Temperature dependence of the kinetics of the urea-induced dissociation of human plasma α 2-macroglobulin into half-molecules : A minimum rate at 15 °C indicates hydrophobic interaction between the subunits

Abstract

The kinetics of the urea-induced dissociation of human plasma α 2-macrogrobulin to half-molecules has been studied as a function of temperature by using small-angle scattering of X-rays and neutrons. The most striking result of the present investigation is that there is a minimum in reaction rate at about 15 °C, and that the rate increases when the temperature is lowered, or raised, from that value. By analyzing the first-order rate constants in terms of transition-state theory it was found that the dissociation is associated with a large and positive change in heat capacity between the activated complex and native α 2-macroglobulin ( ΔC P ‡ , is in the range 5 to 6 kJ mol −1 K −1). In analogy with pure thermodynamic investigations, where a large change in heat capacity normally is interpreted as a melting of hydrophobic interaction, we therefore propose that hydrophobic interaction is involved in the so-called non-covalent interactions between the subunits of α 2-macroglobulin. As a result of the present investigation, it also follows that the free energy of activation ΔG ‡ has a maximum at about 32 °C, whereas the enthalpy of activation ΔH ‡ and the entropy of activation ΔS ‡ are zero at about 15 °C and 32 °C, respectively. These temperatures are slightly dependent upon the concentration of urea and upon whether the reaction is run in a 1H or a 2H medium. Furthermore, from the kinetic point of view, at low temperature the reaction can be characterized as enthalpy driven, whereas at high temperature, it can be characterized as entropy driven.