A kinetic study of the folding of staphylococcal nuclease using size-exclusion chromatography

The kinetics of the hydrodynamic volume change accompanying the reversible unfolding of staphylococcal nuclease have been observed by size-exclusion chromatography at 4 degrees C and pH 7.0 using the denaturant guanidine hydrochloride. The observed chromatographic profiles have been simulated by a six-component unfolding/refolding mechanism using a consistent set of equilibrium and kinetic parameters. The native protein is an equilibrium mixture of the cis and trans isomers of the peptide bond preceding proline-117. The native conformation containing the cis isomer dominates the equilibrium mixture, is more stable, and unfolds more slowly at its transition midpoint. The denatured protein is an equilibrium mixture of at least four components, the cis/trans isomers of proline-117 and one of the five remaining prolines. The dominant refolding pathway is initiated from the denatured component containing the trans isomer of proline-117. The six-component mechanism is consistent with tryptophan fluorescence kinetic measurements of the wild-type protein and with chromatographic measurements of a mutant P117G protein.