Partial molar volume and gel electrophoretic studies of sperm whale myoglobin and apomyoglobin: Native and urea denatured

Abstract

The reaction of urea with sperm whale myoglobin and apomyoglobin generates changes in their volumes which are unique and characteristic for these proteins. The partial molar volume of myoglobin exhibited a positive concave functional dependence on urea concentration reaching a maximum at 6.5 M urea. In contrast, the corresponding parameter for apomyoglobin obeyed a negative curvilinear relationship with increasing urea concentration. Gel electrophoretic analysis revealed a similar dichotomy in the kinetics, products, and reaction mechanism(s). Even though the mechanisms for urea denaturation of myoglobin and apomyoglobin differ, both systems were characterized by the operation of a series of reactions which produce several forms of denatured proteins. The singular partial molar volume relationship observed for myoglobin as a function of urea concentration indicates the occurrence of not only a helix to coil transition, but also additional structural transformations. The negative asymptotic relationship found for the apomyoglobin-urea systems is in accord with a helix to coil conversion. The volume decrease resulting from the transformation of myoglobin from a water-free crystalline state to the completely hydrated state is — 0.039 ml/g protein. The dependence of this parameter on the water content of the crystalline protein indicates that electrostriction is the dominant factor involved.