Proton fourier transform NMR studies of insulin: coordination of calcium to the Glu(B13) site drives hexamer assembly and induces a conformation change
1H Fourier transform NMR investigations of metal ion binding to insulin in 2H2O were undertaken as a function of pH* to determine the effects of metal ion coordination to the Glu(B13) site on the assembly and structure of the insulin hexamer. The C-2 histidyl regions of the 1H NMR spectra of insulin species containing respectively one Ca2+ and two Zn2+/hexamer and three Cd2+/hexamer have been assigned. Both the Cd2+ derivative (In)6(Cd2+)2Cd2+, where two of the Cd2+ ions are coordinated to the His(B10) sites and the remaining Cd2+ ion is coordinated to the Glu(B13) site [Sudmeier, J.L., Bell, S.J., Storm, M. C., & Dunn, M.F. (1981) Science (Washington, D.C.) 212, 560], and the Zn2+-Ca2+ derivative (In)6-(Zn2+)2Ca2+, where the two Zn2+ ions are coordinated to the His(B10) sites and Ca2+ ion is coordinated to the Glu(B13) site, give spectra in which the C-2 proton resonances of His(B10) are shifted upfield relative to metal-free insulin. Spectra of insulin solutions (3-20 mg/mL) containing a ratio of In:Zn2+ = 6:2 in the pH* region from 8.6 to 10 were found to contain signals both from metal-free insulin species and from the 2Zn-insulin hexamer, (In)6(Zn2+)2. The addition of either Ca2+ (in the ratio In:Zn2+:Ca2+ = 6:2:1) or 40 mM NaSCN was found to provide sufficient additional thermodynamic drive to bring about the nearly complete assembly of insulin hexamers. Cd2+ in the ratio In:Cd2+ = 6:3 also drives hexamer assembly to completion. We postulate that the additional thermodynamic drive provide by Ca2+ and CD2+ is due to coordination of these metal ions to the Glu(B13) carboxylates of the hexamer. At high pH*, this coordination neutralizes the repulsive Coulombic interactions between the six Glu(B13) carboxylates and forms metal ion "cross-links" across the dimer-dimer interfaces. Comparison of the aromatic regions of the 1H NMR spectra for (In)6(Zn2+)2 with (In)6(Zn2+)2Ca2+, (In)6(Cd2+)2Cd2+, and (In)6(Cd2+)2Ca2+ indicates that binding of either Ca2+ or Cd2+ to the Glu(B13) site induces a conformation change that perturbs the environments of the side chains of several of the aromatic residues in the insulin structure. Since these residues lie on the monomer-monomer and dimer-dimer subunit interfaces, we conclude that the conformation change includes small changes in the subunit interfaces that alter the microenvironments of the aromatic rings.