Biphasic transition curve on denaturation of chicken cystatin by guanidinium chloride Evidence for an independently unfolding structural region

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Abstract
Far‐ultraviolet circular dichroism and tryptophan fluorescence measurements showed that the reversible unfolding of the cysteine proteinase inhibitor, chicken cystatin, by guanidinium chloride is a two‐step process with transition midpoints at ≈3.4 and ≈5.4 M denaturant. The partially unfolded intermediate had both far‐ and near‐ultraviolet circular dichroism and fluorescence emission spectra comparable to those of the native protein. The largely retained tertiary structure suggests that the intermediate represents a species in which a separate region of lower stability has been unfolded, rather than an intermediate of the ‘molten globule’ type. Such a structurally independent region is apparent in the three‐dimensional structure of the inhibitor.