Structure of the transition state for the folding/unfolding of the barley chymotrypsin inhibitor 2 and its implications for mechanisms of protein folding.

The equilibrium and kinetics of folding of the single-domain protein chymotrypsin inhibitor 2 conform to the simple two-state model. The structure of the rate-determining transition state has been mapped out at the resolution of individual side chains by using the protein engineering method on 74 mutants that have been constructed at 37 of the 64 residues. The structure contains no elements of secondary structure that are fully formed. The majority of interactions are weakened by > 50% in the transition state, although most regions do have some very weak structure. The structure of the transition state appears to be an expanded form of the native state in which secondary and tertiary elements have been partly formed concurrently. This is consistent with a "global collapse" model of folding rather than a framework model in which folding is initiated from fully preformed local secondary structural elements. This may be a general feature for the folding of proteins lacking a folding intermediate and is perhaps representative of the early stages of folding for multidomain or multimodule proteins. The major transition state for the folding of barnase, for example, has some fully formed secondary and tertiary structural elements in the major transition state, and barnase appears to form by a framework process. However, the fully formed framework may be preceded by a global collapse, and a unified folding scheme is presented.