Author: Prof. Dr. med. Peter Altmeyer

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Last updated on: 29.10.2020

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Receptors (from lat. recipere = to take up or receive) belong on a molecular level to a family of cellular proteins or protein complexes whose task it is to mediate the effects of the body's own signal substances. Such receptors are also used, for example, by pharmaceuticals, which can act agonistically, i.e. activating receptors, or antagonistically, inhibiting receptors.

In principle, receptors have 2 functions:

  • Binding of the signalling substance generally according to a lock-and-key principle
  • Initiation of signals that stimulate or inhibit cellular functions.

Receptors protrude from the surface of a biomembrane, if membrane-bound (membrane-bound receptors), or they are positioned inside the cell (intracellular receptors). After their activation, they trigger specific signalling processes.

Thus, a distinction is made at the molecular level:

  • membrane-bound receptors
  • and
  • intracellular receptors (steroid receptors, receptors for vitamin D, thyroid hormones, retinoid receptors)

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Membrane receptors: Membrane receptors are located on the surface of biomembranes (e.g. on cell membranes or on membranes of cell organelles). They consist of proteins, which often have further modifications (e.g. carbohydrate chains). Membrane receptors have a specific fit for small molecules (ligands), or for parts of larger molecules that dock to a specific receptor structure according to the lock-and-key principle. Membrane receptors are used for cell adhesion and/or signal transmission or for the import of substances into the cell. Membrane receptors are also used as docking stations for viruses to enter a host cell.

The following membrane receptors are distinguished (see figure):

  • G-protein coupled receptors
  • Ion channel receptors
  • enzyme receptors
  • Receptors with associated tyrosine kinase

G-protein coupled receptors: There are a variety of G-protein coupled receptors (G-protein stands for guanosine triphosphate binding protein). In contrast to ion channel receptors, G-protein coupled receptors do not form channels or pores, but activate a downstream G-protein or protein kinase when their ligand binds. In this way, they modulate an intracellular signalling cascade by altering the concentration of secondary messenger substances. However, the membrane permeability can also be indirectly altered.

The signal transduction of the G-coupled receptors takes place in 4 phases (see also G-protein-coupled receptors):

  • The agonist triggers a conformational change by binding to the receptor.
  • This conformational change activates the intracellularly associated G-protein.
  • The G-protein breaks down into 2 protein subunits (Gβƴ and Gα), which can increase and decrease the concentration of intracellular messengers (second messengers).
  • The signal ends by hydrolysis of GTP (the alpha subunit has GTPase activity). The G protein returns to its original inactive state.

Examples of G-protein-coupled receptors:

  • G5-associated receptors: The binding of an agonist to the receptor stimulates adenylate cyclase via the alpha subunit of the G5 protein. This leads to an increase in the synthesis of cyclic adenosine 3,5-monophosphate (cAMP). The increase in cAMP leads to an activation of the cAMP-dependent protein kinase A (PKA). PKA substrates are e.g. the L-type -Ca2+ channels in heart muscle cells.
  • Enzymes of the fat and glycogen metabolism
  • Myokinase in smooth muscle cells.
  • Gi/o-protein-associated receptors: activation of this receptor type by an agonist leads to inhibition of the alpha subunit of the Gi/o proteins, the adenyl cyclase, thereby decreasing the synthesis of cAMP. A subunit of this G protein can control several effectors, partly blocking and partly activating.

Ionchannel receptors (ionotropic receptors) These are "very fast receptors" that are effective within milliseconds. This type of receptors has a direct influence on ion channels. Ion channel and receptor are not structurally separate units, but are part of one and the same protein complex. The associated protein complex(see exemplary figure of the nicotinic acetylcholine receptor of the motor end plate) thus comprises the ion channel and the receptor. The ion channel-receptor complex usually consists of 5 subunits which penetrate the cell membrane. They are arranged so that their alpha-helical structure itself or associated channel-forming domains enclose a central pore (membrane channel). The binding site for the endogenous receptor agonist is located extracellularly at one of the channel subunits. Examples of inotropic receptors are the nicotinic acetylcholine receptor, the GABAA receptor whose ligand is gamma-aminobutyric acid. The binding site for the agonist(acetylcholine in the picture) is located extracellularly at one of the channel subunits. When the agonist binds to its receptor, acetylcholine leads to a lightning influx of Na+ ions and an outflow of K+ ions. The activation of the GABA receptor leads to an influx of chloride ions into the cell, to its hyperpolarisation and to a decrease in cellular excitability).

Another important ion channel receptor is the serotonin receptor of type 5-HT3, which causes cellular excitation by increasing the transmembrane conductivity for Na+ and K+.

Enzyme receptors: Enzyme receptors are by definition receptors with "inherent" enzyme activity(see figure with schematic representation). This type of receptor (typical representatives are receptors for natriuretic peptides) binds the agonist with its extracellular n domain. The intracellular domain has an "inherent" enzyme activity, namely guanylate cyclase activity(inherent receptor). The stimulation of this membrane-bound guanylate cyclase leads to the formation of the second messenger cGMP. CGMP-dependent protein kinases (PKG) posphorylate in various ways in smooth muscle cells. protein substrates and thus cause the smooth muscle cells to relax).

Receptors with associated tyrosinase activity: Receptors for erythropoietin, interferons and other cytokines and insulin have no "inherent enzyme activity". They bind their agonist and then secondarily activate a tyrosine kinase associated with the receptor protein, which tyrosine phosphorylates the receptor itself and other intracellular substrates. This creates binding sites for IRS (insulin receptor substrates) which, bound to effector molecules via the small G-protein Ras, activate the protein kinases B and C, which are responsible for the insulin-related changes in carbohydrate, lipid and protein metabolism.

Intracellular receptors: Steroid hormones, thyroid hormones, vitamin D and retinoids are so lipophilic that they can penetrate cell membranes. They can therefore dock to intracellular receptors, which are bound to activating proteins in the cytosol. Docking of the agonist to its receptor initially leads to the detachment of inactivating proteins. The agonist-receptor complex then docks to another agonist-receptor complex and enters the cell nucleus as a dimer. There, this complex binds to specific DNA sequences and promotes or inhibits the transcription of certain target genes.

Receptor-like proteins: This term covers cellular proteins that do not mediate the effects of transmitters, hormones or cytokines, but have other tasks in the cell. These include:

  • enzymes (Na+, K+ or ATPases)
  • transporters (e.g. the neuronal norepinephrine or serotonin transporters blocked by certain antidepressants)
  • Cellular structural proteins (e.g. microtubils blocked by vinca alkaloids or taxanes)

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The term receptor used to be much broader. Receptors also included sensors such as motion sensors, strain sensors, pressure sensors, colour sensors, moisture sensors, joint sensors, hair follicle sensors, pressosensors. This has changed in recent years.

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Last updated on: 29.10.2020