Regeneration of RNase A from the reduced protein: models of regeneration pathways

Two models of protein-folding pathways are proposed on the basis of equilibrium and kinetic data in the literature. One is a growth-type model--i.e., nucleation of the native-like structure occurs in the folding process, in the rate-limiting step(s), and subsequent folding around the nucleation sites proceeds smoothly to form the native disulfide bonds and conformation. The other is a rearrangement-type model--i.e., proper nucleation does not occur in the folding process; instead, non-native interactions play a significant role in the folding pathways and lead to metastable intermediate species. Such non-native interactions, including incorrect disulfide bonds and proline cis-trans isomerization, must be disrupted or rearranged to nucleate the native interactions [a process that is included in the rate-limiting step(s)] for the protein to fold. The rate-limiting steps in the pathways for regeneration of RNase A from the reduced protein are classified as growth- or rearrangement-type pathways. The growth-type pathway is the one accompanying the formation of an intramolecular disulfide bond in the rate-limiting step. The rearrangement-type pathway is the one accompanying the reshuffling or disruption of a disulfide bond in the rate-limiting step. The folding of other proteins, accompanying oxidation of the reduced form, and the folding of denatured proteins with intact disulfide bonds are discussed in terms of the growth- and rearrangement-type models.