SPACE-FILLING MODEL OF 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 It increases the rate at which the sugar is withdrawn from the blood and passed into the tissues.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 image shows a space-filling model of insulin. In this particular example, electron clouds are represented by spheres of prescribed radii. The spheres are colour coded to distinguish different atoms. Generally, the following colour scheme is used:- nitrogen: blue oxygen: red carbon: white sulphur: yellow hydrogen: grey phosphorus: magenta This type of model is sometimes referred to as a CPK model, being named after the chemists Corey, Pauling and Koltun who first used the technique to display different molecular properties. Each sphere represents those electrons which are spinning around the atomic nucleus as well as those which form an inter-atomic bond. i.e. the electrons are shown delocalised around each atom. The radius of this delocalised electron cloud is sometimes called the Van Der Waal's radius. The molecular surface is generated using constructive solid geometry. The atomic coordinates are maintained in a relational data base.