Biology Reference
In-Depth Information
determination, nuclear magnetic resonance (NMR) spectroscopy has
been applied to several globins in solution. NMR spectroscopy, however,
is most powerful in the characterization of local features, in particular
the haem group and its direct environment (
La Mar, Satterlee, & de
Ropp, 2000
). It is also well suited for monitoring the perturbations caused
by a change in conditions, such as pH or temperature, and for describing
the dynamic properties of the structure on a wide range of timescales.
Cyanobacteria and Chlorophyta globins have benefitted from most of these
experimental approaches.
For any globin, the kinetics of ligand binding is of paramount functional
importance. Naturally, the focus is on oxygen, but other small ligands must
be considered as well, either because they are physiological or because they
serve as alternative chemical probes and provide valuable physical insight.
Ligand-binding data are available for a subset of wild-type and variant pro-
teins and are next inspected in light of the structural data. In the same section,
we organize the limited and scattered information published on the reactiv-
ity of the haem group, for example, reduction potential and auto-oxidation
properties. These parameters can be compared with those of other globins to
distinguish trends and guide functional hypotheses. The data, though lim-
ited, suggest many possible directions for deepening the characterization
of the globins from Cyanobacteria and Chlorophyta. In the final section,
we emphasize the most pressing questions and offer comments on the future
of this field of research.
2. HISTORICAL PERSPECTIVE
The year 1992 marked a turning point in the study of the haemoglobin
superfamily.
Potts, Angeloni, Ebel, and Bassam (1992)
, who were then
investigating nitrogen metabolism in the cyanobacterium
Nostoc commune
,
discovered that a globin gene (
glbN
) was present in the
nif
UHD gene cluster
of the nitrogen fixation operon. The amino acid sequence of 'cyanoglobin',
as it became known, was found to be related to that of protozoan globins
(
Iwaasa, Takagi, & Shikama, 1989, 1990
). Protozoan globins had been
detected much earlier by optical measurements on cell suspensions of
Para-
mecium caudatum
(
Sato & Tamiya, 1937
) and
Tetrahymena pyriformis
(
Keilin &
Ryley, 1953
). By 1992, a few sequences of protozoan globins had been
determined (
Iwaasa et al., 1989, 1990
), and it was recognized that they
were shorter than the vertebrate proteins and distantly related to them.
