CPK MODEL OF LYSOZYME WITH SUBSTRATE Lysozyme is an anti-bacterial enzyme found in tears and saliva. It dissolves bacterial cells in the human body. The image shows a space-filling model of lysozyme with a substrate (another molecule reacting with it) shown in 'ball and stick' form. 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.