Simple N-Acylamides and Lipoamino Acids

Many simple fatty amides occur naturally, some of which have profound biological functions. Various amine and fatty acid moieties are involved, and the latter are often saturated or monoenoic in nature. Of these, anandamide or N-arachidonoylethanolamide is of special importance as an endocannabinoid and it is most appropriately discussed under that heading, together with oleamide and N-acyldopamines, which interact with the same receptors. On the other hand, it can be difficult at times to draw the line between what is or is not endocannabinoid activity in a strict sense, since some antagonists for the endocannabinoid receptors (CB1 and CB2) also interact with those for other acyl-ethanolamides, while the endocannabinoids can interact with receptors for other N-acyl-amides.

In addition to these, many simple fatty acyl-amino acid conjugates (lipoamino acids) are present in animal, plant and bacterial tissues and have important biological properties that are now being revealed and are discussed below. Fatty acid conjugates with short peptides, such as glutathione, are discussed here, but proteolipids in which fatty acids are linked covalently to proteins have their own web page. Similarly, bacterial proteolipids and lipopeptides are important natural products, but as these are different in structure and function and constitute a substantial topic in their own right, they are discussed separately in this web site, as are some of the taurolipids and the cysteinyl leukotrienes, which could be considered to be lipoamino acids. Fatty amides are produced synthetically in industry in large amounts (> 300,000 tons per annum) for use as ingredients of detergents, lubricants, inks and many other products, but these are not considered here.

1.  Long-Chain N-Acylethanolamides in Animals

Long-chain N-acylethanolamides are ubiquitous trace constituents of animal and human cells, tissues and body fluids, with important pharmacological properties. For example, in rat plasma, the concentrations of palmitoyl-, oleoyl- and arachidonoylethanolamides were found to be 2.5, 2.0 and 0.2 ng/ml, respectively, in one study and somewhat higher concentrations are reported in brain and other tissues. Similar lipids have been found in fish, molluscs, slime moulds, and certain bacteria also. The fatty acyl ethanolamides other than anandamide and N-docosahexaenoylethanolamide are the subject of this section, and indeed they are the most abundant components of this lipid class often with distinctive biological properties of their own. Although most do not appear to interact with the cannabinoid receptors, they may potentiate the activity of endocannabinoids by minimizing their degradation by competing for the hydrolytic enzymes. As described below, they are derived biosynthetically from precursor N-acyl-phosphatidylethanolamines, which do not have biological activity as lipid mediators (see below).

N-Palmitoylethanolamide, for example, takes part in a signalling system resembling that of the endocannabinoids, which involves and depends on receptors other than the CB1/CB2 receptors characteristic of endocannabinoids. This lipid was first identified in egg yolk more than 50 years ago, and its anti-inflammatory properties were recognized immediately. However, little more was done until there was a resurgence of interest in recent years, during which it has also been shown to have anti-inflammatory, analgesic and neuroprotective actions. It is produced on demand in most mammalian tissues, although most interest has been on its occurrence in the central nervous system where it may act to counter neuroinflammation. In particular, it has been shown to have neuroprotective properties in mast cell-mediated models of stroke, spinal cord injury, traumatic brain injury and Parkinson disease, and it is undergoing clinical trials for the relief of chronic pain. It is available commercially as a nutraceutical in some countries.

Formula of N-palmitoyl- and N-oleoylethanolamide

It is believed that the effects on inflammation and inflammatory pain are mediated mainly through actions upon a specific G‑protein coupled receptor designated GPR55, peroxisome proliferator-activated receptor-α (PPARα) and the transient receptor potential vanilloid 1TPRV1, although other mechanisms have been postulated such as synergistic effects with other acyl amides as part of a complex web of direct and indirect interactions. In addition, palmitoylethanolamide is reported to negatively regulate some anaphylactic responses, such as mast cell degranulation, and in this instance, it may operate through an endocannabinoid-like system that includes CB2, GPR55, and 2-arachidonylglycerol. Incidentally, the most potent endogenous lipid ligand for GPR55 known to date is 2‑arachidonyl lysophosphatidylinositol, which is a precursor of the endocannabinoid 2-arachidonoylglycerol.

N-Oleoylethanolamide is an endogenous regulator of food intake, and it may have some potential as an anti-obesity drug. It is believed to act as a local satiety signal rather than as a blood-borne hormone, and food intake is inhibited in rats following intraperitoneal injection and even after oral administration. Under normal physiological conditions, oleic acid from dietary fat is transported into enterocytes in the small intestine by a fatty acid translocase CD36, and some is converted to oleoylethanolamide via the intermediate N-acylphosphatidylethanolamine (see below), and acts as a sensor for ingestion of fat by a mechanism involving an interaction with the receptor GPR119 to release gut hormones. N-Oleoylethanolamide increases synthesis and secretion of triacylglycerols in enterocytes and of apoB secretion in chylomicrons.

In addition, the effects are mediated in the intestinal brush border of enterocytes by binding with high affinity to PPARα, while TPRV1 may also be involved. This stimulates the vagal nerve via the fatty acid translocase/CD36 and sends neuronal signals to the brainstem, leading to increased lipolysis and β-oxidation of fats. Plasma oleate is not a precursor and the fatty acid translocase CD36 is essential for its synthesis. The effect is highly specific, as linoleoylethanolamide has no such action, although it is produced in tissues in significant amounts and may have other biological effects. On the other hand, sustained ingestion of a high-fat diet abolishes the anorexic signal of oleoylethanolamide, and malfunction of the biosynthesis of this lipid mediator in enterocytes might be responsible for reduced satiety and weight gain. Oleoylethanolamide has anti-inflammatory and antioxidant properties. It does not activate receptors CB1/2 so it is not considered to be an endocannabinoid by the strict definition.

Scottish thistleBasal levels of acylethanolamides are especially high in the gut. Anandamide and N-oleoylethanolamide are selectively decreased and then increased in rat intestine during food deprivation and subsequent re-feeding through remodelling of the original acyl donor phospholipids. However, they have opposing effects upon lipogenesis. In adipose tissue and liver, oleoylethanolamide reduces the triacylglycerol content by stimulating lipolysis and elevating the circulating levels of unesterified fatty acids and glycerol. In liver, it leads to a decrease in the activity and expression of key enzymes involved in triacylglycerol synthesis. It may stimulate thermogenesis in brown adipose tissue. Bioactive amides as products of phospholipid metabolism are thus in a state of dynamic equilibrium as part of the normal system of redistribution of molecular species in phospholipids. Indeed, there is increasing evidence that the balance between the various N-acylethanolamides is important for the correct functioning of innumerable biological systems, with an imbalance leading to pathological conditions. While oleoyl- and palmitoylethanolamides do not activate cannabinoid receptors directly, they can enhance the activity of anandamide by inhibiting its inactivation by fatty acid amide hydrolase ('entourage effects').

Recent research suggests that N-oleoylethanolamide is an important factor that fuels the growth of cells in chronic lymphocytic leukemia and may be involved in drug resistance and the wasting effects of the disease. In addition, it has been demonstrated that oleoylethanolamide by acting as a PPARα agonist has a novel effect in enhancing memory consolidation through noradrenergic activation of specific regions of the brain; it may have an influence on sleep patterns and the effects of stress. It is also reported to have beneficial effects towards non-alcoholic fatty liver disease (NAFLD), the most prevalent of chronic liver diseases, as a PPARα agonist. Preclinical studies have shown that oleoylethanolamide exerts neuroprotective effects in alcohol abuse. In the nematode Caenorhabditis elegans, considered a model for primitive animals, oleoylethanolamide binds to the lysosomal lipid chaperone LBP-8, which promoted longevity by activating specific nuclear hormone receptors and thence activating transcription of key target genes.

Surprisingly, the isomeric N-cis-vaccenoylethanolamide ( i.e., of 11-18:1 rather than 9-18:1) was shown to be the most abundant 18:1 fatty acylethanolamide in rat plasma in unbound form and the second most abundant in human plasma, although its biological properties are as yet unknown. However, there was much more N-oleoylethanolamide in the circulation as the N-acyl phosphatidylethanolamine precursor. As there has been a potential for confusion with N-oleoylethanolamide in certain studies, some re-appraisal of the latter may be necessary.

N-Stearoylethanolamide is an immunomodulator and it induces apoptosis of glioma cells. It down-regulates the expression of liver stearoyl-CoA desaturase-1 mRNA, an anorexic effect, and it also has marked anti-inflammatory properties. By activating the nuclear receptor PPARα, it is able to regulate lipid metabolism.

In some stress situations, increased levels of saturated and mono-unsaturated ethanolamides are produced and in others there is selective stimulation of anandamide synthesis. N-Acylethanolamides in human reproductive fluids may help to regulate many physiological and pathological processes in the reproductive system. Saturated and monoenoic N-acylethanolamides may also function as intracellular messengers by activating specific kinases and interacting with the signalling pathways mediated by ceramide, with which they have some structural similarities; some of these effects may be specific to particular tissues.

Biosynthesis and catabolism. N-Oleoyl- and N-palmitoylethanolamide are produced in tissues from the precursor N-acyl-phosphatidylethanolamines by a similar mechanism to that of anandamide, i.e., by an initial transfer of the fatty acyl group from positions sn-1 of phosphatidylcholine to phosphatidylethanolamine to form N-acyl phosphatidylethanolamine, from which the N-acylethanolamide is released mainly by the action of a phospholipase D specific to this lipid, although other hydrolases are potentially involved. Similarly, as with anandamide, they are hydrolysed if relatively slowly by the fatty acid amide hydrolase, although a lysosomal enzyme that is highly specific for N-palmitoylethanolamide has been characterized (N‑acylethanolamine-hydrolysing acid amidase). Inhibition of the latter enzyme is seen as a promising pharmaceutical target with the potential for anti-inflammatory and analgesic actions. The lysosomal acid ceramidase is also capable of hydrolysing bioactive amides.

2.  Long-Chain N-Acylserotonins

Serotonin or 5-hydroxytryptamine per se is a monoamine neurotransmitter derived from tryptophan, and is synthesised mainly in the enteric nervous system of the gastrointestinal tract (90%), although it is also present in platelets and the central nervous system of animals, where it is popularly known as a contributor to feelings of well-being. A number of N-acylserotonins (16:0, 18:0, 18:1 and 20:4, 20:5 and 22:6) have been detected in intestinal tissue from the rat, pig and humans, especially in the jejunum and ileum where they are believed to regulate intestinal function. In particular, N-docosahexaenoylserotonin in human intestinal tissue is a potent anti-inflammatory mediator that may be relevant to intestinal inflammatory conditions such as Crohn's disease and ulcerative colitis. N‑Arachidonoylserotonin has been found in brain where it is reported to have an analgesic effect; it is an inhibitor of fatty acid amide hydrolase and binds to the vanilloid TRPV1 receptor. Of the other acylserotonins, N‑palmitoylserotonin is reported to have anti-allergy properties and to improve memory in animal models, while N-docosahexaenoylserotonin has potent anti-inflammatory properties. An arylalkylamine N-acyltransferase has been characterized from Drosophila melanogaster that catalyses the formation of long-chain N-acylserotonins from CoA esters and serotonin, but little appears to be known of their biosynthesis in higher animals.

Formula of N-palmitoylserotonin

Such lipids with saturated acyl groups were first detected in the wax layer of green coffee beans, and they have been characterized with 22:0 to 26:0 fatty acid components from the seed oil of the baobab tree (Adansonia digitata), where they may have an antioxidant function.

3.  N-Acylamides in Plants

N-Acylethanolamides are minor but ubiquitous components of plant tissues, and they are especially abundant in desiccated seeds. The fatty acids are representative of those in plants in general with up to three double bonds, and with 12 to 18 carbon atoms. For example, oleoylethanolamide is present naturally at low levels in such food products as oatmeal, nuts and cocoa powder (up to 2 μg/g). In this instance, the precursor N‑acylphosphatidylethanolamine is synthesised by a different mechanism from that in animals, i.e., by direct acylation of phosphatidylethanolamine by an N‑acyl phosphatidylethanolamine synthase. N-acylethanolamides are released from this by the action of two (but not all) isoforms of phospholipase D in response to stress situations.

As in humans, it appears that such compounds have a variety of biological functions in plants with a highly conserved role in cell signalling, although research is still at an early stage in comparison to that with animals. They appear to block essential processes that are part of the transition from seed germination to seedling growth, although some of their effects may be beneficial. For example, N‑linoleoylethanolamine disrupts root development in seedlings, although a specific 9-lipoxygenase metabolite of this (9-hydro(pero)xy-linoleoylethanolamide) may be a more active metabolite working synergistically with abscisic acid to modulate the transition from embryo to seedling. N‑Linolenoylethanolamine inhibits chloroplast development, N-lauroylethanolamine has negative effects on seedling growth and flower senescence, and N‑myristoylethanolamine functions in plant defence against pathogen attack and inhibits stomatal closure. In addition, further metabolites of N‑lauroylethanolamine have been characterized in which glucose is attached to the free hydroxyl of the ethanolamine moiety together with mono- or dimalonylglucosides; these may represent a mechanism for modulating the biological activities of N‑Acylethanolamides in plants. As in animals, a fatty acid amide hydrolase (FAAH) that degrades N‑acylethanolamides in vivo is present in Arabidopsis, and this is also believed to be an important regulatory factor in plants.

Formulae of affinin and capsaicinWhile these simple derivatives have attracted most recent interest because of their similarity to animal lipids, more than 300 different N-acylamides have been identified from eight families of plants and some fungi with as many as 200 different fatty acids and many different amine moieties including propyl, isopropyl, butyl and often isobutyl moieties. The fatty acid moieties are distinctive, and fall into two groups, those with double bonds only and those with double and triple bonds. For example, affinin, an insecticidal amide, has a 10-carbon fatty acid attached to isobutylamine, i.e., it is N‑isobutyl-2E,6Z,8E-decatrienamide. Similarly, capsaicinoids, the pungent components of chili peppers, are N‑acylated phenolic amides in which the fatty acid is nonanoic or decanoic acid, sometimes with an isomethyl group and sometimes a trans-double bond in position 6 or 7. The pungency is mainly determined by the benzene ring and is modified by the acyl chain. Capsaicin per se is the vanillyl derivative, (6E)-N-(4-hydroxy-3-methoxybenzyl)-8-methyl-6-nonenamide, and it exerts its effects in humans, including gastrointestinal discomfort, by binding to the vanilloid receptor subtype 1 (TRPV1). The biosynthetic mechanisms for most plant amides are believed to be quite different from those of the N-acylethanolamines, and the amine moiety is often derived from amino acids. Important biological functions are slowly being revealed, including toxicity to insect predators, bacteria and fungi, while promoting plant growth.

N-Acyl ureas are produced by fungi, often in the form of symmetrical molecular species with palmitoleic, oleic or linoleic acids as the fatty acid constituents. They interact with the human immune system, possibly alerting the host to the presence of a fungal infection.

4.  Simple Lipoamino Acids from Animal Cells

N-acetyl derivatives of amino acids are minor but ubiquitous components of animal tissues, and they may be formed simply as a means of excreting or detoxifying excesses of particular amino acids. Innumerable long-chain fatty acid-amino acid conjugates may exist, and as an example more than 50 different lipoamino acids of this type have been found in rat brain alone, some of which have important biological properties (although few have yet been studied). They are sometimes termed 'elmiric acids'. N-Acyl-taurines are discussed in our web page on lipid sulfates.

N-Acylglycine derivatives of short-chain fatty acids (C2 to C12) have long been recognized as minor constituents of urine and blood, and their compositions in the former especially may have some relation to metabolic diseases. They are formed in the liver partly as a detoxification mechanism for removal of excess acyl-coenzyme A esters, although they can also function as intercellular messengers via cell surface receptors.

N-Arachidonoylglycine is present in bovine and rat brain as well as other tissues at low levels, and it is synthesised by at least three pathways in mammalian cells. One involves sequential oxidation of anandamide by alcohol dehydrogenase and aldehyde dehydrogenase, while in a second arachidonoyl-CoA and glycine combine in a reaction catalysed by glycine N-acyltransferase-like 3 (other glycine N-acyltransferases are responsible for the formation of oleoylglycine and for short-chain fatty acylglycines). In a third pathway, cytochrome C catalyses the formation of arachidonoylglycine from arachidonoyl-CoA and glycine, in the presence of hydrogen peroxide.

Biosynthesis of N-arachidonoyl glycine/serine

N-Arachidonylglycine has been shown to suppress inflammatory pain. It does not bind to the CB1/2 receptors for endocannabinoids, but it is a ligand for other G protein-coupled receptors such as GPR55 (as is lysophosphatidylinositol), GPR18 and GPR92, and it may have a role in regulating tissue levels of anandamide by inhibiting the fatty acid amide hydrolase. In this respect, the dividing line with endocannabinoids appears to be tenuous. Among the reported physiological and functional activities of N‑arachidonylglycine are calcium mobilization, control of apoptosis, anti-inflammatory actions, alleviation of neuropathic pain and regulation of intraocular pressure. It is a substrate for cyclooxygenase-2 (COX-2), producing glycine conjugates of prostaglandins, and it may divert the biosynthetic pathway from the pro-inflammatory PGE2 towards the anti-inflammatory J prostaglandins.

N-Palmitoylglycine is produced in most tissues, but especially skin and spinal cord, and it has a role in sensory neuronal signalling by acting as a modulator of calcium influx and nitric oxide production. There are reports that it is involved in gynecological disorders, and its concentration is elevated in breast cancer. Its signalling is mediated by activation of cation channel such as GPR132. N‑Oleoylglycine was first detected in mouse neuroblastoma cells, and it is now known to have a regulatory effect on body temperature by inducing hypothermia and on locomotion; it is produced by the cytochrome C route. There is a report that this isomer promotes adipogenesis via activation of the CB1 receptor, implying that it is an endocannabinoid with the potential to increase insulin sensitivity and suppress obesity and diabetes. It is a precursor for oleamide by oxidative cleavage in a reaction catalyzed by peptidylglycine α-amidating monooxygenase (other acylglycines may react similarly). Acyl-glycines produced by gut bacteria (see below) may influence the metabolism of the host.

In brown adipose tissue, a secreted enzyme, peptidase M20 domain containing 1 (PM20D1), is able to catalyse both the condensation of fatty acids and amino acids to generate N-acyl amino acids and the reverse hydrolytic reaction. In mice, N-oleoyl-leucine and phenylalanine synthesised in this way have been shown to bind to mitochondria to increase the rate of uncoupled respiration in cells, while treatment with N-oleoyl-leucine improved glucose homeostasis, increased energy expenditure, and reduced body weight.

N-Acylserines have been detected at trace levels in bovine brain with the palmitoyl and stearoyl forms being most abundant. Of these, N‑palmitoylserine is reported to have neuroprotective effects against traumatic brain injury. N-Arachidonoylserine has also been found in tissues, and while it does not bind strongly to cannabinoid receptors, it does have a potent vasodilatory effect on rat arteries in vitro, and it activates certain calcium channels in neurons. In addition, it was found to suppress the formation of reactive oxygen species and production of NO and tumor necrosis factor (TNF) in a murine macrophage cell line. It is presumed to be synthesised by a mechanism similar to that for anandamide but via N‑arachidonoylphosphatidylserine, although mechanisms similar to those described above for arachidonoylglycine are also possible. N‑Oleoylserine stimulates osteoblast proliferation, but it has not been detected in brain. A novel lipid that is presumably derived from palmitoylserine, i.e., N-palmitoyl-O-phosphocholineserine, has been found in patients with the genetic disorder Niemann-Pick disease type C1. It was originally termed "lysosphingomyelin 509", and it appears to be functionally if not structurally related to sphingosylphosphorylcholine (and is illustrated and discussed here..).

At least three other arachidonoyl amino acids, i.e., of γ‑aminobutyric acid, alanine and asparagine, occur naturally and also inhibit pain, suggesting that such biomolecules may be integral to pain regulation and perhaps have other functions in mammals. Like oleoylglycine, they can be converted to primary fatty amides in vitro, and it is possible that this also occurs in vivo. N-linoleoylalanine has anti-inflammatory action that is mediated by a GPR18 response. N-Acylaspartates with 16:0, 18:2 and 20:4 fatty acid moieties attached to the amino group of aspartic acid that may inhibit Hedgehog signalling but do not interact with endocannabinoid receptors have been isolated from animal tissues. N-Stearoyltyrosine fed orally to mice was found to ameliorate obesity by inhibiting absorption of dietary fat, promoting lipolysis and reducing lipogenesis, while the N-palmitoyl isomer has an anti-proliferative action in cancer cells. N‑(17‑Hydroxy)-linolenoyl-L-glutamine (volicitin), N-(17-hydroxy)-linoleoyl-L-glutamic acid and related lipoamino acids have been found in insect larvae. Their presence in oral secretions elicits a defense response in plants.

The N-arachidonoyl amino acid and vanilloid derivatives are minimally oxidized by COX-2, but they are good substrates for the 12S- and 15S‑lipoxygenases. However, it is not yet clear whether this leads to inactivation of these lipids or rather converts them to new bioactive compounds. While the fatty acid amide hydrolase (FAAH) will cleave the N-acyltaurines and N-arachidonoylglycine to the corresponding fatty acid and amino acid, the other N-acyl amino acids are not affected, although they are potent inhibitors of the enzyme (their catabolic fate is uncertain).

5.  Lipid-Glutathione Adducts (Animal)

Glutathione is the tripeptide, γ-L-glutamyl-L-cysteinylglycine, and is most abundant thiol-containing small molecule (3 to 4 mM) in animal cells, where it is located mainly in the cytosol. It has a major defensive role in combating oxidative stress, for example by undergoing oxidation to glutathione disulfide while reducing lipid (and other) hydroperoxides to hydroxides. It also reacts with lipid oxidation products, including peroxy-fatty acids and unsaturated aldehydes, to produce lipid-glutathione adducts, facilitated by the action of glutathione S-transferases. For example, bioactive eicosanoids and α,β‑unsaturated aldehydes (e.g. trans,trans-2,4-decadienal) and malondialdehyde can be deactivated or detoxified by conversion to inactive glutathione conjugates.

In contrast, the eicosanoid 5-hydroperoxyeicosapentaenoic acid (5-HPETE) is converted to the glutathione adduct as an intermediate in the biosynthesis of the cysteinyl leukotrienes or related resolvins/maresins, as described elsewhere on this website.

6.  Simple Lipoamino Acids and N-Acylamides from Bacteria

Formula of an ornithine lipidOrnithine lipids: A variety of lipoamino acids have been isolated from bacterial species, of which the best know is probably the zwitterionic N-acyl-ornithine derivative illustrated, which is widely distributed among prokaryotes (perhaps 50% of all species), but especially Gram-negative bacteria and other eubacteria, where it is located mainly in the outer membrane, although some is located in the cytoplasmic membrane. Ornithine lipids contain a non-hydroxy fatty acid with an estolide linkage to the hydroxyl group of a 3-hydroxy acid (often but not always C16 or C18) and thence via an amide bond to the α-amino group of ornithine. It may be relevant that such fatty acid linkages are also seen in the bacterial endotoxin lipid A. Lyso-ornithine lipids occur in some species, i.e., lacking the O-acyl fatty acid. Although they are normally minor components, ornithine lipids can assume major proportions when phosphate is limiting or in response to stress in some species.

In most bacteria, biosynthesis of ornithine lipids occurs in two steps via sequential acyl-ACP-dependent acylation of ornithine by two different acyltransferases. The first N-acyltransferase transfers a 3-hydroxy fatty acyl residue from acyl carrier protein to the α-amino group of ornithine forming a lyso-ornithine lipid, which is then acylated by an O-acyltransferase to produce the final ornithine lipid. However, at least one species contains a bifunctional acyltransferase in which the N-terminal domain is responsible for the O-acyltransferase reaction, whereas the C-terminal domain carries out the N-acyltransferase reaction. In some bacterial species, either the ester- or amide-linked fatty acid has a hydroxyl group in position 2, and analogous lipids in which the ornithine moiety is hydroxylated in position 4 by a specific hydrolase are known, with both modifications inserted post synthesis of the basic lipid. Indeed, many bacterial species contain similar lipids derived from 4-hydroxy-ornithine in which the 4-hydroxyl group is also esterified by a long-chain fatty acid. In the nodule bacterium Mesorhizobium loti, the ornithine moiety is surprisingly mainly of the D-configuration. Mono, di- and trimethylornithine lipids are formed by sequential methylation of the ornithine moiety by a specific N-methyl transferase requiring S‑adenosylmethionine in Planctomycetes.

Marine bacteria of the family Rhodobacteraceae contain glutamine lipids, which are closely related in structure to the ornithine lipids (some species contain both), while cerilipin characterized from Gluconobacter cerinus has an ornithine-containing lipid core and an additional amide-linked taurine moiety. Structurally related lipids with lysine, serine, glycine, glutamine and taurine residues occur in microorganisms such as the gliding bacterium Cytophaga (Flavobacterium) johnsonae and the Gram-negative marine species Cyclobacterium marinus. (N-(5-Methyl)hexanoyl tyrosine and N-(7-methyl)octanoyl tyrosine or phenylalanine are reported from Olivibacter sp. (Bacteroidetes). Simple N-acyl derivatives amino acids (without a secondary fatty acid constituent) also occur in bacteria, including N-acylleucine (or isoleucine) derivatives in Deleya marina, N-acyl-D-asparagine in Bacillus pumilus and N-acylserine in Serratia sp. In the intestines, commensal Bacteroides vulgatus produces N-acyl-3-hydroxy-palmitoyl glycine ('commendamide'), which resembles the long-chain N-acyl-amides that function as mammalian signalling molecules through activation of G-protein–coupled receptors (GPCR G2A/132); it may facilitate interactions with the immune system of the human host. The marine roseobacter species Ruegeria pomeroyi contains an analogue of the ornithine lipids in which the amine component appears to be aminopropane sulfonic acid, i.e., it is a sulfonolipid. Further lipoamino acid forms of increasing complexity have been characterized, including molecules with a long-chain alcohol moiety linked to the carboxyl group of the amino acid (such as siolipin A from Streptomyces species). The more complex microbial lipopeptides have their own web page.

It appears that such lipoamino acids have a variety of different functions in bacteria depending upon species, and it does not seem possible to generalize. For example, they have been implicated in temperature and stress tolerance in some species, and they appear to be especially important when phosphorus is limiting; they may also be recognized by plant defense systems or be essential for symbiotic relationships. In Gram-negative bacteria, they may stabilize the outer membrane by counteracting the negative charge of the lipopolysaccharides. Some of these lipoamino acids have interesting and potentially useful pharmacological properties.

Dipeptido lipids: Among the lipids found in Flavobacterium species is one containing a glycine-serine dipeptide linked to branched chain acids in a similar manner to the ornithine lipids and termed 'flavolipin'. Biosynthesis is by a similar mechanism to that of the ornithine lipids. Similar lipids (termed 'lipid 654', from the molecular weight of the main isomer, and 'lipid 430' with a single fatty acid constituent) have been found in common oral and intestinal Bacteroidetes bacteria, and they have been detected in and serum and brain samples from healthy subjects, presumably after uptake via the intestines. The former is an agonist for human and mouse Toll-like receptor 2 and may be implicated in the pathogenesis of atherosclerosis through deposition and metabolism in artery walls. A glycine-serine-ornithine tripeptide family of lipids has been isolated from the Bacteroidetes genus Chitinophaga.

Formulae of flavolipin and cerilipin

Formula for an N-acyl-L-homoserine lactoneN-Acyl-L-homoserine lactones are produced by many Gram-negative bacteria. The fatty acid components can vary in chain length from C4 to C18, sometimes with one or two double bonds (in position 2 (trans) and/or more central positions, e.g. ω7 (cis)), with hydroxyl or keto groups in position 3, and/or with methyl branches. For example, the first molecule of this type to be identified was N‑3‑oxohexanoyl-L-homoserine lactone, which induces bioluminescence, from the marine bacterium Vibrio fischeri, although different strains of this organism are now known to produce several isomers with variable chain-lengths and the presence or absence of 3‑oxo or 3‑hydroxyl groups. Some bacteria produced a single highly specific molecular species, but others can secrete many different forms. The photosynthetic bacterium Rhodopseudomonas palustris contains p-coumaroyl-homoserine lactone.

N-Acyl-L-homoserine lactones are used in a form of intercellular signalling termed 'quorum sensing', which controls gene expression in response to the population density of the species, resulting in coordinated regulation of a range of group-level behaviours, including production of secondary metabolites and virulence factors, bioluminescence and biofilm formation. These molecules are able to diffuse freely through the bacterial membrane, and when they reach a threshold concentration in a particular environment (nanomolar range), they bind to their intracellular receptor/activator proteins (LuxR-type) in the inner membrane or in the cytoplasm to induce the expression of innumerable genes, one consequence of which is to establish a feed-forward loop that is believed to promote synchronous gene expression in the population. Homoserine lactones (and their hydrolysis products by ring opening) can promote large-scale remodeling in lipid membranes to generate long microtubules in a reversible manner on the surface of the bilayer and thereby enable molecular transport across the bilayer.

Biosynthesis involves reaction of an acyl carrier protein-linked fatty acid, such as a 3-oxo isomer illustrated, with S-adenosylmethionine (SAM) and catalysed by an acyl-homoserine lactone (AHL) synthase (LuxI). The mechanism is believed to begin with nucleophilic attack on the 1-carbonyl carbon of the fatty acyl moiety by the amine of SAM, followed by nucleophilic attack on the γ-carbon of SAM by its own carboxylate oxygen to produce the lactone.

Biosynthesis of an N-acyl-homoserine lactone

Homoserine lactones are de-activated by specific lactonases, acylases and/or oxidoreductases, so it is hoped that drugs will be found to stimulate these activities and reduce the virulence of pathogenic Gram-negative bacteria. In contrast, certain plants appear to be able to detect quorum sensing signals with the potential to permit them to alter the outcomes to their own benefit via the signalling action of oxylipins and salicylic acid. Also, plants can hydrolyse N-acyl-L-homoserine lactones with fatty acid amide hydrolases to generate L‑homoserine, which encourages plant growth. Such molecules produced by bacteria in the human gut may have an anti-inflammatory effect upon the host intestinal epithelial cells. Other quorum sensing molecules include the lipidic 2-heptyl-4-quinolone and 2-heptyl-3-hydroxy-4(1H)-quinolone.

Other N-acylamides: 13-Docosenamide or erucamide is secreted by many bacterial species when they are grown on a medium containing glucose; it may be relevant to host-bacteria interactions. N-acyl derivatives of homospermidine (undecane with amine groups at the 1-, 6- and 11‑positions) with fatty acids attached each of the amine groups have been found in fruiting bodies of Myxococcus Xanthus, where they may be part of the bacterium’s defense strategy.

8.  Analysis

The main problems in the analysis of simple lipoamino acids relate to the low levels at which they occur naturally, and there is a concern that artefactually high results might be obtained because of the physiological effects of sampling methods and/or artefact formation during sample storage. Until recently, high-performance liquid chromatography with fluorescent detection or gas chromatography-mass spectrometry with selected ion monitoring were most used for the purpose, but liquid chromatography allied to tandem mass spectrometry with electrospray ionization would probably be the preferred method now.

Recommended Reading

Lipid listings Credits/disclaimer Updated: October 19th, 2021 Author: William W. Christie LipidWeb icon