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M family proteins and serve to demonstrate the distinct phylogenetic divi-
sion seen within individual globin families.
3.2.2 The T lineage
The class of globin with the 2/2 structural fold is referred to as the TrHbs as
they are on average 20-40 amino acids shorter than the myoglobin-like
haemoglobins (
Wittenberg et al., 2002; Wu, Wainwright, & Poole,
2003
). These deletions are not uniform truncations from either end of
the peptide but rather are distributed throughout the primary structure.
In addition, they vary widely across the T family. The result is distinct struc-
tural differences between 2/2 proteins and 3/3 proteins, and among 2/2
proteins as a set. Essential structural elements of the 2/2 fold are recapitulated
in the 3/3 fold. These constant parts hold important clues for the construc-
tion of functional haem pockets and the stabilization of bound oxygen,
whereas the variable features allow for the exploration of a broader func-
tional space (
Lecomte, Vuletich, & Lesk, 2005
).
The TrHbs are the only globins found in all three of the Bacteria,
Archaea, and Eukaryota families. Archaea do not appear to have any
M family globins, and Eukaryotes are missing S family globins (except for
SSDgbs in fungi,
Vinogradov et al., 2007, 2013
). This has been used to sug-
gest that the truncated forms of globins may be more ancient than the 3/3
canonical forms (
Vuletich & Lecomte, 2006
), but the alternative view has
since gained much support from additional sequences and in-depth phylo-
genetic analyses (
Vinogradov et al., 2007
).
Regardless of the nature of the common globin ancestor, all phyloge-
netic analyses agree that the TrHbs branch into three distinct groups desig-
nated as TrHb1 (with gene names often ending with an N), TrHb2 (with
gene names often ending in an O) and TrHb3 (with gene names often end-
ing in a P;
Vuletich & Lecomte, 2006; Wittenberg et al., 2002; Wu et al.,
2003
). In Bacteria, the TrHb2 is the dominant group from the T family,
occurring roughly twice as frequently as either TrHb1 or TrHb3. This is
in contrast to Cyanobacteria, where to date only TrHb1s have been iden-
tified. It should be noted that these groupings are based upon phylogeny, and
we do not as yet understand fully what different functional characteristics
each of these globins possess. It is still interesting to note that Cyanobacteria
TrHbs are so divergent from the rest of the bacterial world. In algae both
TrHb1 and TrHb2 have been found; however, TrHb1 are by far the dom-
inant group (
Vinogradov, Fernandez, et al., 2011
).
Figure 6.5
presents a
