Biology Reference
In-Depth Information
strengthened the idea that LI637 is involved in regulation of photosynthesis—
or in mitigation of photosynthetic stressors—as the pyrenoid consists largely
of ribulose 1,5-biphosphate carboxylase/oxygenase (Rubisco), the enzyme
responsible for the critical step in carbon fixation and photosynthetic effi-
ciency (
Meyer et al., 2012
). However, neither direct characterization of
the native protein nor
identification of
its metabolic purpose has
been undertaken.
Biochemical characterization and subsequent crystallization of the
recombinant haem domain of the LI637 protein (CtrHb) resolved the first
structure of a TrHb and demonstrated the existence of a distinctive 2/2 fold
with an associated haem group buried in the core of the protein (
Pesce et al.,
2000
). CtrHb is also unique in that it displays a high O
2
binding affinity
when compared with other TrHbs (such as that from
P. caudatum
) even
though it shares, on the distal site, the tyrosine and glutamine residues inter-
acting with exogenous ligands. Mutational studies of distal residues coordi-
nated to the haem group within CtrHb suggest that the modulation of
substrate binding by the protein is central to its activity (
Das et al., 1999
).
Among possible functions, a protective role against dioxygen damage has
emerged as a plausible hypothesis.
4.5. Chlamydomonas reinhardtii
Both
C. eugametos
and a distantly related species within the same genus,
Chlamydomonas reinhardtii
, have been used as model organisms for the study
of eukaryotic green algae. But
C. eugametos
is an obligate photoautotroph,
whereas
C. reinhardtii
has an added benefit of heterotrophic or mixotrophic
growth on acetate-supplemented medium (
Harris, 2001
). In
C. reinhardtii
,
this has allowed the isolation of many photosynthetic mutants and given rise
to the dominant use of
C. reinhardtii
over
C. eugametos
in algal genetics
(
Grossman et al., 2003
). The nuclear, chloroplastic and mitochondrial
genomes of
C. reinhardtii
have now been completely sequenced
(
Grossman et al., 2010
). In contrast, only a few genes have been isolated
from
C. eugametos
and only partial sequencing of its genome has been
completed. Although initial investigation of TrHbs in algae utilized
C. eugametos
, further genetic characterization of TrHb is focusing on homol-
ogous genetic sequences in
C. reinhardtii
.
Within the nuclear genome of
C. reinhardtii
, a family of 10 putative glo-
bin genes has been identified (
Vinogradov, Fernandez, et al., 2011
). All
10 are categorized as TrHb proteins. Within the annotations of
the
