Zinc–Ligand Interactions Modulate Assembly and Stability of the Insulin Hexamer – A Review

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Abstract
Zinc and calcium ions play important roles in the biosynthesis and storage of insulin. Insulin biosynthesis occurs within the β-cells of the pancreas via preproinsulin and proinsulin precursors. In the golgi apparatus, proinsulin is sequestered within Zn2+- and Ca2+-rich storage/secretory vesicles and assembled into a Zn2+ and Ca2+ containing hexameric species, (Zn2+)2(Ca2+)(Proin)6. In the vesicle, (Zn2+)2(Ca2+)(Proin)6 is converted to the insulin hexamer, (Zn2+)2(Ca2+)(In)6, by excision of the C-peptide through the action of proteolytic enzymes. The conversion of (Zn2+)2(Ca2+)(Proin)6to (Zn2+)2(Ca2+)(In)6 significantly lowers the solubility of the hexamer, causing crystallization within the vesicle. The (Zn2+)2(Ca2+)(In)6 hexamer is an allosteric protein that undergoes ligand-mediated interconversion among three global conformation states designated T6, T3R3 and R6. Two classes of allosteric sites have been identified; hydrophobic pockets (3 in T3R3 and 6 in R6) that bind phenolic ligands, and anion sites (1 in T3R3 and 2 in R6) that bind monovalent anions. The allosteric states differ widely with respect to the physical and chemical stability of the insulin subunits. Fusion of the vesicle with the plasma membrane results in the expulsion of the insulin crystals into the intercellular fluid. Dissolution of the crystals, dissociation of the hexamers to monomer and transport of monomers to the liver and other tissues then occurs via the blood stream. Insulin action then follows binding to the insulin receptors. The role of Zn2+ in the assembly, structure, allosteric properties, and dynamic behavior of the insulin hexamer will be discussed in relation to biological function.