Folding of ribonuclease A from a partially disordered conformation. Kinetic study under folding conditions

Bovine pancreatic ribonuclease A (RNase) was partially disordered with 3.5 M LiClO4 (pH 3.0). The conformation of this partially disordered material was studied by circular dichroism and Raman spectroscopy. Although the partially disordered protein appears to have a lower beta-structure content and disordered tyrosyl side chains, compared to native RNase, it seems to retain some ordered backbone structure that is suggested to be alpha helix. The kinetics of folding of LiClO4-denatured RNase was studied by means of absorption and circular dichroism measurements. For comparison, the kinetics of folding of urea-denatured RNase (which is completely devoid of ordered structure) was examined with the same techniques. Since the kinetics of folding of both denatured species are found to be similar, it appears that the ordered structure present in LiClO4-denatured RNase plays no role in determining the folding pathway. Also, the change in the circular dichroism at 220 nm showed that some of the ordered structure in LiClO4-denatured RNase becomes disordered in the early stages of folding. This implies that all ordered structures in RNase are not equivalent in their influence on the folding pathway; some can play an essential role and some may not.