MOLTEN GLOBULE-LIKE STATE OF CYTOCHROME-C UNDER CONDITIONS SIMULATINGTHOSE NEAR THE MEMBRANE-SURFACE

Methanol-induced conformational transitions in cytochrome c have been studied by near- and far-UV circular dichroism, Trp fluorescence, micr ocalorimetry, and diffusion measurements. The existence of at least tw o cooperative stages of transition has been shown. At the first stage, the native protein is transformed into an intermediate which has only traces of tertiary structure, but has a native-like secondary structu re content and is relatively compact; i.e., it has properties of the m olten globule state. On the second stage, the alcohol-induced molten g lobule is transformed into a more helical state, typical of proteins a t high alcohol concentrations. The conditions at which the alcohol-ind uced molten globule exists (moderately low pH and moderately low diele ctric constant) could be similar to those existing near negatively cha rged membrane surfaces. Consequently, these results might explain how the molten globule state can be achieved under physiological condition s.