Types of receptor

Keywords: receptor, kinetics, effector, transducer, signal, enzyme, ion channel, g-protein, g protein, transcription factor, transmembrane domain, antagonist, partial agonist, inverse agonist, agonist, pharmacodynamics, receptor conformation, active, inactive, down-regulation, up-regulation, law of mass-action
Description: The Worldwide Intensivist

Different people have different ideas about what a receptor is. I rather like the definition of Hucho.

Receptors are proteins interacting with extracellular physiological signals and converting them into intracellular effects

This definition is by no means perfect, but seems far more useful than the blurry definition you find in so many pharmacology textbooks, along the lines of 'a receptor is any functional macromolecular component of an organism to which a drug binds'. We should perhaps avoid semi-religious bickering about the 'essential nature' of receptors, and look at how well the definition works for us!

The most important concept is that the receptor:
  1. Receives a signal; and
  2. transduces the signal to
  3. an effector mechanism.
If we accept Hucho's definition, we can identify at least four groups of receptors:
  • Receptors as enzymes: These receptors usually span the cell membrane just once, and in response to binding of a ligand, usually increase the phosphorylation of intracellular proteins, for example the tyrosine residues on vital regulatory components (Occasionally, serine or threonine residues are phosphorylated).
  • Receptors that activate transmembrane ion channels. These channels are called 'ligand-gated ion channels'.

  • Receptors that use as their transducer the 'G-protein'. Often these receptors can couple promiscuously with several different G-proteins, who are in turn not particularly faithful to a single receptor. The end results are effected by the G protein stimulating one of a variety of pathways, for example adenyl cyclases and phospholipases, or even ion channels that permit certain ions to enter or leave the cell.

  • Perhaps the most interesting of the lot - receptors located within the cell (transcription factors) that once bound to the ligand, either increase or decrease DNA transcription, either by binding DNA or by modulating the effects of histones.
  • Here are the major receptor groups in tabular form:

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