Engineering stability of the insulin monomer fold with application to structure-activity relationships

To evaluate the possible relationship between biological activity and structural stability in selected regions of the insulin molecule, we have analyzed the guanidine hydrochloride induced reversible unfolding of a series of mutant insulins using a combination of near- and far-UV circular dichroism (CD). The unfolding curves are reasonably described on the basis of a two-state denaturation scheme; however, the observation of subtle differences between near- and far-UV CD detected unfolding indicates that intermediates may be present. Three regions of the insulin molecule are analyzed in detail with respect to their contribution to folding stability, i.e., the central B-chain helix, the NH2-terminal A-chain helix, and the B25-B30 extended chain region. Considerable enhancement of folding stability is engineered by mutations at the N-cap of the central B-chain helix and at the C-cap of the NH2-terminal A-chain helix. Mutations that confer increased stability in these regions are identical to those that lead to enhanced biological activity. In contrast, for insulin species modified in the B25-B30 region of the molecule, we observe no correlation between global folding stability and bioactivity. Mutations in the three regions examined are found to affect stability in a nearly independent fashion, and stabilizing mutations are generally found to enhance the cooperativity of the unfolding transition. We conclude that highly potent insulins (i.e., HisA8, ArgA8, GluB10, and AspB10) elicit enhanced activity because these mutations stabilize structural motifs of critical importance for receptor recognition.