Non-native alpha-helical intermediate in the refolding of beta-lactoglobulin, a predominantly beta-sheet protein.

It is generally assumed that folding intermediates contain partially formed native-like secondary structures. However, if we consider the fact that the conformational stability of the intermediate state is simpler than that of the native state, it would be expected that the secondary structures in a folding intermediate would not necessarily be similar to those of the native state. beta-Lactoglobulin is a predominantly beta-sheet protein, although it has a markedly high intrinsic preference for alpha-helical structure. We have studied the refolding kinetics of bovine beta-lactoglobulin using stopped-flow circular dichroism and find that a partly alpha-helical intermediate accumulates transiently before formation of the native beta-sheets. The present results suggest that the folding reaction of beta-lactoglobulin follows a non-hierarchical mechanism, in which non-native alpha-helical structures play important roles.