very similar to conventional
flavoHbs present in other microbes, and display conservation of sequences
including the hydrogen bonding network between HisF8, TyrG5 and
GluH23 that is required for the architecture of the haem pocket in the prox-
imal region (
Bonamore & Boffi, 2008; Frey & Kallio, 2003; Mukai, Mills,
Poole, & Yeh, 2001
). Additionally, the FAD- and NADH-binding sites and
critical residues involved in electron transfer (YS pair in the FAD-binding
domain) are present at the topological positions required for the electron
transfer from the FAD to the haem domain. Thus, Type I flavoHbs of
Mycobacteria are structurally and functionally similar to conventional
flavoHbs.
Type II flavoHbs of Mycobacteria display several differences from con-
ventional flavoHbs in the functionally conserved region of their haem and
FAD-binding sites (
Fig. 5.5
). The most prominent change is the lack of
hydrogen bonding interactions between HisF8, TyrG5 and GluH23 within
the proximal site of its globin domain that may change the redox properties
of
the haem. This indicates that Type II flavoHbs do not have the
