An enkephalin is a pentapeptide involved in regulating

nociception In physiology, nociception , also nocioception; ) is the Somatosensory system, sensory nervous system's process of encoding Noxious stimulus, noxious stimuli. It deals with a series of events and processes required for an organism to receive a pai ...

(pain sensation) in the body. The enkephalins are termed endogenous

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s, as they are internally derived (and therefore

endogenous Endogeny, in biology, refers to the property of originating or developing from within an organism, tissue, or cell. For example, ''endogenous substances'', and ''endogenous processes'' are those that originate within a living system (e.g. an ...

) and bind as

s to the body's

opioid receptor Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. The opioid receptors are ~40% identical to somatostati ...

s. Discovered in 1975, two forms of enkephalin have been found, one containing

leucine Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α-Car ...

("leu"), and the other containing

methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other non-essential amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine play ...

("met"). Both are products of the proenkephalin

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

. *

Met-enkephalin Met-enkephalin, also known as metenkefalin ( INN), sometimes referred to as opioid growth factor (OGF), is a naturally occurring, endogenous opioid peptide that has opioid effects of a relatively short duration. It is one of the two forms of enkep ...

is Tyr-Gly-Gly-Phe-Met. *

Leu-enkephalin Leu-enkephalin is an endogenous opioid peptide neurotransmitter with the amino acid sequence Tyr-Gly-Gly-Phe-Leu that is found naturally in the brains of many animals, including humans. It is one of the two forms of enkephalin; the other is met ...

is Tyr-Gly-Gly-Phe-Leu.

Endogenous opioid peptides

There are three well-characterized families of opioid peptides produced by the body: enkephalins, β-endorphin, and dynorphins. The met-enkephalin peptide sequence is coded for by the enkephalin gene; the leu-enkephalin peptide sequence is coded for by both the enkephalin gene and the dynorphin gene. The proopiomelanocortin gene (

POMC Pro-opiomelanocortin (POMC) is a precursor polypeptide with 241 amino acid residues. POMC is synthesized in corticotrophs of the anterior pituitary from the 267-amino-acid-long polypeptide precursor pre-pro-opiomelanocortin (pre-POMC), by the ...

) also contains the met-enkephalin sequence on the N-terminus of beta-endorphin, but the endorphin peptide is not processed into enkephalin.

Effects on stress

Enkephalin is also considered a

neuropeptide Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like the ...

, which in the human body performs as an important signaling molecule in the brain. Enkephalins are found in high concentration in the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

as well as in the cells of

adrenal medulla The adrenal medulla () is the inner part of the adrenal gland. It is located at the center of the gland, being surrounded by the adrenal cortex. It is the innermost part of the adrenal gland, consisting of chromaffin cells that secrete catecho ...

. In response to pain,

norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...

, a

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

that is activated in fight-or-flight response, is released along with

endorphins Endorphins (contracted from endogenous morphine) are peptides produced in the brain that block the perception of pain and increase feelings of wellbeing. They are produced and stored in the pituitary gland of the brain. Endorphins are endogeno ...

. A 2017 study indicates that this polypeptide may be linked to brain functioning during the stress response, especially in the

hippocampus The hippocampus (: hippocampi; via Latin from Ancient Greek, Greek , 'seahorse'), also hippocampus proper, is a major component of the brain of humans and many other vertebrates. In the human brain the hippocampus, the dentate gyrus, and the ...

and

prefrontal cortex In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. It is the association cortex in the frontal lobe. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, ...

. This research has suggested that, as part of the stress response, several met-enkephalin analogs have increased activity in the

, while leu-enkephalin analogs as well as

somatostatin Somatostatin, also known as growth hormone-inhibiting hormone (GHIH) or by #Nomenclature, several other names, is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G ...

s are downregulated during stress. Stressors may impact neuropeptides whose action is localized to a specific brain region.

Enkephalin receptor

The receptors for enkephalin are the

delta opioid receptor Delta commonly refers to: * Delta (letter) (Δ or δ), the fourth letter of the Greek alphabet * D (NATO phonetic alphabet: "Delta"), the fourth letter in the Latin alphabet * River delta, at a river mouth * Delta Air Lines, a major US carrier ...

s and mu opioid receptors. Opioid receptors are a group of

G-protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large protein family, group of evoluti ...

s, with other

opioid Opioids are a class of Drug, drugs that derive from, or mimic, natural substances found in the Papaver somniferum, opium poppy plant. Opioids work on opioid receptors in the brain and other organs to produce a variety of morphine-like effects, ...

s as

s as well. The other

opioids Opioids are a class of Drug, drugs that derive from, or mimic, natural substances found in the Papaver somniferum, opium poppy plant. Opioids work on opioid receptors in the brain and other organs to produce a variety of morphine-like effects, ...

are dynorphins (that bind to kappa receptors),

endorphin Endorphins (contracted from endogenous morphine) are peptides produced in the brain that block the perception of pain and increase feelings of wellbeing. They are produced and stored in the pituitary gland of the brain. Endorphins are endogeno ...

s (mu receptors), endomorphins, and nociceptin-orphanin FQ. The opioid receptors are ~40% identical to

receptor Receptor may refer to: * Sensory receptor, in physiology, any neurite structure that, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and respond ...

s (SSTRs).

References

External links

* * {{Chocolate Opioid peptides Pentapeptides

Endogenous opioid peptides

Effects on stress

Enkephalin receptor

See also

References

External links