Variation in the glycolytic pathway: The role of evolutionary and physiological processes

We systematically determined the prevalence of altered enzyme expression by examining all the enzymes in a single metabolic pathway: glycolysis. The contributions of evolutionary adaptation and physiological acclimation to this alteration were distinguished by quantifying the variation between populations and determining the effect of acclimation on enzyme expression. Of the 10 cardiac glycolytic enzymes, two enzymes, phosphoglucoisomerase and aldolase, were expressed at a significantly greater level in a northern population of Fundulus heteroclitus than in a southern population but were unaffected by acclimation temperature. The expression of two other enzymes, phosphoglyceromutase and enolase, was the same in both populations at a given temperature but significantly greater at the colder acclimation temperature. These findings indicate that both evolved differences between populations and physiological adjustment within a population occur in a single pathway and can potentially compensate for the differences in the thermal environment. All four enzymes that show variation are equilibrium enzymes that are traditionally considered to be incapable of producing compensatory metabolic changes. Our results thus call into question the paradigm that only nonequilibrium enzymes are important for the regulation of metabolism.