Stepwise formation of α-helices during cytochrome c folding

Two models have been proposed to describe the folding pathways of proteins. The framework model assumes the initial formation of the secondary structures whereas the hydrophobic collapse model supposes their formation after the collapse of backbone structures. To differentiate between these models for real proteins, we have developed a novel CD spectrometer that enables us to observe the submillisecond time frame of protein folding and have characterized the timing of secondary structure formation in the folding process of cytochrome c (cyt c). We found that ~20% of the native helical content was organized in the first phase of folding, which is completed within milliseconds. Furthermore, we suggest the presence of a second intermediate, which has α-helical content resembling that of the molten globule state. Our results indicate that many of the α-helices are organized after collapse in the folding mechanism of cyt c.