Lipids in Action - Their Biological Functions
Energy - Membranes - Signalling
For many years, lipids were considered to be intractable and uninteresting oily materials with two main functions – to serve as a source of energy and as the building blocks of cell membranes. These are indeed vital functions, which continue to be studied. For much of the last century, lipids were certainly not considered to be appropriate candidates for such important molecular tasks as intracellular signalling or local hormonal regulation. In 1929, George and Mildred Burr demonstrated that linoleic acid was an essential dietary constituent for animals, but it was many years before the importance of this finding was recognized by biochemists in general. In the 1960s, it was established that certain glycosphingolipids were blood group determinants, and with the discovery by Bergström, Samuelsson and others in 1964 that the essential fatty acid arachidonate was the biosynthetic precursor of the prostaglandins with their effects on inflammation and other disease states, the scientific world in general began to realize that lipids were much more interesting than they had previously thought.
From the mid-1950s to the 1970s, there arose an awareness of the distinctive biochemistry of phosphatidylinositol and its metabolites and eventually their role in signalling processes, and another milestone was achieved in 1979 with the discovery of a further intact phospholipid with biological activity, platelet-activating factor. Since then, virtually every individual lipid class has been found to have some unique biological role that is distinct from its function as a source of energy or as a simple construction unit of a cellular membrane. Indeed, it is now recognized that lipids in membranes have innumerable functions in the trafficking of cellular constituents, in the regulation of the activities of membrane proteins and in signalling, in addition to their structural roles at the interface between all cells and organisms and their external environment.
All multi-cellular organisms use chemical messengers to send information between organelles and to other cells, and as relatively small hydrophobic molecules, lipids are excellent candidates for this purpose. Unesterified fatty acids and their oxygenated metabolites (oxylipins) have well-defined structural features, such as cis-double bonds or oxygen atoms in particular positions, which can carry information by binding selectively to specific receptors. They can infiltrate membranes or be translocated across them to carry signals to other cells. During transport in the circulation, they are usually bound to proteins and their effective solution concentrations are very low, so they are inactive until they reach their site of action and encounter the appropriate receptor. Intact lipids can sometimes function in this way at sites on cell membranes.
Storage lipids, such as triacylglycerols, in their cellular context are chemically inert in that they only rarely have biological effects per se, and indeed, esterification with fatty acids may be a method of de-activating other lipids such as steroidal hormones, until they are required in a functional state. In contrast, polar phospholipids have both hydrophobic and hydrophilic sites that can bind via various mechanisms to membrane proteins and influence their activities, while glycosphingolipids carry complex carbohydrate moieties that amongst many functions have a part to play in the immune system. Lipids have been implicated in many human disease states, including cancer and cardiovascular disease, sometimes in a beneficial and at others in a detrimental manner. In short, every scientist should now be aware that lipids are just as fascinating as all the other groups of organic molecules that make up living systems.
In this web document, the main biological functions of some key lipids in animals, plants and microorganisms are summarized to give a general overview, but much more information is available in this website on those pages dealing with specific lipid classes.
Fatty Acids and Oxylipins
Fatty acids are one of the defining constituents of all the main lipid classes, and they are in large part responsible for the distinctive physical and metabolic properties of the latter, as discussed in the next sections below. They are vital for life in that they constitute integral components of all tissue membranes, and because they have the highest energy density of all energy substrates. As precursors for many lipid mediators, they modulate innumerable metabolic functions, while through post-translational acylations they control the membrane location and function of many essential proteins (proteolipids), but they are also important in non-esterified form, i.e., as free (unesterified) fatty acids. They are released from triacylglycerols in fat depots during fasting to provide a source of energy and of structural components for cells (see below).
There are a number of more dynamic functions of fatty acids of great interest, and it has long been known that linoleic and linolenic acids are essential fatty acids in that they cannot be synthesised by animals and must come from plants via the diet. They are precursors of arachidonic, eicosapentaenoic and docosahexaenoic acids, which are vital components of all membrane lipids and precursors of oxylipins, i.e., a family of oxygenated fatty acids formed by mono- or dioxygen-dependent oxidation. Linoleic acid is a vital component of skin ceramides and is essential in its own right, not simply as a precursor of higher polyunsaturated fatty acids.
Dietary fatty acids of short and medium chain-length (< C14) are not usually esterified in tissues but are oxidized rapidly as a source of 'fuel' to support all the events necessary to keep organisms functioning, while longer-chain fatty acids are usually present largely in esterified form in triacylglycerols or structural lipids in tissues. Although all lipids are in a state of dynamic flux, membrane lipids are largely conserved in content and composition except under conditions of extreme stress. Triacylglycerols are the primary storage form of long-chain fatty acids for energy and structural purposes, and hydrolysis with release of the free acid components can occur quickly when these are required for metabolic reasons; they are then transported in an appropriate form to the heart, liver and other tissues. Polyunsaturated fatty acids are important as constituents of the phospholipids, where they appear to confer distinctive physical properties to the membranes such as to decrease their rigidity. On the other hand, the presence of saturated and monoenoic acids ensures that there is a correct balance between rigidity and flexibility. Indeed, saturated and 2‑hydroxy fatty acids in sphingolipids appear to give additional rigidity and hydrogen-bonding stability to the sub-domains of membranes termed 'rafts'.
Unesterified fatty acids can act as second messengers required for the translation of external cellular signals, as they are produced rapidly after binding of specific agonists to plasma membrane receptors. Within cells, fatty acids can act to amplify or otherwise modify signals to influence the activities of such enzymes as protein kinases, phospholipases and many more. They are involved in regulating gene expression, mainly targeting genes that encode proteins with roles in fatty acid transport or metabolism via effects on transcription factors, e.g., peroxisome proliferator-activated receptors (PPARs) in the nuclei of cells. Such effects can be highly specific to particular fatty acids, and for example, unesterified arachidonic acid may have some biological importance per se as part of the mechanism by which apoptosis (programmed cell death) is regulated.
The essential fatty acids, linoleic and linolenic acids and their longer-chain polyunsaturated metabolites, such as arachidonic acid, can be found in most lipid classes in animals, and they are the precursors of many different types of eicosanoids (C20) and other oxylipins, including the hydroxyeicosatetraenes ('HETE'), prostanoids (prostaglandins, thromboxanes and prostacyclins) and leukotrienes (and lipoxins), while docosahexaenoic acid is the precursor of docosanoids (C22 - specialized pro-resolving mediators - resolvins, protectins and maresins). These are produced enzymatically with great stereochemical precision by cyclooxygenases (COXs), lipoxygenases (LOXs) and epoxygenases of the cytochrome P-450 family (CYPs). Related isoprostanes are formed by non-enzymic means (autoxidation) from the same precursors. While they are usually treated separately in biochemical textbooks, it should not be forgotten that they are in fact fatty acids.
Some oxylipins are occasionally present in esterified form in phospholipids (and in glycosyldiacylglycerols in plants), and they may even be biologically active in this form (see our web page on oxidized phospholipids). The isoprostanes are all formed in situ in lipids within membranes. The eicosanoids and docosanoids are highly potent at nanomolar concentrations in the regulation of innumerable biological processes, especially in relation to inflammatory responses, pain and fever. Some of these have pro-inflammatory functions and others are anti-inflammatory, so the correct balance between the two groups is essential for the maintenance of health. Fatty acids are the biosynthetic precursors of many insect pheromones and of secondary metabolites in plants, and it is surely of evolutionary significance that plant hormones, such as the jasmonates, are derived from the essential fatty acids and have structural similarities to prostaglandins.
Many other fatty acids are not essential in animal tissues in the above sense but nonetheless have vital functions in tissues even when they are saturated or monoenoic in nature. Palmitic acid is a required biosynthetic precursor of the sphingoid bases and thence of all sphingolipids, while palmitic and myristic acids are key covalently linked constituents of proteolipids and are required for their activity. On the other hand, excessive amounts of these in the human diet are often regarded as harmful (I will not enter into this debate).
Tri-, Di- and Monoacylglycerols
Virtually all the natural fats and oils of commerce from olive oil to lard consist of triacylglycerols, but here we are concerned with their biological functions in tissues. As discussed briefly above, triacylglycerols are the main storage lipid in animal and plant cells where they occur as discrete droplets surrounded by a protective monolayer of phospholipids and key hydrophobic enzymes. Such ‘lipid droplets’ are now considered to be functional organelles. When required, fatty acids are released by hydrolysis reactions catalysed by lipases under the influence of hormones. One specialized form of adipose tissue, brown fat, is highly vascularized and rich in mitochondria, which oxidize fat so rapidly that heat is generated. This appears to be especially important in young animals and in those recovering from hibernation. Triacylglycerols are the main lipid component in the only material designed in nature entirely as a food, i.e., milk, although triacylglycerols in seeds could perhaps be considered as 'food' for the developing plant embryo until it is capable of photosynthesis.
Triacylglycerol depots have other functions. Subcutaneous depots serve as insulation against cold in many terrestrial animals, as is obvious in the pig, which is surrounded by a layer of fat, and it is especially true for marine mammals. In the latter and in fish, the lipid depots are less dense than water and so aid buoyancy with the result that less energy is expended in swimming. More surprisingly perhaps, triacylglycerols together with the structurally related glyceryl ether diesters and wax esters are the main components of the sonar lens used in echolocation by dolphins and some whales. Despite their hydrophobic nature, triacylglycerols are the major transport lipid in plasma in association with lipoproteins where their biological inertness is a virtue.
1,2-Diacyl-sn-glycerols are formed as intermediates in the biosynthesis of triacyl-sn-glycerols and of complex glycerolipids, and they function as second messengers in many cellular processes, modulating vital biochemical mechanisms by activating members of the protein kinase C family of enzymes. They are formed together with the important inositol phosphates by the action of the enzyme phospholipase C on phosphatidylinositol and polyphosphoinositides mainly. Their influence is believed to extend to the pathophysiology of cancer and other disease states.
2-Monoacylglycerols are produced when triacylglycerols are digested in the intestines of animals, but they are re-esterified before they are transported elsewhere in the body. In general, monoacylglycerols are minor components of tissues, which are never permitted to accumulate because their strong detergent properties would have a disruptive effect on membranes.
On the other hand, 2-arachidonoylglycerol, another product of phosphatidylinositol metabolism, is important in animal tissues as an endogenous ligand for cannabinoid receptors, i.e., as an endocannabinoid and mediator of inflammatory responses. In the intestines, 2‑oleoylglycerol has a signalling function by acting as a 'fat sensor'.
Waxes form a thin hydrophobic layer over all the green tissues of plants that is both a chemical and a physical barrier. Wax esters are usually major components, and they are accompanied by many different aliphatic compounds. This layer serves many purposes; it limits the diffusion of water and solutes, while permitting a controlled release of volatiles that may deter pests or attract pollinating insects, it provides protection from disease and predators and helps the plants resist drought. Waxes have a waterproofing and protective role on the external surface of insects in the same way.
Waxes can have a storage function as in marine organisms and in the seeds of the jojoba plant. Bees use wax to produce the rigid structures of honeycombs, while the uropygial (preen) glands of birds secrete waxes, which they use to provide waterproofing for feathers.
Some Other Simple Lipids
Before a fatty acid can be metabolized in tissues, it must usually be activated by conversion to a coenzyme A ester or acyl-CoA, with the fatty acid group linked to the terminal thiol moiety. The thiol ester is a highly energetic bond that permits a facile transfer of the acyl group to receptor molecules such as free hydroxyl or amine groups during the biosynthesis of virtually all lipid classes. Acyl-carnitines assist the transport and metabolism of fatty acids in and out of mitochondria, where they are oxidized for energy production. In so doing, carnitine maintains a balance between free and esterified coenzyme A in cells.
Long-chain N-acylethanolamines are ubiquitous trace constituents of animal and human cells with important pharmacological properties. The nature of the fatty acid controls the biological functions. Anandamide or N‑arachidonoylethanolamine has attracted special interest because of its marked biological activities, exerting its effects through binding to and activating specific cannabinoid receptors. Like 2‑arachidonoylglycerol, discussed above, it is an endogenous cannabinoid or ‘endocannabinoid’. In contrast, oleoylethanolamide is an endogenous regulator of food intake with potential as an anti-obesity drug, while palmitoylethanolamine is an anti-inflammatory agent, and stearoylethanolamine is a pro-apoptotic agent. Changing the nature of the amide moiety changes the function, and the simple oleamide molecule or cis-9,10-octadecenamide, isolated from the cerebrospinal fluid of sleep-deprived cats, has been identified as the signalling molecule responsible for causing sleep. Many simple fatty acid derivatives of amino acids are now known, and their biological functions are slowly being revealed.
Cholesterol is a ubiquitous component of all animal tissues, where much of it is located in the membranes. It occurs in the free form and esterified to long-chain fatty acids (cholesterol esters) in animal tissues, including the plasma lipoproteins. It is generally believed that the main physical function of cholesterol is to modulate the fluidity of membranes by interacting with their complex lipid components, specifically the phospholipids such as phosphatidylcholine and sphingomyelin, increasing the degree of order by promoting a 'liquid-ordered phase'. More intimate protein-cholesterol interactions may regulate the activities of certain membrane proteins such as ion channels. It is of course a key biosynthetic precursor of bile acids, vitamin D and steroidal hormones. While it may have negative connotations in the popular press, it is of crucial importance for life.
In plants, cholesterol tends to be a minor component only of a complex phytosterol fraction that includes campesterol, β-sitosterol, stigmasterol, Δ5‑avenasterol and brassicasterols, while yeasts and fungi have ergosterol as their main sterol component. Plant sterols are able to regulate membrane fluidity and permeability, and they modulate the activity of membrane-bound enzymes in a similar manner to cholesterol in animal membranes. Some bacterial species produce structurally and functionally related lipids, the hopanoids.
Cholesterol is a polyisoprenoid molecule or triterpene, and many more related terpenoids, including tocopherols, retinoids and dolichols, are important to life as vitamins, antioxidants, cofactors, and so forth; many of these are discussed in separate web pages on this site.
Complex Lipids in Membranes
Cellular membranes are semi-permeable barriers that enclose and define cells and their sub-cellular compartments by separating and protecting the interiors from the outside environment, although they can deform to enable budding, fission and fusion. They control the transport of materials, including signalling molecules, between cells and organelles, and indeed, many biochemical reactions occur within membranes, including energy production and biosynthesis of cellular components. It is evident that the specific lipid compositions of membranes have evolved to provide a barrier to the diffusion of ionic solutes and other molecules into cellular compartments where they may not be required. At the same time, the membrane environment for each organelle is distinct and provides a stable molecular platform for essential metabolic events and for intense signalling activity. Cellular membranes are the first site for receipt of extracellular signals, they recruit and activate effector molecules, and they are the launch pad for activated effector molecules throughout the cell.
The characteristic feature of membrane lipids that is essential for all these functions is that they contain both hydrophobic and hydrophilic constituents, i.e., they are amphiphilic. As such, they are weak surfactants, and they tend to form aggregates in bilayer or hexagonal-II arrangements in aqueous media in the normal temperature ranges that prevail in living cells. In natural membranes, there is a mixture of lipid types, which determine that bilayer structures predominate.
Glycerophospholipids, such as phosphatidylcholine, phosphatidylethanolamine and so forth, together with the sphingolipids, such as sphingomyelin and the glycosphingolipids, and cholesterol (sterols) are essential structural elements of all biological membranes. In the conventional model of the plasma membrane, polar lipids form a bilayer with the polar head groups oriented towards the aqueous phase while the hydrophobic fatty acyl moieties are arranged internally.
Schematic representation of a cellular membrane. Released into the public domain by its author, LadyofHats, and Wikipedia Creative Commons and gratefully acknowledged.
It is important to recognize that lipids are only part of the bilayer, as proteins, such as enzymes, transport systems or signalling receptors, span or intercalate into the bilayer and take up much of the membrane surface. Proteins interact via their basic amino acid residues with the ionic groups of polar lipids via electrostatic interactions, generating a net charge that is mainly negative or zwitterionic, and through interactions by Van der Waals forces between their hydrophobic amino acids and the fatty acid components of lipids. The lipids surrounding a membrane protein are often crucial for its tertiary structure and function, either because of direct interaction between a specific lipid and a protein or because of the physical properties of the membrane matrix surrounding the protein, e.g., its fluidity, affect its function indirectly. Defined lipid species are required to stabilize protein structures, to control the insertion and folding of proteins in membranes, and even for the assembly or polymerization of enzyme complexes with direct effects on their functions. Lipids both determine the plasticity of membranes and actively regulate the composition and function of their protein constituents. In addition, elements of the inner membrane layer are linked to the underlying cytoskeleton.
These membrane structures are not static, and free movement is possible within each leaflet (lateral diffusion) and between leaflets (vertical or flip-flop diffusion), while lipid molecules can rotate around their principal axis (rotational diffusion). The lateral and rotational diffusions are responsible for the liquid characteristics of membranes, with the constraint that the hydrophobic chains remain parallel to each other and perpendicular to the surface of the bilayer.
The distribution of lipids in each of the membrane leaflets in animals is asymmetric with phosphatidylcholine and sphingolipids located in the outer leaflet of the plasma membrane, while phosphatidylethanolamine and anionic phospholipids such as phosphatidylinositol (and polyphosphoinositides) and phosphatidylserine occur primarily in the inner leaflet. Cholesterol is believed to occur in roughly equal proportions in both faces, where it modulates the fluidity of membranes by its interaction with phospholipids. A membrane translocation machinery, which consumes large amounts of energy, is required to maintain this asymmetry. Each glycerophospholipid with its distinctive polar head group and characteristic fatty acid composition modifies the properties of a membrane in a unique manner and contributes to its overall properties. Phosphatidylcholine is often the most abundant lipid in membranes, and it has a cylindrical shape, which does not induce curvature. On the other hand, an increased concentration of cone-shaped lipids on one side and inverted cones on the other side of a membrane will bring about curvature, which may be required for membrane transport and fusion processes. Each membrane and membrane leaflet requires a specific lipid composition, including characteristic molecular species of every lipid class, to maintain structural integrity and function. The same principles govern the distributions of lipids in membranes of plants and microorganisms.
The balance between saturated, monoenoic and polyunsaturated fatty acids is important in maintaining the optimum degree of fluidity of a given membrane. Docosahexaenoic acid adopts a more flexible and compact conformation than fatty acids with few double bonds with an average length only 60% of that for oleic acyl chains, and this in turn increases the conformational disorder of saturated chains in mixed-chain phospholipids. In bacterial membranes, branched-chain and cyclopropane fatty acids modify the fluidity in an analogous manner. When ether and plasmalogen forms of lipids are considered, eukaryotic membranes can contain a thousand distinct molecular species of phospholipids. It is obviously impossible to quantify the relative importance of each of these to the physical and biological properties of membranes, and some general assessments only are possible.
While the need to form stable bilayers is a primary prerequisite for all membranes, there is also a requirement for a potential ability of the lipids to form non-bilayer structures for some membrane-associated cell processes, and short-lived non-bilayer structures with specific lipid components are probably formed in the processes of fusion and fission of lipid bilayers and for cell division. The activities of certain membrane-associated proteins can be modulated by lipids that do not form lamellar layers.
Many proteins are directed to membranes by covalent linkages to lipids, such as the glycosyl phosphatidylinositol anchors, or by modification with myristoyl, palmitoyl, prenyl or sterol moieties (proteolipids - see below). The sphingolipids together with cholesterol arrange themselves into distinct sub-domains or 'rafts' (see below) with certain membrane enzymes, and they act to compartmentalize these to exert their various biochemical functions optimally.
Phospholipids have multiple roles in cells that add to their essential function in establishing structural barriers as membrane components. They provide a matrix for the assembly and function of a wide variety of enzymes, they participate in the synthesis of macromolecules, and they act as molecular signals to influence metabolic events. Anionic lipids like phosphatidylinositol and its phosphorylated derivatives, which are concentrated on the cytoplasmic leaflet of membranes, exert a control on the properties of the membrane-cytosol interface and consequently on many aspects of membrane trafficking, including vacuole formation, transport and fusion. Specific lipids of this kind are associated with particular organelles in cells, where in combination with other signalling molecules they can recruit effector proteins with appropriate functions for each cellular compartment.
Phosphatidylcholine is a zwitterionic lipid and is usually the most abundant phospholipid in membranes of animals and plants, constituting a high proportion of the outer leaflet of the plasma membrane, and it is an integral component of the lipoproteins in plasma. It can serve as a source of diacylglycerols with a signalling function, while the plasmalogen form especially may provide arachidonate for eicosanoid production (though secondary to phosphatidylinositol). As phosphatidylcholine is the biosynthetic precursor of sphingomyelin and thence of many signalling molecules, it has an influence on innumerable metabolic pathways.
Platelet-activating factor or 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine, a closely related lipid, was one of the first biologically active phospholipids to be discovered. Amongst the innumerable activities that have been documented, it effects the aggregation of platelets at concentrations as low as 10-11M, it is a mediator of inflammation, and it is involved in the mechanism of the immune response.
Phosphatidylethanolamine is another major component of membranes, especially in bacteria, with distinctive physical properties because of its small head group and hydrogen bonding capacity. In the bacterium Escherichia coli, it supports active transport by the lactose permease, and other transport systems may require or be stimulated by it. In animal and plants, it acts as a 'chaperone' during the assembly of membrane proteins to guide the folding path for the proteins and to aid in the transition from the cytoplasmic to the membrane environment.
Phosphatidylinositol is an acidic or anionic phospholipid, a high proportion of which in animal membranes consists of the 1-stearoyl,2-arachidonoyl molecular species, which is of considerable biological importance. Phosphatidylinositol mono-, di- and triphosphates, reversibly phosphorylated on the 3, 4, or 5 positions, have major roles in cellular processes such as signalling and trafficking, and specific isomers are often identified with particular cellular compartments. They are primary precursors of 1,2-diacyl-sn-glycerols with their distinctive signalling functions (see above), together with water-soluble inositol phosphates with many different biological activities. Among innumerable other functions, phosphatidylinositol 4,5-bisphosphate can be hydrolysed by phospholipase C to produce the second messengers, inositol trisphosphate and diacylglycerol, or it can be phosphorylated to generate phosphatidylinositol 3,4,5-trisphosphate, which activates pathways required for cell growth and survival. The production of these various metabolites constitutes phosphoinositide and phosphatidylinositol cycles. In addition, phosphatidylinositol and its metabolites are the main source of arachidonic acid for eicosanoid and endocannabinoid biosynthesis. In all eukaryotes, phosphatidylinositol can serve as an anchor to link proteins covalently to the external leaflet of the plasma membrane via complex glycosyl bridges, i.e., glycosyl-phosphatidylinositol(GPI)-anchored proteins, to enable and regulate their functions.
A further acidic lipid, phosphatidylserine, contributes substantially to non-specific electrostatic interactions in the inner leaflet of membranes. This normal distribution is disturbed during platelet activation and in the process of cellular apoptosis when the lipid is transferred from the inner to the outer leaflet of the plasma membrane and acts as an "eat me" signal to scavenger cells. Phosphatidylserine chelates with calcium to act as the foundation for bone growth, and it is an essential cofactor for the activation of many enzymes, including protein kinase C, a key enzyme in signal transduction.
Cardiolipin or diphosphatidylglycerol is a unique acidic phospholipid with four acyl groups. In the mitochondria of cells, its primary location, many biological functions of this lipid have been identified, but the main ones involve activation of those enzymes concerned with oxidative phosphorylation. Indeed, it is integrated into their quaternary structure, where it is an essential component of the interface between the enzymes and their environment and may stabilize the active sites. In higher plants, cardiolipin is an integral constituent of the photosystem II complexes, which are involved in energy processes, and where it may be required for the maintenance of structural and functional properties.
Phosphatidic acid is generally a minor component of cells, but it is a key intermediate in the biosynthesis of all other phospholipids. It is known to have signalling functions in animal cells by binding to specific proteins, and it may be even more important in higher plants where it is formed rapidly in response to stresses of all kinds.
Bis(monoacylglycero)phosphate has a unique stereochemistry and a distinctive occurrence and biological function in the endosomal membranes of cells, where it is relatively resistant to enzymatic hydrolysis.
Lysophospholipids, i.e., with only one mole of fatty acid per mole of lipid, were long thought to be merely intermediates in the biosynthesis of phospholipids that if allowed to accumulate were potentially disruptive to cells because of their powerful detergent properties. Lysophosphatidic acid is of special interest and has been shown to have signalling and other biological effects that are dependent on receptor mechanisms. It is produced by a wide variety of cell types with most mammalian cells expressing receptors for it, and it is involved in the activation of protein kinases, adenyl cyclase and phospholipase C, in the release of arachidonic acid for eicosanoid synthesis, and much more. Interest was stimulated especially by a finding that lysophosphatidic acid is elevated significantly in the plasma of ovarian cancer patients compared to healthy controls, so that it may represent a useful marker for the early detection of the disease. Other lysophospholipids, including the sphingolipid analogue, sphingosine-1-phosphate (below), exhibit their own biological activities.
Mono- and digalactosyldiacylglycerols and sulfoquinovosyldiacylglycerol (a sulfonolipid) are important components of membranes of chloroplasts and related organelles, and indeed these are the most abundant lipids in all photosynthetic tissues, including those of higher plants, algae and cyanobacteria. They may substitute in part for phospholipids, especially when phosphorus is limiting, although the distinctive ability of monogalactosyldiacylglycerols to form inverted micelles may be important for membrane structure and for interactions with specific proteins. The thylakoid membrane where photosynthesis occurs in plants has an asymmetric distribution of glycolipids, with much of the digalactosyldiacylglycerol on the luminal leaflet, where it may assist the movement of protons along the membrane surface to the ATPase. While many different functions have been ascribed to these lipids, their primary importance lies in their interactions with the photosynthetic apparatus.
Although glycosyldiacylglycerols have been found in animal tissues, they are usually present in rather small amounts, and their role in mammalian membranes is poorly understood. On the other hand, the lipid sulfate seminolipid or 1-O-hexadecyl-2-O-hexadecanoyl-3-O-β-D-(3'-sulfo)-galactopyranosyl-sn-glycerol, which was first found in mammalian spermatozoa and testes, is known to be essential for spermatogenesis and may have a role in myelination in the central nervous system.
Sphingolipids are characterized by the presence of a long-chain or sphingoid base, such as sphingosine, to which a fatty acid is linked by an amide bond and usually with the primary hydroxyl group attached to complex phosphoryl or carbohydrate moieties. They have an immense range of functions in tissues that are quite distinct from those of the complex glycerolipids. While sphingomyelin has structural similarities to phosphatidylcholine, it has very different physical and biological properties, and the complex oligoglycosylceramides and gangliosides have no true parallels among the glycerolipids.
Free sphingoid bases are found at trace levels only in tissues, but they are mediators of many cellular events; they inhibit the enzyme protein kinase C, and they are inhibitors of cell growth, although they stimulate cell proliferation and DNA synthesis. Some of the structural features of the long-chain bases are only introduced after they are esterified with long-chain fatty acids to form ceramides, which are the primary precursors of the complex sphingolipids. Ceramides have an important role in cellular signalling and especially in the regulation of apoptosis and of cell differentiation, transformation and proliferation. In contrast, sphingosine-1-phosphate is a vitally important sphingosine metabolite that promotes cellular division (mitosis) as opposed to apoptosis, so that the balance among the former and ceramide, ceramide-1-phosphate and sphingosine levels in cells is critical. In fact, the biosynthesis and catabolism of sphingolipids involves numerous metabolites, many of which have distinctive biological activities. In animals the relationships between these metabolites have been rationalized in terms of a 'sphingomyelin cycle', in which each of the various compounds has characteristic metabolic properties. Some comparable sphingolipid pathways occur in plants (although sphingomyelin is not involved).
Sphingomyelin is by far the most abundant sphingolipid in animal tissues. As well as serving as a source of key cellular metabolites, it is an important building block of membranes and like its glycerolipid analogue phosphatidylcholine tends to be most abundant in the plasma membrane of cells and especially in the outer leaflet. The sphingolipids in general contain high proportions of longer-chain saturated and monoenoic fatty acids, often accompanied by high proportions of 2-hydroxy but not polyunsaturated fatty acids.
Sphingomyelin and other sphingolipids together with cholesterol are often located in an intimate association in specific sub-domains or 'rafts' (or related structures termed 'caveolae') of membranes, but especially the plasma membrane. These are laterally segregated regions that form because of selective affinities between sphingolipids and membrane proteins. As sphingolipids containing long saturated acyl chains, they pack more tightly together, thus giving sphingolipids much higher melting temperatures than glycerophospholipids. This tight acyl chain packing is essential for raft lipid organization, since the differential packing facility of sphingolipids and cholesterol in comparison with glycerophospholipids leads to spontaneous phase separation in the membrane, thus giving rise to the sphingolipid-rich regions ('liquid-ordered' phase) surrounded by glycerophospholipid-rich domains ('liquid-disordered' phase). The ordered phases are relatively resistant to attack by detergents, a property that was once used to define them. An important result of this process is that rafts contain a variety of different proteins, including glycosyl-phosphatidylinositol (GPI)-anchored proteins and tyrosine receptor kinases. These provide much of the important biological properties of rafts and are essential to maintain their stability. Comparable micro-domains or rafts that are enriched in sphingolipids (other than sphingomyelin), sterols and specific proteins have been detected in the plasma membrane of plant cells.
Monoglycosylceramides or cerebrosides are common constituents of membranes of animals and plants. Galactosylceramide is the principal glycosphingolipid in brain tissue and myelin, while glucosylceramide is a major constituent of skin lipids and is the source of the unusual complex ceramides that are found in the stratum corneum. In plants, glucosylceramides can elicit defence responses against fungal attack, and they appear to assist plants to withstand stresses brought about by cold and drought.
The membranes of animals contain a wide range of complex oligoglycosylceramides for which glucosylceramides are the biosynthetic precursor; forms with several hundred different head groups that differ in the numbers, types and arrangements of the carbohydrate moieties have been characterized. Most of these occur on the external leaflet of the plasma membrane in rafts, where they are important components of the body's immune defence system, both as cellular immunogens and as antigens. Certain glycosphingolipids are involved in the antigenicity of blood group determinants, while others bind to specific toxins or bacteria. Some function as receptors for cellular recognition, and they can be specific for particular tissues or tumours.
Glycosphingolipid sulfates are highly polar acidic molecules that are important in the transport of sodium and potassium ions and osmoregulation in animal tissues, and they may have a role in the protection of the intestinal mucosa against digestive enzymes. Gangliosides are complex oligoglycosylceramides containing sialic acid residues and are highly polar and acidic. As cell-type specific antigens that control the growth and differentiation of cells, they have a significant role in the interactions between cells especially in the immune defence system. They are especially important for myelination in brain and other nervous tissues, where they are most abundant. As gangliosides act as receptors for interferon, epidermal growth factor, nerve growth factor, insulin and many other metabolites, they are able to regulate cell signalling. Certain gangliosides bind to specific bacterial toxins, and they mediate interactions between microbes and host cells during infections.
Proteolipids and Lipoproteins
Proteins that contain covalently bound fatty acids or other lipid moieties, such as isoprenoids, cholesterol and glycosylphosphatidylinositol, are widespread in nature with many important functions. The term proteolipid can be used to define such complexes and to differentiate them from the plasma lipoproteins, which have very different structures and functions. Two main types of protein with a fatty acid modification have been described, i.e., those with only myristoyl and those with predominantly palmitoyl moieties, each with a distinctive type of linkage, amide or thiol ester, respectively. The prenylated lipids contain an isoprenoid group, farnesyl or geranylgeranyl, linked via a sulfur atom (thiol ether bond) to the protein, while the so-called “hedgehog” proteins, which are important in cellular development, are modified covalently by both cholesterol and N-palmitoyl moieties. The hormone ghrelin and the Wnt proteins, important in the development of animal tissues, are O-acylated.
It is now clear that such modifications are important in determining the activities of proteins and in targeting them to specific subcellular membrane domains, including the rafts in plasma membranes. Thus, both myristoylated and palmitoylated proteins are targeted to rafts (as are the GPI-anchored proteins), but prenylated lipids are not. It is noteworthy that many signalling proteins are modified by lipids with implications for events at cell surfaces.
Lipoproteins are complex aggregates of lipids and proteins (not bound covalently) that render the lipids compatible with the aqueous environment of body fluids and enable their transport throughout the body of all vertebrates, beginning with uptake of dietary lipids in the intestines. Within the circulation, these aggregates are in a state of constant flux, changing in composition and physical structure as the peripheral tissues take up the various components before the remnants return to the liver. The most abundant lipid constituents are triacylglycerols, free cholesterol, cholesterol esters and phospholipids (phosphatidylcholine and sphingomyelin especially), although fat-soluble vitamins and some antioxidant molecules are transported in the same way. Free (unesterified) fatty acids and lysophosphatidylcholine are bound to the protein albumin by hydrophobic forces in plasma.
Ideally, the lipoprotein aggregates should be described in terms of the different protein components or apoproteins (or ‘apolipoproteins’), as these determine the overall structures and metabolism, and the interactions with receptor molecules in liver and peripheral tissues. However, the practical methods that have been used to separate different lipoprotein classes for study have determined the nomenclature. Thus, the main groups are classified as chylomicrons (CM), very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL), based on the relative densities of the aggregates on ultracentrifugation. Lipoproteins deliver nutrients to the peripheral tissues and are the key to maintaining a healthy balance of cholesterol, triacylglycerols (or their fatty acid constituents) and other lipids within the body.
Many more lipids occur in nature with important biological functions than can be described in this brief account, and while lipopeptides, lipopolysaccharides, betaine lipids, fat-soluble vitamins, rhamnolipids, arsenolipids and so forth are not discussed in this document, you will encounter them elsewhere on this website. There are a host of lipids that are unique to specific organisms from bacteria to marine invertebrates that are not described above, and readers will find information on the chemistry, biochemistry and functions of most of these in other web pages on this site.
I trust that this brief commentary and the various pages in this website will provide an insight into the importance of lipids to the health and well-being of all living creatures and will stimulate further reading. The various documents dealing with individual lipid classes on this website have reading lists attached that should help readers obtain much more information, but the following books and review articles would form a good basis for any lipid library -
- Casares, D., Escribá, P.V. and Rosselló, C.A. Membrane lipid composition: effect on membrane and organelle structure, function and compartmentalization and therapeutic avenues. Int. J. Mol. Sci., 20, 2167 (2019); DOI
- Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Woodhead Publishing and now Elsevier) (2010) - see Science Direct.
- Gunstone, F.D., Harwood, J.L. and Dijkstra, A.J. (Editors), The Lipid Handbook (3rd Edition). (CRC Press, Boca Raton) (2007) - see CRC Press.
- Gurr, M.I., Harwood, J.L., Frayn, K.N., Murphy, D.J. and Michell, R.H. Lipids: Biochemistry, Biotechnology and Health (6th Edition). (Wiley-Blackwell) (2016).
- Ridgway, N.D. and McLeod, R.S. (Editors) Biochemistry of Lipids, Lipoproteins and Membranes (6th Edition). (Elsevier, Amsterdam) (2016) - see Science Direct - now in a 7th edition (2021).
- Yang, Y.B., Lee, M. and Fairn, G.D. Phospholipid subcellular localization and dynamics. J. Biol. Chem., 293, 6230-6240 (2018); DOI.
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