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Revised Manuscript Received August 17, 1995® abstract: Leucine zipper peptides provide simple model systems for studying both the intramolecular and intermolecular interactions that govern protein folding. The synthetic 33-residue peptide GCN4-pl, derived from the yeast transcriptional activator GCN4, forms a stable bimolecular coiled-coil structure [O’Shea, E. K „Klemm, J. D „Kim, P. S „& Alber, T.(1991) Science 254, 539-544], The guanidine-HC1 induced equilibrium unfolding of this peptide at 5 C and pH 7.0 yields a standard state free energy of 10.49±0.23 kcal (mol dimer)-1 when fit to a two-state model involving the nativedimer and the unfolded monomer. The unfolding and refolding kinetics of GCN4-pl were monitored by stopped-flow circular dichroism spectroscopy as a function of both peptide concentration and final denaturant concentration. The unfolding kinetics displayed single-exponential behavior …