THE 2-ZINC INSULIN HEXAMER IS A CALCIUM-BINDING PROTEIN

Publisher Summary This chapter outlines X-ray diffraction structures for the 3Cd-insulin hexamer (at 1.9 A spacing) the 5Pb-insulin hexamer (at 2.5 A spacing) and the metal-free insulin hexamer (at 2.5 A spacing). Most spectra shown are the average of 200 to 400 transients collected over a time period of 20 to 40 m. The electron density section taken through the His(B10) site indicates a distorted octahedral ligand field, consisting of three histidyl imidazolyl nitrogens and three water molecules. Other changes in the aromatic region occur upon assembly of the metal-substituted insulin hexamers. The changes because of assembly involve perturbation of the environments of Phe and Tyr residues as the dimer-dimer interfaces are formed. Binding of metal ions to the Glu(B13) site stabilizes the insulin hexamer and causes a conformational perturbation that influences the microenvironments of distant residues along the dimer-dimer interfaces.