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and eukaryotic microorganisms, including bacteria, yeasts, algae, protozoa
and fungi, suggests that the globin superfamily is exceptionally flexible in
terms of biological roles and possible functions. The number of “globin-
like” proteins is currently increasing as different genomes from microorgan-
isms are sequenced and annotated ( Vinogradov et al., 2005; Vinogradov
& Moens, 2008; Vinogradov, Tinajero-Trejo, Poole, & Hoogewijs, 2013 ).
Non-vertebrate globins display high variability in primary and tertiary
structures, which probably indicates their adaptations to additional functions
with respect to their vertebrate homologues ( Vinogradov et al., 2005;
Vinogradov & Moens, 2008; Vinogradov et al., 2013 ).
Themost recent bioinformatic survey of globin-like sequences in prokary-
otic genomes revealed that over half of the more than 2200 bacterial genomes
sequenced so far contain putative globins ( Vinogradov et al., 2013 ). A new
global nomenclature including prokaryotic and eukaryotic globins has been
proposed, and globins have been classified within three families:
(i) myoglobin (Mb)-like family (M) (displaying the classical three-on-three
(3/3) a -helical sandwich motif ) containing flavohaemoglobins (FHbs) and
single-domain globins (SDgb); (ii) sensor globins family (S); and
(iii) truncated haemoglobins family (T), showing the two-on-two (2/2)
a -helical sandwich motif ( Vinogradov et al., 2013 ).
Although there are still some uncertainties about the evolutionary
relationship between these three classes of microbial globins, it has been
proposed that prokaryotic and eukaryotic globins, including vertebrate a /
b globins, Mb, neuroglobin (Ngb), cytoglobin (Cygb), and invertebrate
and plant Hbs, emerged from a common ancestor ( Vinogradov et al., 2005 ).
4.1. Flavohaemoglobins
Chimeric globins seem to have kept their original enzymatic functions in
prokaryotes, plants and some unicellular eukaryotes. Therefore, the FHb
sub-family has been the only one able to adapt to different functions more
extensively than the other two globin families. Moreover, the presence of
Hbs in unicellular organisms suggests that O 2 transport in metazoans is a rel-
atively recent evolutionary acquisition and that the early Hb functions have
been enzymatic and O 2 sensing ( Vinogradov & Moens, 2008 ).
FHbs are widely present in bacteria, yeasts and fungi and belong to the
ferredoxin reductase-like protein family. They consist of an N-terminal
haem-globin domain fused with C-terminal reductase domain binding
NAD(P)H and FAD ( Bolognesi, Bordo, Rizzi, Tarricone, & Ascenzi,
1997; Bonamore & Boffi, 2008 ).
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