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Receptor (biochemistry)

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In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand) such as a neurotransmitter or a hormone or other substance, and initiates the cellular response to the ligand. Ligand-induced changes in the behavior of receptor proteins result in physiological changes that constitute the biological actions of the ligands.

Overview Edit

The shapes and actions of receptors are newly investigated by the X-ray crystallography and computer modelling. This increases the current understanding of drug action at binding sites on the receptors.

Transmembrane receptor

Transmembrane receptor:E=extracellular space; I=intracellular space; P=plasma membrane

Receptors exist in different types, dependent on their ligand and function:

Binding and activation Edit

Ligand binding to a receptor is an equilibrium process: Ligands bind to an empty receptor and they dissociate from it (according to the law of mass action):

:\left[\mathrm{Ligand}\right] \cdot \left[\mathrm{Receptor}\right]\;\;\overset{ K_d}{\rightleftharpoons}\;\;\left[\mbox{Ligand-receptor complex}\right]
(the brackets stand for concentrations)

A measure of how well a certain molecule fits into a given receptor is the binding affinity which is measured as the dissociation constant Kd (good fit means high affinity and a low Kd). The activation of the second messenger cascade and the final biological response is achieved only when at a certain time point a significant number of receptors are activated by bound ligands.

If the receptor exists in two states (see this picture), then the ligand binding must account for these two receptor states. For a more detailed discussion of two-state binding, which is thought to occur as an activation mechanism in many receptors see this link.

Constitutive activity

Receptors which are active in the absence of a ligand. The constitutive activity of these receptors may be reversed by inverse agonist binding. Mutations in receptors that result in increased constitutive receptor signaling underlie some heritable diseases, such as precocious puberty (luteinizing hormone receptor mutations) and hyperthyroidism (thyroid-stimulating hormone receptor mutations). Psychostimulants act as inverse agonists at dopamine receptors.

For the use of statistical mechanics in a quantitative study of the ligand-receptor binding affinity, see the comprehensive article[1].

Agonists versus antagonists Edit

Not every ligand that binds to a receptor also activates the receptor. The following classes of ligands exist:

  • (Full) agonists are able to activate the receptor and result in a maximal biological response. Most natural ligands are full agonists
  • Partial agonists are not able to activate the receptor maximally, resulting in a partial biological response compared to a full agonist.
  • Antagonists bind to the receptor but do not activate it. This results in a receptor blockade that inhibits the binding of agonists.
  • Inverse agonists are antagonists that are able to further reduce the receptor activation by decreasing its basal activity

Peripheral membrane protein receptors Edit

Transmembrane receptorsEdit

Main article: Transmembrane receptor

Metabotropic receptors Edit

Main article: Metabotropic receptor

G protein-coupled receptors Edit

Main article: G protein-coupled receptor

These receptors are also known as seven transmembrane receptors or 7TM receptors.

Receptor tyrosine kinases Edit

Main article: Receptor tyrosine kinase

These receptors detect ligands and propagate signals via the tyrosine kinase of their intracellular domains. This family of receptors includes;

Guanylyl cyclase receptors Edit

Ionotropic receptors Edit

Ionotropic receptors are heteromeric or homomeric oligomers [2]. They are receptors that respond to extracellular ligands and receptors that respond to intracellular ligands.

Extracellular ligandsEdit

Receptor Ligand Ion current
Nicotinic acetylcholine receptor Acetylcholine, Nicotine Na+, K+, Ca2+ [2]
Glycine receptor (GlyR) Glycine, Strychnine Cl- > HCO-3 [2]
GABA receptors: GABA-A, GABA-C GABA Cl- > HCO-3 [2]
Glutamate receptors: NMDA receptor, AMPA receptor, and Kainate receptor Glutamate Na+, K+, Ca2+ [2]
5-HT3 receptor Serotonin Na+, K+ [2]
P2X receptors ATP Ca2+, Na+, Mg2+ [2]

Intracellular ligandsEdit

Receptor Ligand Ion current
cyclic nucleotide-gated ion channels cGMP (vision), cAMP and cGTP (olfaction) Na+, K+ [2]
IP3 receptor IP3 Ca2+ [2]
Intracellular ATP receptors ATP (closes channel)[2] K+ [2]
Ryanodine receptor Ca2+ Ca2+ [2]

The entire repertoire of human plasma membrane receptors is listed at the Human Plasma Membrane Receptome (http://receptome.stanford.edu).

Intracellular receptorsEdit

Main article: Intracellular receptor

Transcription factorsEdit

VariousEdit

Role in Genetic DisordersEdit

Many genetic disorders involve hereditary defects in receptor genes. Often, it is hard to determine whether the receptor is nonfunctional or the hormone is produced at decreased level; this gives rise to the "pseudo-hypo-" group of endocrine disorders, where there appears to be a decreased hormonal level while in fact it is the receptor that is not responding sufficiently to the hormone.

Receptor RegulationEdit

Cells can increase (upregulate) or decrease (downregulate) the number of receptors to a given hormone or neurotransmitter to alter its sensitivity to this molecule. This is a locally acting feedback mechanism.

Receptor desensitization

Ligand-bound desensitation of receptors was first characterized by Katz and Thesleff in the nicotine acetylcholine receptor[4][5] Prolonged or repeat exposure to a stimulus often results in decreased responsiveness of that receptor for a stimulus. Receptor desensitization results in altered affinity for the ligand.[4] Receptor desensitization can modeled by a two-state model that also predicts that antagonists combined with agonists can prevent receptor desensitization [6] See this link [1] for detailed molecular description

Desensitation may be accomplished by

  • Receptor phosphorylation.[7]
  • Uncoupling of receptor effector molecules.
  • Receptor sequestration (internalization).[7]

See also Edit

ReferencesEdit

  1. Vu-Quoc, L., Configuration integral (statistical mechanics), 2008.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Medical Physiology, Boron & Boulpaep, ISBN 1-4160-2328-3, Elsevier Saunders 2005. Updated edition. Page 90.
  3. Gobeil F, et al. (2006) G-protein-coupled receptors signalling at the cell nucleus: an emerging paradigm. Can J Physiol Pharmacol. 2006 Mar-Apr;84(3-4):287-97. PMID 16902576
  4. 4.0 4.1 Y. Sun, R. Olson, M. Horning, N. Armstrong, M. Mayer and E. Gouaux. (2002) Mechanism of glutamate receptor desensitization Nature 417, 245-253
  5. S. Pitchford, J.W. Day, A. Gordon and D. Mochly-Rosen. (1992) Acetylcholine receptor desensitization is Regulate by activation-induced extracellular adenosine accumulation. The Journal of Neuroscience, 1.311): 4540-4544.
  6. Lanzara, "Optimal Agonist/Antagonist Combinations Maintain Receptor Response by Preventing Rapid Beta-1 adrenergic Receptor Desensitization" Intl. J. Pharmacol., 1(2): 122-131, 2005.[http://www.bio-balance.com/ijp.pdf
  7. 7.0 7.1 G. Boulay, L. Chrbtien, D.E. Richard, AND G. Guillemettes. (1994) Short-Term Desensitization of the Angiotensin II Receptor of Bovine Adrenal Glomerulosa Cells Corresponds to a Shift from a High to a Low Affinity State. Endocrinology Vol. 135. No. 5 2130-2136

External linksEdit

Template:Receptor tyrosine kinases


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