Folding pathway of guanidine-denatured disulfide-intact wild-type and mutant bovine pancreatic ribonuclease A

The refolding kinetics of guanidine-denatured disulfide-intact bovine pancreatic ribo-nuclease A (RNase A) and its proline-42-to-alanine mutant (Pro42Ala) have been studied by monitoring tyrosine burial and 2-cytidine monophosphate (2CMP) inhibitor binding. The folding rate for wild-type RNase A is faster in the presence of the inhibitor 2CMP than in its absence, indicating that the transition-state structure in the rate-determining step is stabilized by 2CMP. The folding rate monitored by 2CMP binding to the major slow-folding species of Pro42Ala RNase A is faster than the folding rate monitored by tyrosine burial; however, the folding rate monitored by inhibitor binding to the minor slow-folding species is decreased significantly over the folding rate monitored by tyrosine burial, indicating that the major and minor slow-folding species of Pro42Ala fold to the native state with different transition-state conformations in the rate-determining step.