Figure 6.Assembly and disassembly of the 2-Zn insulin hexamer. The A-chain of insulin is illustrated as a thickened black line, and the B-chain as a thinner line. As explained in the text, monomers assemble into dimers as the concentration of protein increases, and into hexamers in the presence of divalent cations such as zinc. The side chains of the six HisB10 residues are shown coordinating with the zinc ions in the hexamer. The pathway is reversed when the hexamer is secreted from the ï¢-cells and diffuses down its concentration gradient into the blood.