A ligand is a signal-triggering molecule that fits to specific receptor protein sites located on cell walls in the living organisms body. When a ligand fits into its specific protein site, it causes the protein's physical shape to change. This change in the receptor protein’s shape might then activate or inhibit another biological mechanism linked to that specific interaction. The manner in which the ligand and protein fit together and their resultant product are called protein-ligand interactions.
Protein-ligand interactions are vastly complicated. All molecules are held together by strong electronic forces that surround them in their three-dimensional configuration in space. These forces can repel or attract other molecules, depending on their electronic structure. Biological molecules often act like magnets of either like charges or opposite charges, repelling one another or attracting each other.
In addition, the actual physical configuration of the biological molecules affect whether they will fit together and how precise the fit must be. Some biological molecules are flexible and will bend in their environments, and others are more rigid. Each receptor also has an active site, which is the specific spot that triggers the biological change. The location of this active site is important in determining how likely it will be to activate. Ligands and proteins are interacting while floating around on the surfaces of cells in the body, so the fit will not always be straight on, like a key fits into a lock.
Understanding protein-ligand interactions is vital for the creation of novel therapeutic drugs to treat disease, as well as for the understanding of the biological causes of the disease itself. For the researcher interested in ligand-protein interactions for drug development, additional factors must be considered. Some such considerations include the study of how effective the novel drug will be in binding to the receptor protein of interest, potential side effects and the mechanism by which the body will break down the drug.
There are numerous chemical and computer methods available for studying these interactions, all of which are based on finding the complimentary ligand shape and charge to the receptor that the researcher wants to behaviorally modify. Computer-aided design programs are commonly used to study protein-ligand interactions and develop novel drugs for treating human diseases. This is possible because many physical and chemical traits of molecules are well known, such as their electronic structures.