Receptors are proteins that receive and interpret information carried by signaling molecules. Regardless of the distance between communicating cells, a message is received by a responding cell when the signaling molecule binds to a receptor protein on or in the responding cell. For this reason, the signaling molecule is often referred to as a ligand (from the Latin ligare, which means “to bind”). The signaling molecule binds to a specific part of the receptor protein called the ligand-binding site. The bond is noncovalent and highly specific: The signaling molecule binds only to a receptor with a ligand-binding site that recognizes the molecule.

Almost without exception, the binding of a signaling molecule to the ligand-binding site of a receptor causes a conformational change in the receptor. We say that the conformational change “activates” the receptor because it is through this change that the receptor passes the message from the signaling molecule to the interior of the cell. In many ways, this change in receptor shape is similar to the change that occurs when a substrate binds to the active site of an enzyme (Chapter 6). The conformational change in the receptor ultimately triggers chemical reactions or other changes in the cytosol, and is therefore a crucial step in the reception and interpretation of communications from other cells.