# Receptor (biochemistry)

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, 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.

## Binding and activation

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):

${\displaystyle \left[Ligand\right]\cdot \left[Receptor\right]{\overset {K_{d}}{\rightleftharpoons }}\left[Ligand-receptorcomplex\right]}$
(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.

## Agonists versus antagonists

Not every ligand that binds to a receptor l 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

## Overview

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

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

## Transmembrane receptors

### Metabotropic receptors

#### G protein-coupled receptors

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

#### Receptor tyrosine kinases

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

### Ionotropic receptors

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

## Role in Genetic Disorders

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 Regulation

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.