CHEMICAL BONDS OF THE INSULIN HEXAMER Insulin is a polypeptide hormone which, roughly speaking, enables the body to use sugar. When the level of sugar in the blood rises, insulin is secreted into the blood. It has two actions. It increases the rate at which the sugar is withdrawn from the blood and passed into the tissues. It also decreases the rate at which sugar is added to the blood by the liver. The molecule is hexameric, having six units arranged around a central zinc atom. The rotatable line-drawing shows the chemical bonds in the structure. The position of the atoms are inferred from the bonds. The picture gives an indication of the complexity of the structures with which protein chemist deal.