Carboxylate Ions Are Strong Allosteric Ligands for the HisB10 Sites of the R-State Insulin Hexamer

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Abstract
The insulin hexamer is an allosteric protein which displays positive and negative cooperativity and half-site reactivity that is modulated by strong homotropic and heterotropic ligand binding interactions at two different loci. These loci consist of phenolic pockets situated on the dimer−dimer interfaces of T−R and R−R subunit pairs and of anion sites comprising the HisB10 metal ion sites of the R3 units of the T3R3 and R6 states. In this study, we show that suitably tailored organic carboxylates are strong allosteric effectors with relatively high affinities for the R-state HisB10 metal sites. Methods of quantifying the relative affinities of ligands for these sites in both Co(II)- and Zn(II)-substituted insulin hexamers are presented. These analyses show that, in addition to the electron density on the ion, the carboxylate affinity is influenced by polar, nonpolar, and hydrophobic interactions between substituents on the carboxylate and the amphipathic protein surface of the narrow tunnel which controls ligand access to the metal ion. Since the binding of anions to the HisB10 site makes a critically important contribution to the stability of the T3R3 and R6 forms of the insulin hexamer, the design of high-affinity ligands with a carboxylate donor for coordination to the metal ion provides an opportunity for constructing insulin formulations with improved pharmaceutical properties.