Enkephalins and Proenkephalins

Wha is the Enkephalin?

Endogenous opioid pentapeptides known as enkephalins are primarily synthesized in peripheral tissues, the adrenal medulla, and the central nervous system. One is Met-enkephalin (YGGFM), while the other is Leu-enkephalin (YGGFL). Their structures are distinct. These are produced through posttranslational proteolytic cleavage of a proenkephalin precursor protein. Proenkephalin in mammals has seven enkephalin sequences, including six Met-enkephalin copies and one Leu-enkephalin copy. Thus, by producing several enkephalin peptides, the proenkephalin gene is able to enhance its activity. A preference for the δ-opioid receptor exists among the three traditional opioid receptors, which are ν-opioid, μ-opioid, and δ-opioid, according to the binding of enkephalin peptides.

Enkephalin structure

One of two enkephalins, met-enkephalin or leu-enkephalin, can be produced by cleaving the precursor molecule pro-enkephalin. Three exons and two introns make up the pro-enkephalin gene. As a result of processing pro-enkephalin, animals like humans produce six copies of met-enkephalin and one copy of leu-enkephalin. Several parts of the brain, the spinal cord, and the adrenal medulla are home to enkephalins. The adrenal medulla may be the primary source of the enkephalins found in plasma, according to some theory. Hydrolysis, a process that enkephalins go through in their biodegradation, breaks the pentapeptide at the Tyr-Gly bond. The molecules are subsequently broken down into peptides ranging in length from two to four amino acids by enkephalinases and aminopeptidases.

The enkephalins are pentapeptides with leucine or methionine as the carboxy-terminal amino acid, which divides them into two structural categories. It follows that the enkephalins are either met-encephalins or leu-encephalins. Tyr-Gly-Gly-Phe-Met is the amino acid sequence found in Met-enkephalins. Tyr-Gly-Gly-Phe-Leu is the amino acid sequence found in leu-enkephalins. At the molecular level, the receptor is bound by Tyr and Phe, with the glycine pair serving as a spacer.

Met enkephalin

An endogenous peptide with 5 amino acids, met-enkephalin, agonistically engages opioid receptors. Opiopeptins, of which it is a congener of Leu-enkephalin, were among the first endogenous opioid peptides to be identified. It binds more strongly to receptors for mu opioid peptide and delta opioid peptide than to receptors for kappa opioid peptide. There doesn't seem to be any therapeutic utility for it because endogenous proteases digest it very quickly. The enkephalins seem to be involved in a wide variety of physiological processes, such as the sense of pain, the body's reaction to stress, and gastrointestinal (GI) function. The smaller peptides proenkephalin, prodynorphin, and proopiomelanocortin (POMC) are the building blocks of the bigger met-enkephalin. There is little hope for the clinical application of met-enkephalin because of how quickly peptidases break it down.

Leucine enkephalin function

An endogenous peptide with five amino acids, leu-enkephalin, agonistically engages opioid receptors. It and its congener, Met-enkephalin, were among the first endogenous opioid peptides (opiopeptins) to be identified. While it binds weakly to kappa opioid receptors, it binds more strongly to receptors for mu and delta opioid peptides. Endogenous peptidases digest it quickly, therefore it doesn't seem to have any therapeutic benefit. The enkephalins seem to be involved in a wide variety of physiological processes, such as the sense of pain, the body's reaction to stress, and GI function. The chemicals proenkephalin and prodynorphin are the building blocks of leu-enkephalin.

Enkephalin mechanism of action

The widespread distribution of enkephalin-specific opioid receptors allows them to exert their physiological effects. There are three primary types of opioid receptors: mu, which is mostly found in the central nervous system, delta, which is equally expressed in the spinal cord and the SNC, and kappa, which is mostly found in the spinal cord. Not included in the aforementioned tripartite group, sometimes dubbed the classical opioid receptors, is the fourth class of opioid receptors, nociceptin, which was found in 1994. Among the opioid receptors, enkephalins bind most strongly to the delta-opioid and mu-opioid sites, whereas their affinity for the kappa-opioid site is rather modest.

The opioid receptor is a member of the G-protein coupled receptor family that is easily identifiable by its seven membrane-spanning motifs that share about 60% of its sequence. The selectivity of these proteins is determined by their extracellular domains, which exhibit a sequence similarity ranging from 34% to 49%. Reducing K⁺ and Ca²⁺ influx is the mechanism by which enkephalins exert their substantial inhibitory effects. Ligand binding initiates signal transduction by causing the Gα and Gβγ subunits to dissociate. Cellular hyperpolarization is caused by the direct interaction of the Gα subunit with inward rectifying potassium channels. Reducing the cAMP-dependent Ca²⁺ influx, the Gα subunit also hinders adenylyl cyclase action, leading to a decrease in cAMP production. The direct binding of the Gβγ to different types of Ca²⁺ channels further decreases calcium influx.

Enkephalin function

There is extensive expression of enkephalins and the opioid receptors they target in several organ systems, peripheral and central neurological systems, endocrine tissues, and the organs themselves. Looking at a subset of these systems that have been tested experimentally helps to understand the different impacts of enkephalins. Among the many physiological effects of enkephalin that have been extensively studied in the scientific literature are its functions in analgesia, angiogenesis, regulation of blood pressure, embryonic development, feeding, hypoxia, modulation of the limbic system (related to emotions), memory processes, neuroprotection, peristalsis, pancreatic secretion, wound healing, control of respiration, and hepatoprotective mechanisms. It appears that leu-enkephalin is involved in controlling gonadal function once again. Modulation of peristalsis, regulation of the stress response, and analgesia are the primary roles.

(1) Analgesia

Inhibiting the transmission of nociceptive signals in the dorsal horn is one of the main ways that Enkephalins influence pain. Because they inhibit the activity of ascending neurons, which are stimulated by excitatory neurotransmitters including glutamate, substance P, and calcitonin gene-related peptide (CGRP), which are secreted by primary afferent neurons, Enkephalins, which are released by spinal interneurons, have postsynaptic effects inside the central nervous system. Additionally, Enkephalins regulate pain intensity by indirectly activating noradrenergic neurons in the locus coeruleus and serotonergic neurons in the raphe nucleus, which promotes the activation of the descending pathway. To sum up, Enkephalins have the ability to change the balance of inhibitory and excitatory neurotransmission in pain pathways by influencing the activity of glutamatergic, noradrenergic, serotoninergic, and GABAergic neurons.

(2) Stress response regulation

Stress causes the secretion of Corticotropin-Releasing Factor (CRF), which in turn triggers the synthesis of catecholamines and endogenous opioids like endorphins and enkephalins. The premise is that there are various ways in which the endogenous opioid system controls the length and severity of the stress reaction. For example, research has shown that enkephalin regulates CRF release from the hypothalamic paraventricular nucleus. The reduction of pain perception is the primary mechanism by which ENKs modulate mood. When exogenous opioids like morphine or fentanyl are administered, these effects are comparable to what is seen. Furthermore, enkephalins influence the release of cortisol, the primary stress hormone, via altering the hypothalamic-pituitary-adrenal (HPA) axis, therefore contributing to the stress response.

(3) Peristalsis

By influencing neuronal excitability, endogenous opioids and enkephalins reduce motility in the gastrointestinal tract. One way it works is by blocking potassium and calcium channels. This makes the cell more hyperpolarized, which stops it from conducting an action potential and releasing the neurotransmitter that the gut motility relies on.

Enkephalin synthesis

The transcription of the proenkephalin gene (Penk) and the subsequent enzymatic cleavage of the proenkephalin A protein (pENK; 243 αα) are the steps that produce Enkephalins in various parts of the central nervous system (CNS), peripheral nervous system (PNS), immunological cells, and adrenal glands. Due to the sequence homology of these peptides, Enkephalins can be synthesized through posttranslational processing from β-endorphin (β-END) and/or dynorphin (DYN). A complex network of peptidases, including prohormone convertase 1 and 2, carboxypeptidase E, cathepsin H, and four copies of Met-Enkephalin and one copy of Leu-Enkephalin, must work in unison for pENK to mature into functional opioid peptides. Aminopeptidase N, neprilysin, and angiotensin-converting enzyme (ACE) are the three peptidases that govern the half-life of ENKs once they are released.

Endorphins vs enkephalins

The body produces its own opioids, called endorphins, which are peptides. Of the four families of endorphins, the enkephalins and beta-endorphins are the most well-known. In contrast to the widespread production of enkephalins in the brain, the pituitary gland is responsible for producing beta-endorphin. In addition to modulating the stress response, endorphins are involved in the pain response. Exercising and laughing with others both trigger the release of endorphins. Additionally, lymphocytes can alleviate pain in inflammatory areas by producing opioids. Out of the four opioid receptors—mu, delta, kappa, and nociceptin—the mu receptor, which is most well known as the morphine receptor, is the one that endorphins bind to. Opioids have an immune-suppressing effect on lymphocytes through binding to opioid receptors. Penta peptides, of which enkephalins are a subset, are endorphins. They are both part of the beta-lipotropin compound. The most potent morphine-like peptide that has been isolated thus far is beta-endorphin, which is the C-terminal segment that has 31 amino acids. Naloxone and other morphine antagonists can reverse several of these drugs' effects. It was intriguing to study the role of narcotic antagonists in mood disorders because of the significant impact that morphine-like drugs have on mood. Because of the discovery that these chemicals may induce catatonic-like states when administered into the brains of animals, they were also of interest in the field of schizophrenia.

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