6
1 The Subject Matter and Methods of Comparative Biochemistry
adaptive sense of differences in molecular structure or activity found in comparative investigations.
1.4.2 Molecular Adaptation
Adaptive molecular evolution consists, on the
one hand, of the adaptation of macromolecules,
such as RNAs, enzymes and proteins, by various
external and internal conditions and, on the other
hand, of evolutionary changes in the unfortunately poorly understood mechanisms regulating
gene expression, upon which the adaptive evolution of the complex characters of form, metabolism and behaviour is based. The evolutionary
process involves thousands of generations; other
events of molecular adaptation occur within the
lifetime of an individual, requiring anything from
a few milliseconds to several weeks. These processes also finally affect macromolecules: even
when biochemical adaptation ends in changes in
the concentration of small molecules or factors,
these are always the result of changes in enzyme
activity. Conversely, perturbations in the concentration of substrates or allosteric effectors have
effects upon enzymes. In individual, physiological adaptation events, the properties or amounts
of enzymes or proteins are modified. Alterations
in the properties of proteins can arise through
allosteric effects or chemical modifications, such
as substitution or limited proteolysis, and changes
in protein quantity arise mainly through the regulation of gene expression; here the distinction
must be drawn between alterations in protein
concentration and the appearance of new proteins or isoforms following induction.
Biochemical adaptation can serve two purposes: it can compensate for change in external or
internal conditions (compensatory adaptation),
or allow colonization of a new ecological niche
(exploitative adaptation). Hochachka and
Somero indicate the relevant problems and give
examples in their book Biochemical Adaptation
[15]; because of its close association with ecology,
biochemical adaptation has been the subject of
several recent reviews [8,16,29,30]. Processes of
biochemical adaptation occurring in single individuals have major consequences for the regulation
of metabolism. In the field of comparative biochemistry, these events are themselves of minor
interest compared with the underlying mechanisms, their variety, and changes in evolution. In
the adaptive, molecular evolution of an enzyme,
changes in regulatory characters play at least as
great a role as changes in catalytic and physicochemical characters.
Many examples of regulatory and evolutionary
molecular adaptation events will be described in
particular chapters of this book, among them, for
example, regulatory changes in connection with
reproduction, exercise [28], hunger, and during
inflammation and defence processes or other
stress reactions, but most of all in connection with
regulatory and evolutionary adaptation to various
external conditions. Here is included, for example, the osmolality of the extracellular fluids to
which the cell must adapt by isosmotic intracellular regulation. Amongst other changes occurring
during this process is that of the intracellular concentration of specific osmotically active, organic
substances, in particular the free amino acids and
other nitrogen compounds, and thus the environment of intracellular proteins. The cartilaginous
fish enrich their blood and other tissues with
osmotically active substances, such as urea and
trimethylaminoxide, to such an extent that specific protein adaptation is necessary. High extra
and intracellular urea concentrations also occur
during aestivation in the lungfish and certain
frogs. The biochemical adaptations to transient or
continuously low oxygen partial pressure are well
known; the properties of the respiratory pigments
may be changed or, in some animals, there are
variedly effective mechanisms for anaerobic
energy recovery; these mainly concern catabolism of carbohydrates and will be dealt with in
Chapter 14. The use of different energy-yielding
substrates in the muscles of various animals
should also be considered as an adaptive mechanism to specific external and internal conditions
(p.581).
One external factor of critical biological importance is temperature [3, 5]. The homeothermal
mammals and birds are able to maintain a constant body temperature by regulation of energyproducing metabolism. Many mammals possess a
specialized tissue for heat production, the brown
fat, in the mitochondria of which the Krebs cycle
and ATP production are uncoupled by the action
of a specific uncoupling protein (p. 702). In mammals and birds that can reduce their body temperature to the level of their surroundings during
sleep (bats, hummingbirds, young swifts) or
hibernation (bats, several rodents), the biochemical organization of the cells must be adapted to
the combination of low temperature, reduced
metabolism, acidification, and often increased
urea concentration. The poikilothermal (coldblooded) animals are at the mercy of the tem-
1 The Subject Matter and Methods of Comparative Biochemistry
adaptive sense of differences in molecular structure or activity found in comparative investigations.
1.4.2 Molecular Adaptation
Adaptive molecular evolution consists, on the
one hand, of the adaptation of macromolecules,
such as RNAs, enzymes and proteins, by various
external and internal conditions and, on the other
hand, of evolutionary changes in the unfortunately poorly understood mechanisms regulating
gene expression, upon which the adaptive evolution of the complex characters of form, metabolism and behaviour is based. The evolutionary
process involves thousands of generations; other
events of molecular adaptation occur within the
lifetime of an individual, requiring anything from
a few milliseconds to several weeks. These processes also finally affect macromolecules: even
when biochemical adaptation ends in changes in
the concentration of small molecules or factors,
these are always the result of changes in enzyme
activity. Conversely, perturbations in the concentration of substrates or allosteric effectors have
effects upon enzymes. In individual, physiological adaptation events, the properties or amounts
of enzymes or proteins are modified. Alterations
in the properties of proteins can arise through
allosteric effects or chemical modifications, such
as substitution or limited proteolysis, and changes
in protein quantity arise mainly through the regulation of gene expression; here the distinction
must be drawn between alterations in protein
concentration and the appearance of new proteins or isoforms following induction.
Biochemical adaptation can serve two purposes: it can compensate for change in external or
internal conditions (compensatory adaptation),
or allow colonization of a new ecological niche
(exploitative adaptation). Hochachka and
Somero indicate the relevant problems and give
examples in their book Biochemical Adaptation
[15]; because of its close association with ecology,
biochemical adaptation has been the subject of
several recent reviews [8,16,29,30]. Processes of
biochemical adaptation occurring in single individuals have major consequences for the regulation
of metabolism. In the field of comparative biochemistry, these events are themselves of minor
interest compared with the underlying mechanisms, their variety, and changes in evolution. In
the adaptive, molecular evolution of an enzyme,
changes in regulatory characters play at least as
great a role as changes in catalytic and physicochemical characters.
Many examples of regulatory and evolutionary
molecular adaptation events will be described in
particular chapters of this book, among them, for
example, regulatory changes in connection with
reproduction, exercise [28], hunger, and during
inflammation and defence processes or other
stress reactions, but most of all in connection with
regulatory and evolutionary adaptation to various
external conditions. Here is included, for example, the osmolality of the extracellular fluids to
which the cell must adapt by isosmotic intracellular regulation. Amongst other changes occurring
during this process is that of the intracellular concentration of specific osmotically active, organic
substances, in particular the free amino acids and
other nitrogen compounds, and thus the environment of intracellular proteins. The cartilaginous
fish enrich their blood and other tissues with
osmotically active substances, such as urea and
trimethylaminoxide, to such an extent that specific protein adaptation is necessary. High extra
and intracellular urea concentrations also occur
during aestivation in the lungfish and certain
frogs. The biochemical adaptations to transient or
continuously low oxygen partial pressure are well
known; the properties of the respiratory pigments
may be changed or, in some animals, there are
variedly effective mechanisms for anaerobic
energy recovery; these mainly concern catabolism of carbohydrates and will be dealt with in
Chapter 14. The use of different energy-yielding
substrates in the muscles of various animals
should also be considered as an adaptive mechanism to specific external and internal conditions
(p.581).
One external factor of critical biological importance is temperature [3, 5]. The homeothermal
mammals and birds are able to maintain a constant body temperature by regulation of energyproducing metabolism. Many mammals possess a
specialized tissue for heat production, the brown
fat, in the mitochondria of which the Krebs cycle
and ATP production are uncoupled by the action
of a specific uncoupling protein (p. 702). In mammals and birds that can reduce their body temperature to the level of their surroundings during
sleep (bats, hummingbirds, young swifts) or
hibernation (bats, several rodents), the biochemical organization of the cells must be adapted to
the combination of low temperature, reduced
metabolism, acidification, and often increased
urea concentration. The poikilothermal (coldblooded) animals are at the mercy of the tem-