Thermodynamic stability of the molten globule states of apomyoglobin.
Whereas horse apomyoglobin is fully unfolded at pH 2 in the absence of salt, addition of a salt such as sodium chloride or sodium trichloroacetate stabilizes the molten globule state. Thermal unfolding of the salt-stabilized molten globule states of horse apomyoglobin at pH 2 measured by far-UV circular dichroism occurs not only on heating (i.e. heat-denaturation) but also on cooling (i.e. cold-denaturation). This demonstrates that a hydrophobic interaction contributes to the stability of the molten globule state and suggests that the unfolding transition can be represented by a cooperative two-state mechanism. To clarify the mechanism of conformational transition, we investigated the thermal unfolding of the chloride-stabilized molten globule state by differential scanning calorimetry. We observed a broad but distinct excess heat capacity peak, which is consistent with the unfolding transition measured by circular dichroism. To further characterize the molten globule states, we examined by far-UV circular dichroism the denaturant-induced unfolding transitions of the molten globule states stabilized by sodium chloride or sodium trichloroacetate. The urea-induced unfolding transitions of the molten globule states were explained by the two-state mechanism. The guanidine-hydrochloride-induced unfolding experiments clarified that the trichloroacetate-stabilized molten globule state is distinct from the chloride-stabilized one and that the former involves additional helical segment(s). These results support a view that the thermal unfolding of the molten globule states at pH 2 can be approximated by a two-state transition. However, several results suggested that a combined mechanism incorporating the two-state transition and a gradual structural change would be more general in describing the conformational transition of the molten globule states.
Nishii
I
I
Department of Biology, Faculty of Science Osaka University, Toyonaka, Japan.
Kataoka
M
M
Goto
Y
Y
D000818:Animals; D001059:Apoproteins / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D002942:Circular Dichroism; D019791:Guanidine; D006146:Guanidines; D006736:Horses; D009211:Myoglobin / Q000737:chemistry*; D011489:Protein Denaturation; D017510:Protein Folding; D013816:Thermodynamics; D014508:Urea
D001059:Apoproteins; D006146:Guanidines; D009211:Myoglobin; C010857:apomyoglobin; D014508:Urea; D019791:Guanidine