Mechanisms of disease
3. Targets for action of medicines
The main target is ‘receptors. These are protein molecules found on the surface of the cell. They are often the first stage of the molecular processes, which lead to reactions within the cell. These reactions may cause symptoms of a disease. See Figure 5. A receptor sits on the surface of a cell. It receives a chemical signal when another molecule called the ‘ligand’ binds to it. A ligand is a small molecule that triggers signals by binding to receptors.
A medicine behaves like a ligand. Binding only takes place when the two molecules fit in each other like ‘the key to a lock’. The binding of the ligand causes the receptor to send a message to other molecules inside the cell - causing the cell to start working. It can be a chemical reaction like secretion, a mechanical reaction, such as a change of cell shape. This might in turn bring about cell division or a movement (displacement) of ions causing an electric impulse from that cell (called an action potential).
By interacting with the first stage of the cascade reaction (i.e. the receptor), it is possible to change the end process. When medicines are used, they are designed to act on a large number of molecules with a minimum amount of medicine. This is so that a large amount of secretion (e.g. a hormone like insulin or enzymes for digestion like pepsin in the stomach) can be produced.