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In biochemistry and pharmacology, a receptor is a protein molecule, that receives chemical signals from outside the cell. When such chemical signals bind to a receptor, they cause some form of cellular/tissue response, e.g. a change in the electrical activity of the cell. In this sense, a receptor is a protein molecule that recognises and responds to endogenous chemical signals, e.g. the acetylcholine receptor recognizes and responds to its endogenous ligand, acetylcholine. However, sometimes in pharmacology, the term is also used to include other proteins that are drug targets, such as enzymes, transporters and ion channels. Receptor proteins are embedded in the cell's plasmatic membranes; facing extracellular(cell surface receptors), cytoplasmic (cytoplasmic receptors), or in the nucleus (nuclear receptors). A molecule that binds to a receptor is called a ligand, and can be a peptide (short protein) or another small molecule such as a neurotransmitter, hormone, pharmaceutical drug, toxin, or parts of the outside of a virus or microbe. The endogenously designated molecule for a particular receptor is referred to as its endogenous ligand. E.g. the endogenous ligand for the nicotinic acetylcholine receptor is acetylcholine but the receptor can also be activated by nicotine and blocked by curare. Each receptor is linked to a specific cellular biochemical pathway. While numerous receptors are found in most cells, each receptor will only bind with ligands of a particular structure, much like how locks will only accept specifically shaped keys. When a ligand binds to its corresponding receptor, it activates or inhibits the receptor's associated biochemical pathway. == Structure == The structures of receptors are very diverse and can broadly be classified into the following categories: * Type 1: L (ionotropic receptors)– These receptors are typically the targets of fast neurotransmitters such as acetylcholine (nicotinic) and GABA and activation of these receptor results in changes in ion movement across the membrane. They have a hetero structure. Each subunit consists of the extracellular ligand-binding domain and a transmembrane domain where the transmembrane domain in turn includes four transmembrane alpha helixes. The ligand binding cavities are located at the interface between the subunits. * Type 2: G protein-coupled receptors (metabotropic) – This is the largest family of receptors and includes the receptors for several hormones and slow transmitters e.g. dopamine, metabotropic glutamate. They are composed of seven transmembrane alpha helices. The loops connecting the alpha helices form extracellular and intracellular domains. The binding site for larger peptidic ligands is usually located in the extracellular domain whereas the binding site for smaller non-peptidic ligands is often located between the seven alpha helices and one extracellular loop. These receptors are coupled to different intracellular effector systems via G-proteins. * Type 3: kinase linked and related receptors (see "Receptor tyrosine kinase", and "Enzyme-linked receptor") - These receptors are composed of an extracellular domain containing the ligand binding site and an intracellular domain, often with enzymatic function, linked by a single transmembrane alpha helix. e.g. the insulin receptor. * Type 4: nuclear receptors – While they are called nuclear receptors, these are actually located in the cytosol and migrate to the nucleus after binding with their ligands. They are composed of a C-terminal ligand binding region, a core DNA-binding domain (DBD) and an N-terminal domain that contains the ''AF1''(activation function 1) region. The core region has two zinc fingers that are responsible for recognising the DNA sequences specific to this receptor. The N-terminal interacts with other cellular transcription factors in a ligand independent manner and depending on these interactions it can modify the binding/activity of the receptor. Steroid and thyroid hormone receptors are examples of such receptors. Membrane receptors may be isolated from cell membranes by complex extraction procedures using solvents, detergents, and/or affinity purification. The structures and actions of receptors may be studied by using biophysical methods such as X-ray crystallography, NMR, circular dichroism, and dual polarisation interferometry. Computer simulations of the dynamic behavior of receptors have been used to gain understanding of their mechanism of action. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Receptor (biochemistry)」の詳細全文を読む スポンサード リンク
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