Destabilization of Insulin Hexamer in Water-Ethanol Binary Mixture.

We report combined experimental and simulation studies which reveal that the structural integrity of insulin hexamer, the storehouse of the important hormone in our body, is compromised by the interactions with ethanol. X-ray crystal structures suggest that ethanol replaces water molecules inside the insulin hexamer cavity. At the maximum physiologically tolerable concentration of ethanol (∼0.6% v/v), molecular dynamics simulations show that ethanol molecules get exchanged between the bulk and the cavity with a free energy cost of ∼5 kcal mol-1. However, biological time scales are orders of magnitude longer than that achievable by molecular dynamics simulations. Hence, to accelerate the process we investigate insulin hexamer in ∼30% v/v ethanol concentration. We find that the entrance and exit of ethanol from the hexamer cavity lead to the modification of atomic contacts in the protein. This causes large-scale fluctuations that force the protein out of its native state free energy minimum. Structural perturbations are also observed at lower ethanol concentration. The computational findings are consistent with dynamic light scattering experiments that suggest an abrupt reduction in the population of insulin hexamers at a critical ethanol concentration. The structural changes triggered by interaction of ethanol with the insulin hexamer are likely to represent a general dynamic event of amphiphilic cosolvent induced changes in macromolecular assemblies with the consequent effects on cellular homeostasis.