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et al., 2005
). The fact that globins evolved long before the advent of aerobic
respiration—before the oxygenation of our atmosphere—indicates far more
diverse roles for these proteins than the oxygen transport or storage that is
typically associated with myoglobin and haemoglobin (
Vinogradov &
Moens, 2008
). Although stable and reversible oxygen binding is likely to
be at the root of many of these functions (
Shikama & Matsuoka, 2004
), it
is also possible that activities unrelated to dioxygen stabilization were, or still
are, represented in the superfamily.
The currently available genomic data for cyanobacteria and algae have
identified globins that show strong homology to both the M family with
its canonical 3/3 fold and the T family identified by its unique 2/2 topology
(
Vinogradov, Fernandez, et al., 2011
). Until recently, cyanobacteria and
algae were not known to contain genes for globins from the S family. This
has now changed with the discovery of SSDgb in three separate cyano-
bacteria (
Vinogradov et al., 2013
). However, not all cyanobacteria have glo-
bins; in fact only about half of the cyanobacterial genomes that have been
sequenced to this date contain one or more globin genes (
Vinogradov
et al., 2013
). In this section, we will discuss the common phylogeny of pro-
teins predicted to be globins or to contain globin domains.
A succinct list of globins from cyanobacteria and algae compiled from the
UniProt Knowledgebase suffices to assess the state of the field. In cyano-
bacteria, 52 species with 75 predicted proteins possessing either canonical
or 2/2 predicted structures are listed. The green algae of Chlorophyta are
represented by eight species within the databases accounting for a total of
33 known or predicted globins. We have no doubt that by the time of pub-
lication of this chapter these numbers will be outdated owing to the ever-
increasing volume of genomic databases. With this reservation,
Fig. 6.3
shows a simple phylogenetic tree constructed from the simultaneous align-
ments of these
100 proteins.
3.2.1 The M lineage
Within land plants, the canonical Hbs are often associated with nitrogen fix-
ing metabolism (
Vazquez-Limon et al., 2012
). Nitrogen fixation in land
plants takes place through a symbiotic relationship between the plants and
certain bacteria. Because legumes were among the first plants recognized
to have this symbiotic relationship, these canonical haemoglobins are
referred to as leghaemoglobin (
Virtanen & Laine, 1946
)orassymbiotic
canonical haemoglobins (sHb;
Vinogradov,Fernandez,etal.,2011
). They
are classified as oxygen transport proteins. Land plants also harbour nsHbs
