Enkephalin

An enkephalin is a pentapeptide involved in regulating nociception in the body. The enkephalins are termed endogenous ligands, or specifically endorphins, as they are internally derived and bind to the body's opioid receptors. Discovered in 1975, two forms of enkephalin were revealed, one containing leucine ("leu"), and the other containing methionine ("met"). Both are products of the proenkephalin gene.


 * Met-enkephalin is Tyr-Gly-Gly-Phe-Met.
 * Leu-enkephalin has Tyr-Gly-Gly-Phe-Leu.

Endogenous opioid peptides
There are three well-characterized families of opioid peptides produced by the body: enkephalins, endorphins, and dynorphins. The met-enkephalin peptide sequence is coded for by both the enkephalin gene and the endorphin gene (also known as the POMC gene); the leu-enkephalin peptide sequence is coded for by both the enkephalin gene and the dynorphin gene.

Enkephalin receptor
The receptors for enkephalin are the opioid receptors (epsilon receptor). These are a group of G-protein-coupled receptors, with other opioids as ligands as well. The other endogenous opioids are dynorphins (that bind to kappa receptors), endorphins (mu receptors), endomorphins, and nociceptin/orphanin FQ. The opioid receptors are ~40% identical to somatostatin receptors (SSTRs).