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  • Title: Spectroscopic evidence for preexisting T- and R-state insulin hexamer conformations.
    Author: Choi WE, Borchardt D, Kaarsholm NC, Brzovic PS, Dunn MF.
    Journal: Proteins; 1996 Dec; 26(4):377-90. PubMed ID: 8990494.
    Abstract:
    The insulin hexamer is an allosteric protein exhibiting both positive and negative cooperative homotropic interactions and positive cooperative heterotropic interactions (C. R. Bloom et al., J. Mol. Biol. 245, 324-330, 1995). In this study, detailed spectroscopic analyses of the UV/Vis absorbance spectra of the Co(II)-substituted human insulin hexamer and the 1H NMR spectra of the Zn(II)-substituted hexamer have been carried out under a variety of ligation conditions to test the applicability of the sequential (KNF) and the half-site reactivity (SMB) models for allostery. Through spectral decomposition of the characteristic d-->d transitions of the octahedral Co(II)-T-state and tetrahedral Co(II)-R-state species, and analysis of the 1H NMR spectra of T- and R-state species, these studies establish the presence of preexisting T- and R-state protein conformations in the absence of ligands for the phenolic pockets. The demonstration of preexisting R-state species with unoccupied sites is incompatible with the principles upon which the KNF model is based. However, the SMB model requires preexisting T- and R-states. This feature, and the symmetry constraints of the SMB model make it appropriate for describing the allosteric properties of the insulin hexamer.
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