An enkephalin is a
pentapeptide involved in regulating
nociception in the body. The enkephalins are termed endogenous
ligand
In coordination chemistry, a ligand is an ion or molecule ( 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 elec ...
s, as they are internally derived and bind to the body's
opioid receptors. 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- ...
("leu"), and the other containing
methionine ("met"). Both are products of the proenkephalin
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
.
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Met-enkephalin
Met-enkephalin, also known as metenkefalin (INN), sometimes referred to as opioid growth factor (OGF), is a naturally occurring
A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In ...
is Tyr-Gly-Gly-Phe-Met.
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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,
B-endorphin, and
dynorphins
Dynorphins (Dyn) are a class of opioid peptides that arise from the precursor protein prodynorphin. When prodynorphin is cleaved during processing by proprotein convertase 2 (PC2), multiple active peptides are released: dynorphin A, dynorphin B, ...
. 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 re ...
) 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
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...
as well as in the cells of
adrenal medulla. In response to pain,
norepinephrine
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad' ...
, a
hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are require ...
that is activated in fight-or-flight response is released, along with
endorphins.
A 2017 study indicates that this polypeptide may be linked to brain functioning during the
stress response
The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first des ...
, especially in the
hippocampus
The hippocampus (via Latin from Greek , ' seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, ...
and
prefrontal cortex
In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, BA13, BA14, BA24, BA25, BA32, BA44, BA45, BA46 ...
. This research has suggested that, as part of the stress response, several met-enkephalin analogs have increased activity in the
hippocampus
The hippocampus (via Latin from Greek , ' seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, ...
, while leu-enkephalin analogs as well as
somatostatins are
downregulated
In the biological context of organisms' production of gene products, downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external stimulus. The complementary proc ...
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 receptors 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 group of evolutionarily-related p ...
s, with other
opioid
Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioid use ...
s as
ligand
In coordination chemistry, a ligand is an ion or molecule ( 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 elec ...
s as well. The other
endogenous opioids are
dynorphin
Dynorphins (Dyn) are a class of opioid peptides that arise from the precursor protein prodynorphin. When prodynorphin is cleaved during processing by proprotein convertase 2 (PC2), multiple active peptides are released: dynorphin A, dynorphin ...
s (that bind to kappa receptors),
endorphin
Endorphins (contracted from endogenous morphine) are chemical signals in the brain that block the perception of pain and increase feelings of wellbeing. They are produced and stored in an area of the brain known as the pituitary gland.
Hist ...
s (mu receptors), endomorphins, and nociceptin/orphanin FQ. The opioid receptors are ~40% identical to
somatostatin receptors (SSTRs).
See also
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Endorphin
Endorphins (contracted from endogenous morphine) are chemical signals in the brain that block the perception of pain and increase feelings of wellbeing. They are produced and stored in an area of the brain known as the pituitary gland.
Hist ...
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Enkephalinase
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Opioid peptide
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Racecadotril
Racecadotril, also known as acetorphan, is an antidiarrheal medication which acts as a peripheral enkephalinase inhibitor. Unlike other opioid medications used to treat diarrhea, which reduce intestinal motility, racecadotril has an antisecret ...
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RB-101
RB-101 is a drug that acts as an enkephalinase inhibitor, which is used in scientific research.
RB-101 is a prodrug which acts by splitting at the disulfide bond once inside the brain, to form two selective enzyme inhibitors and blocking both ty ...
References
External links
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Opioid peptides
Pentapeptides
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