Electrophoretic investigations of the acid conformational change of alpha-lactalbumin.

In order to clarify how the electrophoretic behavior reflects the conformational transition of globular proteins, moving boundary electrophoresis was applied to analysis of the acid conformational change of alpha-lactalbumin. The appearance of only a single electrophoretic boundary in the transition region of the protein suggests a very rapid transition with a half-time estimated to be smaller than 7 min on the basis of the theory of isomerizing systems in electrophoresis. The transition is clearly reflected in the dependence of the mobility on the protein net charge, which shows a sigmoidal curve closely similar to that obtained by a Linderstrøm-Lang pH-tritration plot for the carboxyl groups of alpha-lactalbumin. It was also concluded from the transition curves that the acidfication does not result in complete unfolding, but that a compact structure is maintained in the acidic region with an apparently expanded form as compared to the native state of the protein. All results obtained by electrophoresis were also supported by the results of pH-jump studies, analytical gel chromatography, and CD measurements.