Receptor binding redefined by a structural switch in a mutant human insulin

Abstract

CRYSTAL structures of insulin have been determined in various distinct forms^1–5, the relevance of which to receptor recognition has long been the subject of speculation^2,6,7. Recently the crystal structure of an inactive insulin analogue has been determined and, surprisingly, found to have a conformation identical to native insulin^8,9. On this basis Dodson and colleagues have suggested that the known insulin crystal structures reflect an inactive conformation, and that a change in conformation is required for activity—specifically, the carboxy terminal residues of the B-chain are proposed to separate from the amino terminal residues of the A-chain⁸. Here we report the solution structure of an active insulin mutant^7,10, determined by two-dimensional NMR, which supports this hypothesis. In the mutant, the carboxy terminal β-turn and β-strand of the B-chain are destabilized and do not pack across the rest of the molecule. We suggest that analogous detachment of the carboxy terminal region of the B-chain occurs in native insulin on binding to its receptor. Our finding that partial unfolding of the B-chain exposes an alternative protein surface rationalizes the receptor-binding properties of a series of anomalous insulin analogues^7,11–13, including a mutant insulin associated with diabetes mellitus in man^14,15.