The roles of partly folded intermediates in protein folding.

Proteins can fold very rapidly, undoubtedly because they do not do so simply by random searching. The stable, partly folded species that can be detected during protein refolding are, however, of uncertain kinetic significance. The available kinetic evidence indicates that the intermediates that are most responsible for the rapidity of folding are extremely unstable and not populated detectably; they are less extreme versions of the transition state for folding. Protein folding is most readily studied when it is coupled to disulfide formation, which has the advantages that the intermediates can be characterized in detail and their kinetic roles determined unambiguously. The most important aspects of the disulfide folding pathway of BPTI are understood to at least a first approximation, and several other protein disulfide folding pathways are known in outline. These pathways demonstrate that disulfide folding is not intrinsically different from that not involving disulfide formation. Partly folded conformations can increase the rate of folding somewhat by causing productive disulfide bonds to be populated preferentially, but the most important folding intermediates are not detectable. The essence of folding is to build up the cooperativity between the individual interactions that is necessary for a stable conformation.