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Thermodynamic Contributions to the Stability of the Insulin Hexamer

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journal contribution
posted on 10.06.2014, 00:00 by George P. Lisi, Chien Yi M. Png, Dean E. Wilcox
The insulin hexamer is resistant to degradation and fibrillation, which makes it an important quaternary structure for its in vivo storage in Zn2+- and Ca2+-rich vesicles in the pancreas and for pharmaceutical formulations. In addition to the two Zn2+ ions that are required for its formation, three other species, Zn-coordinating anions (e.g., Cl), Ca2+, and phenols (e.g., resorcinol), bind to the hexamer and affect the subunit conformation and stability. The contributions of these four species to the thermodynamics of insulin unfolding have been quantified by differential scanning calorimetry and thermal unfolding measurements to determine the extent and nature of their stabilization of the insulin hexamer. Both Zn2+ and resorcinol make a significant enthalpic contribution, while Ca2+ primarily affects the protein heat capacity (solvation) by its interactions in the central cation-binding cavity, which is modulated by the surrounding subunit conformations. Coordinating anions have a negligible effect on the stability of the hexamer, even though subunits shift to an alternate conformation when these anions bind to the Zn2+ ions. Finally, Zn2+ in excess of the two that are required to form the hexamer further stabilizes the protein by additional enthalpic contributions.