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proteins from these two 3/3 lineages raises interesting phylogeny questions.
In green algae, SDgbs (M lineage) are found but no protein from the
S lineage. These eukaryotes contain TrHb1s, which outnumber TrHb2s,
unlike in higher plants where rare TrHb1s are present (
Vazquez-Limon,
Hoogewijs, Vinogradov, & Arredondo-Peter, 2012
). The distribution of
globins, discussed in greater detail in
Section 3.2
, continues to provide
models of evolution and the transition between anaerobic and aerobic life.
Meanwhile, a novel structural aspect of the Hb superfamily was slowly
emerging. In the canonical vertebrate myoglobin, the proximal histidine
is the only proteinaceous ligand (
Fig. 6.1
). The deoxy state, that is, the
ferrous protein without added exogenous ligand, has a 'distal' coordination
site either emptied or occupied by a loosely bound water molecule readily
displaced by dioxygen. Coordination of a residue on the distal side
interferes with dioxygen binding and is a non-functional, occasionally
pathological, situation. In many of the recently identified haemoglobins,
however, endogenous hexacoordination, that is, the ligation of the iron
by two protein residues, is observed. Examples of these so-called
hexacoordinate globins include cytoglobin (
Trent & Hargrove, 2002
), neu-
roglobin (
Fuchs et al., 2004
),
Drosophila melanogaster
intracellular
haemoglobin (
Hankeln et al., 2002
),
Oryza sativa
non-symbiotic
haemoglobin (nsHb;
Arredondo-Peter et al., 1997
),
Hordeum
sp. (
Duff,
Wittenberg, & Hill, 1997
) and
Solanum lycopersicon
nsHb (
Ioanitescu
et al., 2005
), to name just a few.
Synechocystis
sp. PCC 6803 GlbN (hereafter
Synechocystis
6803 GlbN;
Scott & Lecomte, 2000
),
Synechococcus
sp. PCC 7002 GlbN (hereafter
Syn-
echococcus
7002 GlbN;
Scott et al., 2002
) and
C. eugametos
LI637 Hb
(
Couture & Guertin, 1996
) are also hexacoordinate globins.
Synechocystis
6803 GlbN and
Synechococcus
7002 GlbN use two histidines as axial ligands,
as in cytochrome
b
5
, whereas
C. eugametos
LI637 Hb uses a tyrosine as the
distal ligand. Thus, on the basis of these three instances, the classical myo-
globin coordination scheme does not seem to hold well in the world of
cyanobacterial and algal proteins. The presence of a labile protein ligand
on the distal side has several important consequences. The work of Hargrove
and colleagues, among others, in this specialized area of globin research, is
shedding light on the control of iron coordination by the globin fold (
Kakar,
Hoffman, Storz, Fabian, & Hargrove, 2010
).
Haem post-translational modification (PTM) is non-existent in the glo-
bin superfamily, except in
Synechocystis
6803 and
Synechococcus
7002 GlbNs,
