Molecular Adaptations of Antarctic Fish Hemoglobins
349
The Molecular Modeling and Phylogenetic Approaches
Antarctic fish Hbs display large differences in functional characteristics
despite high identity in primary structure. Thus the ensemble of Antarctic
Hbs is a greatly simplified system, which allows us to apply molecular
modeling as a useful approach to structure/function relationships, to tackle
not only intriguing questions pertaining to the Antarctic extreme
environment (e.g. the lack of subunit cooperativity found in Hbs of
Artedidraconidae), but also questions of general bearing (e.g. the
molecular basis of the Root effect, a very important functional feature of
fish Hb).
The major Hbs of T bernacchii and T newnesi only differ by four
residues in the a chain and ten in the ~ [17,41]. Despite this high identity,
the functional behavior of these Hbs is very different. T newnesi Hb 1 has
no Root effect and a weak Bohr effect, whereas both effects are very large
in T bernacchii Hb 1. These Hbs are therefore an exceptional model for
the study of Hb structure-function relationship and, in particular, for the
study of the molecular basis of the Root effect. The differences in the
sequences of the two proteins include neither the residues considered
responsible for the Root effect nor those involved in the binding of
organophosphates [42].
Moreover, the analysis of the two structures does not evidentiate
substantial structural differences, as also established by crystallographic
studies of T newnesi Hb 1 in the Rand T states (D'A vino et al.,
unpublished). Space limitations do not allow a detailed description of these
results, which will be reported elsewhere.
The above considerations imply that the structure-function interplay may
entail very refined and often unpredictable mechanisms. The limited
number of residues involved makes these two functionally distinct Hbs the
simplest system so far available. Site-directed mutagenesis, currently being
initiated, is likely to become another tool of choice - in close combination
with molecular modeling - to tackle the molecular basis of the Root effect,
as well as to shed light on other open questions.
Notothenioid cladograms and phylogenetic trees have been built from
the amino acid sequences of Antarctic and nonAntarctic Hbs (Starn et al.,
this Vol.; [43,44]). The trees agree with those obtained by morphological
analysis and other molecular approaches (see Bargelloni and Lecointre,
this Vol.); they give strong support to the monophyly of Antarctic
notothenioids, with nonAntarctic P. urvillii as ancestral taxon. In
agreement with our Hb studies, they also indicate that Bovichtidae (and/or
Pseudaphritidae [33]) diverged before the formation of the Antarctic Polar
Front. The grouping of nonAntarctic N angustata with two Antarctic
349
The Molecular Modeling and Phylogenetic Approaches
Antarctic fish Hbs display large differences in functional characteristics
despite high identity in primary structure. Thus the ensemble of Antarctic
Hbs is a greatly simplified system, which allows us to apply molecular
modeling as a useful approach to structure/function relationships, to tackle
not only intriguing questions pertaining to the Antarctic extreme
environment (e.g. the lack of subunit cooperativity found in Hbs of
Artedidraconidae), but also questions of general bearing (e.g. the
molecular basis of the Root effect, a very important functional feature of
fish Hb).
The major Hbs of T bernacchii and T newnesi only differ by four
residues in the a chain and ten in the ~ [17,41]. Despite this high identity,
the functional behavior of these Hbs is very different. T newnesi Hb 1 has
no Root effect and a weak Bohr effect, whereas both effects are very large
in T bernacchii Hb 1. These Hbs are therefore an exceptional model for
the study of Hb structure-function relationship and, in particular, for the
study of the molecular basis of the Root effect. The differences in the
sequences of the two proteins include neither the residues considered
responsible for the Root effect nor those involved in the binding of
organophosphates [42].
Moreover, the analysis of the two structures does not evidentiate
substantial structural differences, as also established by crystallographic
studies of T newnesi Hb 1 in the Rand T states (D'A vino et al.,
unpublished). Space limitations do not allow a detailed description of these
results, which will be reported elsewhere.
The above considerations imply that the structure-function interplay may
entail very refined and often unpredictable mechanisms. The limited
number of residues involved makes these two functionally distinct Hbs the
simplest system so far available. Site-directed mutagenesis, currently being
initiated, is likely to become another tool of choice - in close combination
with molecular modeling - to tackle the molecular basis of the Root effect,
as well as to shed light on other open questions.
Notothenioid cladograms and phylogenetic trees have been built from
the amino acid sequences of Antarctic and nonAntarctic Hbs (Starn et al.,
this Vol.; [43,44]). The trees agree with those obtained by morphological
analysis and other molecular approaches (see Bargelloni and Lecointre,
this Vol.); they give strong support to the monophyly of Antarctic
notothenioids, with nonAntarctic P. urvillii as ancestral taxon. In
agreement with our Hb studies, they also indicate that Bovichtidae (and/or
Pseudaphritidae [33]) diverged before the formation of the Antarctic Polar
Front. The grouping of nonAntarctic N angustata with two Antarctic