104
G.N. Somero et al.
Adaptation in Enzyme Kinetic Properties: Lactate
Dehydrogenase-A Orthologs of Notothenioid Fishes
Many structural and functional properties of proteins reflect adaptation to
temperature [27-29]. When orthologous homologs (orthologs) of a protein
from differently thermally adapted species are compared, their thermal
stabilities typically vary directly with adaptation temperature. Furthermore,
temperature-sensitive kinetic properties such as Michaelis-Menten
constants (Km values) are highly conserved among species at their
physiological temperatures. Km values generally provide an index of the
ability of an enzyme to bind its substrate(s) and cofactor(s). Rapid
increases in Km with changing temperature are a sign that the enzyme is
losing its ability to bind ligands and is becoming impaired in function. Km
values generally increase with rising temperature [29], although for some
enzymes steep rises in Km with falling temperature have been observed [4].
The enzymes of eurythermal species typically have Km values that are less
perturbed by temperature than those of enzymes from stenothermal species
[30,31 ).
To obtain further insights into the biochemistry of cold adaptation and
stenothermy in Antarctic notothenioid fishes, we compared the effects of
temperature on the Km of pyruvate for A-type (muscle isoform) lactate
dehydrogenases (LDH-As) from six nototheniid and channichthyid species
(Fig. 3; Fields and Somero, unpublished observations).
All of the notothenioid species' LDH-As exhibited similar responses of
Km of pyruvate to temperature. At temperatures near 0 °e, the Km of
pyruvate for all orthologs from the notothenioids was in the range of 0.15
to 0.25 mM pyruvate, i.e., within the range of Km values at physiological
temperatures typical of LDH-As of other vertebrates [29]. (Note that the
box enclosing the Km values for LDH-As of the notothenioid species
includes the 95% confidence limits around the Km values.) At temperatures
only a few degrees above 0 °e, however, the Km of pyruvate began to rise
sharply, an index of the stenothermy of the LDH-A orthologs of the
notothenioids. In contrast, the slope of the Km of pyruvate versus
temperature plots for the LDH-As of two goby fishes, the extremely
eurythermal species Gillichthys seta, which encounters habitat eurythermal
congener, Gillichthys mirabilis, is relatively shallow, illustrating that the
eurythermality observed at the level of the whole organism may also be
reflected in the kinetic properties of individual enzymes.
G.N. Somero et al.
Adaptation in Enzyme Kinetic Properties: Lactate
Dehydrogenase-A Orthologs of Notothenioid Fishes
Many structural and functional properties of proteins reflect adaptation to
temperature [27-29]. When orthologous homologs (orthologs) of a protein
from differently thermally adapted species are compared, their thermal
stabilities typically vary directly with adaptation temperature. Furthermore,
temperature-sensitive kinetic properties such as Michaelis-Menten
constants (Km values) are highly conserved among species at their
physiological temperatures. Km values generally provide an index of the
ability of an enzyme to bind its substrate(s) and cofactor(s). Rapid
increases in Km with changing temperature are a sign that the enzyme is
losing its ability to bind ligands and is becoming impaired in function. Km
values generally increase with rising temperature [29], although for some
enzymes steep rises in Km with falling temperature have been observed [4].
The enzymes of eurythermal species typically have Km values that are less
perturbed by temperature than those of enzymes from stenothermal species
[30,31 ).
To obtain further insights into the biochemistry of cold adaptation and
stenothermy in Antarctic notothenioid fishes, we compared the effects of
temperature on the Km of pyruvate for A-type (muscle isoform) lactate
dehydrogenases (LDH-As) from six nototheniid and channichthyid species
(Fig. 3; Fields and Somero, unpublished observations).
All of the notothenioid species' LDH-As exhibited similar responses of
Km of pyruvate to temperature. At temperatures near 0 °e, the Km of
pyruvate for all orthologs from the notothenioids was in the range of 0.15
to 0.25 mM pyruvate, i.e., within the range of Km values at physiological
temperatures typical of LDH-As of other vertebrates [29]. (Note that the
box enclosing the Km values for LDH-As of the notothenioid species
includes the 95% confidence limits around the Km values.) At temperatures
only a few degrees above 0 °e, however, the Km of pyruvate began to rise
sharply, an index of the stenothermy of the LDH-A orthologs of the
notothenioids. In contrast, the slope of the Km of pyruvate versus
temperature plots for the LDH-As of two goby fishes, the extremely
eurythermal species Gillichthys seta, which encounters habitat eurythermal
congener, Gillichthys mirabilis, is relatively shallow, illustrating that the
eurythermality observed at the level of the whole organism may also be
reflected in the kinetic properties of individual enzymes.