A spectroscopic investigation of the conformational dynamics of insulin in solution

A conformational change, termed the T --> R transition, which can be detected by visible, circular dichoric, and fluorescence spectroscopy, occurs in native insulin and tryptophan substituted insulin analogs ([TrpB25]-, [TrpB26]-, [GlyB24,TrpB25]-, and [GlyB24,TrpB26]insulin) upon binding specific alcohol ligands, including phenol and cyclohexanol. In these studies we have demonstrated that changes in the visible absorbance spectrum of an insulin6(Co2+)2 solution are not a definitive means of determining the occurrence of T --> R transitions in the presence of alcohol ligands. We also have presented evidence that fast protein liquid chromatography (FPLC) can be used to determine the aggregation state of insulin and that des-octapeptide(B23-30)insulin (DOI) forms Zn(2+)-coordinated hexamers that appear to be stabilized by the T --> R transformation. Using fluorescence spectroscopy, we have shown that in the presence of specific alcohol ligands the B-chain COOH-terminal residues, particularly position B25, of hexameric, as well as monomeric insulin undergo a conformational change which appears to be related to the T --> R transformation. Circular dichroic studies indicate that a conformation similar to the R-state of metal-coordinated hexameric insulin can be induced by binding cyclohexanol; however, this new conformational state (RI-state) exists independent of divalent metal ion coordination, and therefore of hexamer formation. We further show that monomeric insulin can be induced to assume the RI-state upon alcohol binding, therefore illustrating the first defined conformational change described for monomeric insulin. We suggest that this new conformation may be an intermediate state in the T --> R transformation in metal-coordinated hexameric insulin, such that T --> RI --> R. The model presented here of the structural adjustments undergone by insulin upon binding cyclohexanol provides further insight into the conformational flexibility of insulin in solution.