Heparin Binding

This little video clip that I put together shows how a growth factor binds to its receptor. Heparin mediates the process. The growth factors bind to receptors in pairs with the heparin threaded through all four proteins and holding the complex together. The receptors would be embedded in the membrane at the bottom. This clip starts with a single receptor to which the heparin binds first, followed by the first growth factor.  The basic amino acids, arginine and lysine are shown in blue.  In the first image, the amino acid backbone is shown as a ribbon with blue bars.  In the second, the atoms of the amino acid side chains are filled in and the blue bars become blue atomic balls that reveal the surface of the protein.  The blue areas are positively charged and groups on the surface are heparin-binding domains.  The heparin is shown as a polysaccharide with atoms colored by element, O-red, S-orange, N-blue.  The proteins bind to the heparin like beads on a string.  Pairing of the receptors in a particular orientation determined by the heparin and growth factors, is responsible for triggering the cytoplasmic signal that the growth factor is present.