Ligand binding to wild-type and E-B13Q mutant insulins: a three-state allosteric model system showing half-site reactivity.

By using ultra-violet and visible absorbance in conjunction with high field 1H-nuclear magnetic resonance spectroscopy, the insulin hexamer has been shown to undergo two allosteric transitions in solution involving three allosteric states (T6<-->T3 R3<-->R6). A simple mathematical model consisting of four variables has been derived that quantitatively describes the complex homotropic and heterotropic interactions that modulate these allosteric transitions. The mutation of one residue, Glu-B13 to Gln, results in an unexpected change in the T3R3 to R6 equilibrium by a factor of 10(7).