Receptors are integral membrane proteins that serve as the binding sites for ligands, which are molecules that mediate signal transduction between cells. Ligand-receptor interactions can be either reversible or irreversible, depending on the type of receptor involved. Receptors can be classified according to their structure and function. Structural classification is based on the number of subunits that compose the receptor protein, while functional classification is based on the type of signaling pathway that is activated by ligand binding.
The most common type of receptors are G protein-coupled receptors (GPCRs), which activate heterotrimeric G proteins in response to ligand binding. GPCRs are found in all cell types and play a role in a wide variety of physiological processes, including vision, taste, smell, blood pressure regulation, and neurotransmission. The next most common type of receptors are ionotropic receptors, which are composed of a ligand-binding domain and an ion channel. Ionotropic receptors are found in both excitable and nonexcitable cells and mediate fast signaling between cells. Examples include voltage-gated ion channels and ligand-gated ion channels.
Other types of receptors include enzyme-linked receptors (which activate enzymes in response to ligand binding) and nuclear hormone receptors (which regulate gene transcription in response to hormones). Receptors can also be classified according to their location within the cell. Membrane-bound receptors are found on the cell surface or within intracellular membranes, while soluble receptors circulate in the blood or extracellular fluid spaces. Finally, some receptors exist as part of multi-protein complexes within cells (e.g., Toll-like receptor complexes).
Receptor function is often regulated by post-translational modifications, such as phosphorylation or glycosylation. These modifications can alter receptor affinity for ligands or affect downstream signaling pathways . Receptor trafficking is another important mechanism for regulating receptor function; this process determines where in the cell a particular receptor will be located (e..g., at the cell surface or intracellularly) and how long it will remain there . Receptor density can also be regulated through endocytosis , whereby plasma membrane -bound receptors are internalized into endosomes following ligand binding .