Thermodynamic activation parameters of fish myofibrillar ATPase enzyme and evolutionary adaptations to temperature

Abstract

INTERSPECIFIC compensatory adaptations to environmental temperature which occur at the molecular level have been demonstrated for several enzyme systems¹. Most of these studies have been concerned with either kinetic parameters such as K_m (refs 2, 3) or thermodynamic parameters such as activation energy^2,4. The significance of changes in these parameters in the overall mechanism of evolutionary temperature compensation is controversial¹. In the case of activation energy (E_a), as calculated from Arrhenius' equation, a correlation exists with habitat temperature for some enzymes^2,5,6 but not others³. Studies of activation energy are principally concerned with the enthalpy of activation (ΔH^‡). There have been comparatively few studies of the free energy of activation (ΔG^‡) between homologous enzymes from animals of different thermal environments^7,8. Low et al.⁸ showed a correlation between ΔG^‡ for muscle type (M₄) lactate dehydrogenase and body temperature. The relative importance of enthalpic (ΔH^‡) and entropic (ΔS^‡) activation between poikilotherms and homoeotherms was also shown to be different⁸. Similar results have been obtained for skeletal muscle myofibrillar ATPase activity⁷. Since these studies deal with homologous enzymes from animals with very different phylogenetic positions it is difficult to assess directly the adaptive significance of changes in the magnitude of these parameters.