G protein-coupled receptors (GPCRs) are a large family of cell membrane proteins that play a crucial role in signal transduction. They are involved in many different physiological functions and are targeted by a large percentage of currently marketed drugs, including those used to treat conditions such as hypertension, asthma, schizophrenia, and Parkinson's disease.
The basic structure of a GPCR includes seven transmembrane alpha helices, an extracellular N terminus, and an intracellular C terminus. They are called "G protein-coupled" because when they bind to their ligand (such as a hormone or neurotransmitter) on the outside of the cell, they activate an intracellular G protein. This G protein then triggers a series of downstream events inside the cell, leading to the cell's response to the original signal.
There are several different types of G proteins, and the specific G protein that a GPCR interacts with determines the downstream effects. For example, some G proteins activate enzymes that increase the production of cyclic AMP (cAMP), a common second messenger, while others inhibit this process. Other G proteins activate different signaling pathways altogether.
The GPCR family is incredibly diverse, with receptors for a wide variety of signals. These include light-sensitive receptors in the retina, smell receptors in the nose, and a wide range of hormone and neurotransmitter receptors throughout the body. Because of their involvement in so many different physiological processes, they are a major focus of pharmaceutical research.
