NONNATIVE ALPHA-HELICAL INTERMEDIATE IN THE REFOLDING OF BETA-LACTOGLOBULIN, A PREDOMINANTLY BETA-SHEET PROTEIN

It is generally assumed that folding intermediates contain partially f ormed native-like secondary structures. However, if we consider the fa ct that the conformational stability of the intermediate state is simp ler than that of the native state, it would be expected that the secon dary structures in a folding intermediate would not necessarily be sim ilar to those of the native state. beta-Lactoglobulin is a predominant ly beta-sheet protein, although it has a markedly high intrinsic prefe rence for alpha-helical structure. We have studied the refolding kinet ics of bovine beta-lactoglobulin using stopped-flow circular dichroism and find that a partly alpha-helical intermediate accumulates transie ntly before formation of the native beta-sheets. The present results s uggest that the folding reaction of beta-lactoglobulin follows a non-h ierarchical mechanism, in which non-native alpha-helical structures pl ay important roles.