Biology Reference
In-Depth Information
(
Das et al., 2000
). Its function is unknown. However, the high concentra-
tions of globins seen in ciliates (up to 0.9% of total protein) and the
extremely high oxygen affinities (
Smith et al., 1962
) suggest that their role
may be to trap O
2
under O
2
-limited conditions.
6.5. Algae
The only eukaryotic alga from which a globin has been characterized in any
detail is
Chlamydomonas
, a genus of unicellular green algae (Chlorophyta).
These algae are found universally.
Chlamydomonas
is a unicellular flagellate
and used as a model organism for molecular biology. The most widely used
laboratory species is
Chlamydomonas reinhardtii
. Two genes of
Chlamydomonas eugametos
encode chloroplast haemoglobins related to those
of
P
.
caudatum
,
T. pyriformis
and the cyanobacterium
Nostoc commune
(
Couture, Chamberland, St-Pierre, Lafontaine, & Guertin, 1994
). The dis-
sociation rate for O
2
is extremely low and results in part from at least two
hydrogen bonds stabilizing the bound ligand (
Couture et al., 1999
). How-
ever, within the chloroplast the globin concentration is estimated to be only
130 nM, far too low to store significant O
2
or facilitate O
2
transport. The
high oxygen affinity argues against a metabolic function and the function
of the globin remains unknown. It is suggested that its role may be to safe-
guard photosynthesis
from oxygen 'leaks'
in the chloroplast
(
Das
et al., 1999
).
6.6. Perspectives
The protective function against nitrosative stress makes the FHbs of patho-
genic microorganisms attractive targets for antibiotic design. In particular,
antimicrobial imidazoles bind to
S. cerevisiae
and
C. albicans
FHbs and inhi-
bit their NOD function (
Helmick et al., 2005
). Recently, the structural
aspects of this inhibition have been investigated by
El Hammi et al.
(2011)
,
El Hammi, Houee-Levin, et
al.
(2012)
and
El Hammi,
Warkentin, et al. (2012)
.
A very recent report by
Rosic, Leggat, Kaniewska, Dove, and Hoegh-
Guldberg (2013)
has suggested a role for the SDgbs of
Symbiodinium
sp.
clade
206D
, a coral endosymbiont, as stress biomarkers, based on the increase in
transcription levels in the presence of thermal and nutrient stress. The pro-
posed function is reminiscent of some of the functions suggested for the non-
symbiotic Hbs of plants (
Hill, 2012
).
