receptor

Molecular receptors are usually proteins that have binding sites for neurotransmitters, hormones or growth factors. Neurotransmitters are ligands that bind to neurotransmitter receptors. Any small molecule can function as a ligand that binds tightly to a receptor if the shape of the receptor binding site matches the shape of the ligand in a lock-and-key fashion.

Example. Cardiac glycosides are ligands that bind to a specific receptor protein in cells. The receptor for cardiac glycosides is the N⁺.K⁺ pump protein. When cardiac glycosides bind to the pump, the activity of the pump is inhibited. In general, ligand binding to receptor causes a conformation (shape) change in the receptor protein that changes the physiological function of the cell.