Proline peptide isomerization and protein folding.

ABSTRACT

The unfolding-refolding of proteins is a cooperative process and, as judged by equilibrium properties, occurs in one step involving the native, N, and the unfolded U, conformational states. Kinetic studies have shown that the denatured protein exists as a mixture of slow-(US) and fast-(UF) refolding forms produced by proline peptide cis-trans isomerization. Proline residues in UF are in the same configuration as in the native protein while they are in non-native configuration in US. For protein folding to occur quickly US must be converted into UF. The fact that the equilibrium and kinetic properties of US UF are the same as those found for proline cis-trans isomerization taken together with the absence of slow phase in the kinetics of refolding of a protein devoid of proline, support this view. However, the absence of a linear correlation between half-time of reactivation of denatured enzymes and their proline-contents, as well as the dissimilarities in the kinetic properties of US UF in unfolding and refolding experiments are not consistent with the model. Conformational energy calculation and experimental results on refolding of proteins suggest that some proline residues are non-essential. They will not block protein folding even in wrong isomeric form. The native-like folded structure with incorrect proline isomers will serve as intermediate state(s) in which these prolines will more readily isomerize to the correct isomeric form. The picture becomes more complex when one considers the consequence of cis-trans isomerism of non-proline residues on protein folding.