lipid

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Any of a group of organic compounds, including the fats, oils, waxes, sterols, and triglycerides, that are insoluble in water but soluble in nonpolar organic solvents, are oily to the touch, and together with carbohydrates and proteins constitute the principal structural material of living cells.

[French lipide : Greek lipos, fat; see lipo– + French -ide, -ide.]

One of a class of compounds which contains long-chain aliphatic hydrocarbons (cyclic or acyclic) and their derivatives, such as acids (fatty acids), alcohols, amines, amino alcohols, and aldehydes. The presence of the long aliphatic chain as the characteristic component of lipids confers distinct solubility properties on the simpler members of this class of naturally occurring compounds.

The lipids are generally classified into the following groups:

1. Simple lipids

   1. Triglycerides or fats and oils are fatty acid esters of glycerol. Examples are lard, corn oil, cottonseed oil, and butter.

   2. Waxes are fatty acid esters of long-chain alcohols. Examples are beeswax, spermaceti, and carnauba wax.

   3. Steroids are lipids derived from partially or completely hydrogenated phenanthrene. Examples are cholesterol and ergosterol.

   B.

   Complex lipids

   1. Phosphatides or phospholipids are lipids which contain phosphorus and, in many instances, nitrogen. Examples are lecithin, cephalin, and phosphatidyl inositol.

   2. Glycolipids are lipids which contain carbohydrate residues. Examples are sterol glycosides, cerebrosides, and plant phytoglycolipids.

   3. Sphingolipids are lipids containing the long-chain amino alcohol sphingosine and its derivatives. Examples are sphingomyelins, ceramides, and cerebrosides.

   Lipids are present in all living cells, but the proportion varies from tissue to tissue. The triglycerides accumulate in certain areas, such as adipose tissue in the human being and in the seeds of plants, where they represent a form of energy storage. The more complex lipids occur closely linked with protein in the membranes of cells and of subcellular particles. More active tissues generally have a higher complex lipid content; for example, the brain, liver, kidney, lung, and blood contain the highest concentration of phosphatides in the mammal. See also Fat and oil; Fat and oil (food); Glycolipid; Sphingolipid; Steroid; Triglyceride; Vitamin; Wax, animal and vegetable.

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(Also sometimes lipides, lipins.) A general term for fats and oils (chemically triacylglycerols), waxes, phospholipids, steroids, and terpenes. Their common property is insolubility in water and solubility in hydrocarbons, chloroform, and alcohols.

An organic compound, insoluble in water, but which dissolves readily in other lipids and in organic solvents such as alcohol, chloroform, and ether. Lipids contain carbon, hydrogen, oxygen, and sometimes phosphorus. They are classified according to their solubility and include neutral fats (triglycerides), phospholipids, and steroids. See also fat.

A heterogeneous group of substances related actually or potentially to the fatty acids that are soluble in nonpolar solvents such as benzene, chloroform, and ether and are relatively insoluble in water. Included are the fatty acids, acylglycerols, phospholipids, cerebrosides, and steroids.

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Organic compound, insoluble in water, but which dissolves readily in other lipids and in organic solvents, such as alcohol, chloroform, and ether. Lipids contain carbon, hydrogen, and oxygen, and sometimes phosphorus. They are commonly classified into three groups: simple lipids (e.g. neutral fats, triacylglycerol or triglyceride, and waxes), compound lipids (e.g. phospholipids such as lecithin, glycolipids, and lipoproteins), and derived lipids (e.g. fatty acids such as oleic acid and stearic acid, steroids such as cholesterol and oestrogen, and hydrocarbons).

Lipids (fats and oils) have borne the brunt of the blame for the degenerative diseases (heart disease and cancer) that are the major causes of death in the developed world. The negative view of lipids has obscured their essentiality for human health. If a problem exists, it is one of quantity, in general, and specific lipids in particular.

Lipids are important for maintenance of human health and well-being in a number of ways. Probably the most important function of lipids is provision of an efficient energy source. Fat provides 9 calories of energy per gram or 2.25 times as much as either carbohydrate or protein. Carbohydrate is not stored in the body and protein stores are predominantly muscle, whose breakdown entails serious health consequences. Fat is stored as such and can be easily mobilized if needed. In primitive times survival may have been possible because of energy provided by metabolic use of stored fat (Gurr and Harwood, 1991).

Lipids are a group of substances of diverse structures that share the common trait of being soluble in solvents such as ether or benzene. The major lipids of the body are triglycerides, which comprise a molecule of glycerol to which three fatty acids are bonded. Phospholipids are substances in which glycerol carries only two fatty acids plus phosphoric acid and an organic base such as choline or serine. Cholesterol is a member of the family of large complex molecules generically called steroids. It has the capacity to carry one molecule of fatty acids (cholesteryl ester). Cell membranes are predominantly composed of phospholipids and cholesterol. Cell membranes confer stability to cells and control entry or release of chemicals into or from the cell. Lipids serve as effective insulators and help in maintaining body temperature. Important organs such as the heart, kidneys, and reproductive organs are cushioned by fat. Nerves are protected by a sheath (myelin) that contains cholesterol, phospholipids, and other lipids.The animal organism carries a number of essential substances that catalyze chemical reactions in cells. These are called vitamins and are designated by letters. The B and C vitamins are soluble in water; the others, vitamins A, D, E, and K, are insoluble in water but soluble in fats. They are transported in lipids in the blood and stored in fat in the body.

Chemistry

Cholesterol is a molecule that is found in the membrane of every cell. About 0.2 percent of the average body weight is cholesterol. Most of this cholesterol is present in the muscle (cell membrane) or brain (as insulation against trauma). The functions of cholesterol in the brain are still poorly understood. Most of the cholesterol in the body is manufactured in the liver, and the diet makes a relatively small contribution to this pool. Cholesterol, in turn, is the parent substance of a number of vital compounds. Among these are the bile acids that are necessary for proper absorption and digestion of fat; the corticosteroids such as cortisol and hydrocortisone that are essential to life; progesterone which is required for normal reproduction, and the male and female sex hormones. The involvement of cholesterol in the etiology of coronary heart disease will be discussed below.

Fatty acids are chains of carbon acids that culminate in an acidic group called a carboxyl group. Each carbon atom has the capacity to bind four other atoms. In the fatty acid chain, two of those binding elements are bound to the carbon atoms on either side, and the other two are bound to hydrogen atoms. If the hydrogen atoms on adjacent carbon atoms are missing, the two carbons (which are already bound by one bond) form a second bond, and these are called double bonds. A fatty acid lacking the maximum number of hydrogen atoms is called an unsaturated fatty acid. The most common fatty acid in the human body is palmitic acid (16:0, which designates sixteen carbon atoms and no double bonds). Oleic acid (18:1) is the next common fatty acid. The diet provides linoleic (18:2) and linolenic (18:3) acids, which are called "essential fatty acids," meaning fatty acids that are essential to life and health and cannot be synthesized by the human body. Linoleic acid is converted via arachidonic acid to a series of compounds with hormonal activity called prostaglandins. The prostaglandins are usually made within the tissue in which they act and are involved in diverse functions such as control of inflammation, uterine contraction during labor, and blood platelet aggregation. An important group of long-chain polyunsaturated acids (polyunsaturated fatty acids [PUFAs]) occur in the fats of cold-water fish such as salmon and cod. The two principal PUFAs are eicosapentaenoic acid (20:5) and docosahexaenoic acid (22:6). While these fatty acids do not necessarily affect blood cholesterol levels, their presence in the diet has been associated with a reduced risk of cardiovascular disease.They have been shown to be essential to development of normal vision and also to influence brain development in newborns (Innis, 1991).

Phospholipids are glycerol derivatives in which two of the hydroxyls are esterified to fatty acids and the third to phosphoric acid, which is, in turn, esterified to a base. In lecithin, the most abundant phospholipid, the base is choline. The fatty acid in the 2 position of a phospholipid is usually polyunsaturated. It is often arachidonic acid (20:4), a product of metabolism of essential fatty acid, and a direct precursor of prostaglandins.

Biochemistry

Blood is an aqueous medium that contains an appreciable amount of lipid. Normal blood serum or plasma appears as a pale yellow, clear liquid, because the fat has been emulsified to give water-soluble fat-protein aggregates. These aggregates are designated as lipoproteins and have a lipid core and a protein coat. Fat enters the lymph in the form of chylomicrons, which are large triglyceride-rich particles. In the course of circulation the triglyceride is deposited in or metabolized by cells and the particles become smaller in size. The lipoproteins can be separated physically on the basis of their hydrated density and are designated as very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Although estimations of the lipid composition of the various lipoproteins are available, their size and shape may vary.

The proteins surrounding the lipid core (apoproteins) have been characterized and their biological functions catalogued. Thus, apolipoprotein AI (ApoAI) and apolipoprotein AII (ApoAII) are present only in HDL and are required for metabolism of the lipid portion of HDL. ApoAI activates lecithin-cholesterol acyltransferase, which is active in the synthesis of cholesterol esters, and ApoII is required for breakdown of the triglycerides by lipoprotein lipase.

Apolipoprotein B (ApoB) occurs only in LDL and is required for secretion of the triglyceride-rich lipoproteins. The exclusivity of ApoA and ApoB to HDL and LDL, respectively, is often used for determination of LDL/HDL ratios. Apolipoprotein E (ApoE) is present in both VLDL and HDL. It occurs in several modifications (isoforms), which may determine level of success in treatment of hypercholesterolemia and which have been hypothesized to influence susceptibility to Alzheimer's disease. An LDL variant, Lp(a), appears to confer increased susceptibility to atherosclerosis, and its presence in serum is often used as an additional diagnostic indicator. The principal lipoproteins, LDL and HDL, are known popularly as the "bad" and the "good" cholesterol. Elevated levels of LDL are a risk factor for heart disease, hence LDL is considered to be a "bad" lipoprotein. Elevated HDL levels lower the risk of heart disease, hence the designation '"good" cholesterol. LDL is rich in cholesterol and delivers cholesterol into cells, whereas HDL, which is about 50 percent protein, aids in cholesterol egress from cells.

Table 1

Heart Disease

There is a roster of risk factors that are associated with an increased chance of succumbing to heart disease, but none of these factors is an unequivocal risk. Risk in places like Las Vegas is called "odds." There are a number of well-documented risk factors for development of coronary heart disease. Heredity and age are beyond control. The others are elevated blood pressure, elevated blood cholesterol, smoking, obesity, diabetes, physical inactivity, and stress. Each factor exerts its effects differently in each individual. These factors may also interact. It is now becoming accepted that the initial injury in atherosclerosis may be inflammation, which complicates the risk picture (Ross, 1993). There are suggestions that infection in some way prepares the arterial tissue for the subsequent metabolic events. At present we must monitor the various controllable risk factors, bearing in mind the possibility that a prior event may determine the extent to which the risk factors affect risk. In the United States, deaths from heart disease (cases per 100,000, adjusted for age) peaked in 1968 and have been falling since then. Between 1960 and 1998 mortality from all causes in men fell by 33.8 percent and coronary heart disease mortality by 51.0 percent. In women the reductions were 33.7 and 50.1 percent, respectively. Incidence of the disease may be rising as population increases and other modes of demise diminish or disappear. A century ago the major causes of death were related to infection, while a half-century ago the average age of victims of coronary disease was considerably below what it is today. This is a public health triumph due to improved diagnosis and treatment. The aim now should be to achieve productive and healthy aging.

Of the risk factors cited above none has received more attention than blood cholesterol. Dietary studies related to atherogenesis were conducted early in the twentieth century; they usually involved a combination of dietary alterations plus physical stress. The earliest purely nutritional study was carried out by Ignatowski in 1909. He observed aortic atherosclerosis when weanling rabbits were fed milk and egg yolk or when adult rabbits were fed meat. A few years later Anitschkow (1913) fed rabbits cholesterol and reported atherosclerotic lesions and fat deposition. Anitschkow's work established dietary cholesterol as the modality for establishment of atherosclerotic-like lesions, and this was carried over to human nutrition; consequently dietary cholesterol was presumed to be the principal contributor to cardiovascular disease. Relatively mild interest in cholesterol and atherosclerosis was evinced in the research and medical communities for the few decades after Anitschkow's publications. In the late 1940s and early 1950s interest in cholesterol intensified. The reasons for this renewed interest were an increase in death from coronary disease, as death from infectious causes waned and new research findings, especially Gofman's demonstration of the separation of different lipoprotein classes, which differed in their chemistry (Gofman et al., 1950). The cholesterol-rich lipoproteins were associated with greater susceptibility to heart disease. Subsequently the research area developed the concept of risk factors, of which elevated blood cholesterol was the first clearly defined one. At about the same time epidemiological studies, many conducted by Ancel Keys, began to show that populations whose diets were rich in cholesterol and fat demonstrated high death rates from heart disease.

At this point it might be important to distinguish between the effects of dietary cholesterol and dietary fat. While there is no argument that blood cholesterol is a risk factor for coronary disease, the connection with dietary cholesterol is not strong.The connection between dietary cholesterol and blood cholesterol is controversial. The data show that the amount of dietary cholesterol plays a lesser role in affecting blood cholesterol than does the type of dietary fat. Dietary cholesterol plus saturated fat is much more cholesterolemic than the same amount of cholesterol plus unsaturated fat (McNamara, 1987). Since dietary cholesterol is often accompanied by saturated fat, it is considered prudent to limit its intake. Gertler et al. (1950) reported a study in which they had segregated from a large cohort of coronary patients and controls four groups of ten men each, those who ate the most cholesterol and those who ate the least, and those with highest or lowest plasma cholesterol levels. In every subgroup the coronary patients exhibited significantly higher plasma cholesterol levels than did the controls—thus confirming the role of cholesterol as a risk factor. However, in no group did the investigators find any correlation between dietary cholesterol intake and blood cholesterol level. Thirty years later an attempt was made to correlate diet with coronary disease in three large populations under continuous study. The populations were in Framingham, Massachusetts; Puerto Rico; and Hawaii. Diets of men who had had a coronary event and those who had not differed significantly in total calories (lower in cases), complex carbohydrate (lower in cases), and alcohol intake (lower in cases). Intake of fat or cholesterol was the same in cases and controls (Gordon et al., 1981).

Type of dietary fat affects atherogenesis in rabbits and cholesterolemia in humans. Keys (1965) and Hegsted (1965) and their colleagues showed that fats rich in saturated fatty acids promoted cholesterolemia. They developed formulas to predict changes in blood cholesterol based on dietary saturated and/or unsaturated fatty acids. Since the publication of the original formulas many revised and refined versions have appeared. The new formulas provide coefficients for specific fatty acids, but none has proved to be more serviceable or useful than the originals. It should be pointed out that even the most saturated dietary fat, coconut oil, contains oleic (about 7 percent) and linoleic (about 2 percent) acids, and that one of the most unsaturated fats, safflower oil, contains about 7 percent palmitic acid and 2 percent stearic acid. In the Keys and Hegsted formulas stearic acid is viewed as "neutral" because it has no effect on blood cholesterol.

An issue that has been debated for several decades is the role of trans-fatty acids. In most naturally occurring unsaturated fatty acids the hydrogen atoms attached to the carbons that constitute the double bond are spatially on the same side of the molecule (cis); when they are on opposite sides, they are designated as "trans." There are many trans fats in nature but not many in our usual diet. However, trans double bonds may be formed during hydrogenation of fat used for margarines. The major source of trans fat in the diet is margarine and baked goods made with margarines or margarine stock. Concerns over diets high in trans fats were aired in the 1940s and 1950s. It was found then that in rabbits fed atherogenic diets trans fat elevated cholesterol levels but did not increase severity of atherosclerosis (McMillan et al., 1963). The question of trans fat effects is complicated because hydrogenation may provide fats with double bonds anywhere from carbon 4 to carbon 14 of the fatty acid. Recent research shows that trans fat lowers levels of HDL-cholesterol in humans. It has also been demonstrated that trans fats have little effect in diets containing high levels of polyunsaturated fat. Because of health concerns margarine manufacturers have begun to produce products containing little or no trans-unsaturated fat (Kritchevsky, 1999b).

Ingestion of cholesterol per se appears to have little effect on cholesterolemia. Numerous studies have shown that eggs, the richest source of cholesterol, have little effect on blood cholesterol (McNamara, 2000). However, most cholesterol in the diet is associated with animal fats, which are more saturated than plant fats. Hence the admonition to exercise prudence in ingestion of cholesterol.

The field of fat and cholesterol is still active and as new fats and new facts emerge dietary suggestions will be modified. At one time we were admonished to eat a virtually fat-free diet, but fat is a necessary nutrient. Very low-fat diets present their own problems, since diets too high in carbohydrate may affect insulin metabolism and can lead to triglyceridemia (Lichtenstein and Van Horn, 1998). In the 1950s high plasma triglyceride levels were considered to be an independent risk factor for coronary disease. For a long while triglyceride levels were virtually ignored, but they are beginning to reassume importance as new clinical and epidemiological data appear. Similarly, the appreciation of specific aspects of fatty acid effects has led to changes in recommendations regarding their intake. At one time the entire emphasis was on polyunsaturated fat, but it was shown that this type of fat lowered both LDL and HDL cholesterol whereas monounsaturated fat (olive oil, for instance) reduced only the "bad" lipoprotein (LDL), leading to a more acceptable LDL-cholesterol/HDL-cholesterol ratio (Mattson and Grundy, 1985). These observations have led to support of the "Mediterranean diet," which is rich in monounsaturated fat but also contains more vegetables and fruit than does the present American diet.

In general terms, current recommendations suggest a diet containing 30 to 35 percent calories from fat with no more than 7 to 10 percent being saturated fat and about 30 to 40 percent carbohydrate, with adequate levels of dietary fiber. Liberal intakes of vegetables and fruit (five to seven servings per day) are also recommended as we begin to find that various plant constituents (carotenoids, flavonoids, phytosterols) may contribute to cardiovascular health. The role of caloric intake is not always addressed directly, but obesity is looked upon as a risk, and daily physical activity is encouraged (Krauss et al., 1996, 2001).

Our view of coronary disease keeps changing with new research findings. Whereas it was originally thought to be simply fat deposition, we now view it as an inflammatory process that can be stimulated by oxidized cholesterol and specific growth factors (Ross, 1993). The initial inflammation may be caused by viral or bacterial infection. The size of the LDL particle may be important; thus small, dense LDL particles may indicate increased risk even in the face of normal lipid levels (Krauss and Burke, 1982). Lipoprotein (a), a slightly altered LDL, affects blood clotting and may be an independent risk factor (Loscalzo, 1990).

The question of established and emerging risk factors has been addressed. The well-established, major risk factors continue to be cigarette smoking, hypertension, elevated serum cholesterol, elevated LDL cholesterol, low-HDL cholesterol, diabetes, and aging. Additional factors that predispose to coronary disease are family history of premature coronary disease (genetics), obesity, physical inactivity, and psychosocial factors (stress, for instance). Other risk factors are also beginning to appear—some are general and the causative actions of some are not clear. Among these are elevated serum homocysteine levels, first suggested over thirty years ago and possibly connected with metabolism of folic acid and vitamins B₆ and B₁₂ (Malinow et al., 1999). C-reactive protein (CRP) is a general marker of inflammation produced in the liver in response to bacterial infection or physical trauma. The risk of coronary events is elevated in subjects with elevated levels of cholesterol and CRP (Ridker et al., 1999).Coronary heart disease is related to elevated serum lipids, diabetes, and obesity. All may be influenced by diet but the view of diet becomes more sophisticated and goes beyond dietary fat, although fat still plays a significant role. There is a plethora of risk factors of varying significance, and we still have no unequivocal indication of which subject's risk is affected by which particular factor.

Cancer

The role of fat in cancer has also been the subject of much research inquiry. In a classic study, Armstrong and Doll (1975) investigated the effects of diet on a number of cancers. Positive associations were found between total fat consumption and colorectal or breast tumors. Animal studies showed that a high-fat diet was more co-carcinogenic than a low-fat diet and that unsaturated fat was more co-carcinogenic than saturated fat. The latter result were due to the fact that linoleic acid is a growth factor for tumors (Carroll and Khor, 1971).

The data concerning fat and cancer risk are inconsistent. High intake of fat is a marker for a high-calorie diet and it is possible that it is the caloric contribution of fat rather than fat itself that is the culprit. Hoffman (1913) suggested that "erroneous diet" was a factor in the etiology of cancer. Excess body weight has been correlated with cancer mortality (Garfinkel, 1985). Animal studies dating to 1909 show that caloric restriction leads to reduced tumor growth. Lavik and Baumann (1943) showed that the incidence of methylcholanthrene-induced skin tumors in mice fed a diet high in fat but low in calories was 52 percent lower than that seen in mice fed a diet high in calories but low in fat. It has also been shown that incidence of dimethylbenz(a)anthracene induced mammary tumors in rats fed 5 percent fat ad libitum is lower than in rats fed a diet containing 20 percent fat but whose energy intake is restricted by 20 percent (Klurfeld et al., 1989).

Epidemiological studies have shown a positive correlation between energy intake and breast or colon cancer risk. The factors underlying the cancer-inhibiting effects of energy restriction are under study. Energy restriction leads to reduction in circulating insulin, and insulin is a growth factor for tumors. Energy restriction also reduced oncogene expression and leads to enhanced DNA repair (Kritchevsky, 1999a).

Diet

When all of the above has been said, the question each of us must answer remains, "What should I eat?" Dietary suggestions have ranged from the four food groups (meat, carbohydrates, dairy, and fruits and vegetables) to the United States Department of Agriculture (USDA) pyramid. The USDA pyramid is an attempt to illustrate which foods should be eaten in which amounts. The broad base of the pyramid represents large quantitites of grains and starches, and the narrow peak represents small quantities of fats and oils. Other dietary components are displayed between the peak and the base and their position in the pyramid represents the relative suggested levels of intake. The idea is to incorporate the best dietary information of the day into a healthful eating pattern. The "Dietary Guidelines for Americans" are written by select committees appointed by the United States Departments of Agriculture and Health and Human Services, and the publication is disseminated under their joint sponsorship. The guideline recommendations have changed relatively little in the past few decades, but the changes that appear reflect current findings and opinion. We are told to maintain ideal weight, although nobody is certain what that means. Originally we were advised to eat a diet that would provide protection against the ravages of infection, but now we are intent on protection against degenerative diseases, heart disease, and cancer, for which we have developed a catalog of risk factors but have no unequivocal diagnoses. Another general factor that we did not have to deal with in the past is the rise in obesity.

Vegetables and fruits provide chemicals that, in the laboratory, protect against cancer and heart disease and provide little or no fat. Grains are part of a healthful diet because they provide complex carbohydrate and fiber. Meat provides high-grade protein, necessary trace minerals (zinc, manganese, iron) and vitamin B₁₂, but fear of its fat content is reflected in advice to limit its consumption. Dietary fats are limited because of their caloric content, but they contain the essential fatty acids. Advice about dietary components is presented with the implied view that they are metabolized in a similar manner despite their quantity or presence of other nutrients in the diet. There is virtually no information concerning interaction of individual nutrients.

Fat is feared because of its caloric density and its connection with the risk of heart disease or cancer. The food industry is capable of producing foods that address current concerns. We have available a host of fat-free snacks, but their caloric content is rarely different from the fatrich food they are replacing. Thus, influence on a risk may be diminished but there is no effect on body weight. Very low-fat diets are criticized as unhealthy. Diets high in carbohydrate may affect insulin metabolism, and there are some investigators who believe that insulin resistance may underlie both cancer and coronary disease.

General dietary advice—enough essential nutrients to maintain health—is constant but the specifics are distributed on an ad hoc basis depending on current knowledge. A case in point is the avocado. Thirty or so years ago this fruit was not recommended because of its fat content. Today we know the fat is monounsaturated ("good") and the avocado also contains generous quantities of various carotenoids. The avocado is now recommended by nutritionists everywhere. Fat content?—well, just don't eat too much of it. Carotenoids are a family of chemicals that occur in highly colored fruits and vegetables. Some may be precursors of vitamin A. The most common carotenoid is lycopene, which occurs in tomatoes.

To return to the specifics—namely, what we should eat—we still mean a "well-rounded" diet, to be taken in quantities that do not influence body weight. Suggestions to exercise regularly are also becoming part of dietary advice, again for purposes of weight control. Sugary snacks and sugar-rich beverages should be kept to a minimum. The ideal diet, in addition to its content, requires input from the consumer—namely, a measure of discipline.

Healthful diets go beyond "one size fits all." Growing children have different requirements than adults. The elderly may require different levels of various nutrients, and the active elderly have different needs than do the infirm elderly.

So we come down to the general advice of a little of everything but not too much of anything. The advice has to consider age, activity, and health status. Eating should be a pleasurable, social activity and not feared as the specific arbiter of life and death. The best advice for the average healthy person is variety, balance, and moderation. The watchword should be: Moderation, not Martyrdom.

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—David Kritchevsky

A group of organic molecules that includes fats, oils, and waxes. Lipids do not dissolve in water. In animals, including humans, lipids store energy and form parts of cell structures, such as cell membranes.

A group of substances comprising fatty, greasy, oily and waxy compounds that are insoluble in water and soluble in nonpolar solvents, such as hexane, ether and chloroform.
Simple lipids are the triglycerides or neutral fats. Each triglyceride molecule is composed of one molecule of glycerol joined by ester linkages to three fatty acid molecules. They are an important source of oxidizable substrate to the body and have a greater caloric density (2.25 times) than carbohydrate.
Compound lipids are important structural components of cell membranes. Phospholipids include lecithin and the cephalins, which are composed of fatty acids linked to phosphatidic acid, and the sphingomyelins, which are composed of fatty acids linked to sphingosine. Glycolipids are composed of a carbohydrate chain and fatty acids linked to sphingosine or ceramide. Cholesterol is a steroid alcohol. Another important function of the phospholipids is as lung surfactants.

• l. A — a component of the cell wall of gram-negative bacteria, responsible for their toxic properties.
• l. pneumonia — see lipid pneumonia.
• protected l's — fats treated to protect them against microbial degeneration in the rumen.
• l. transport disease — a group of diseases in which there is a disorder of lipid metabolism with abnormal levels or types of lipoproteins in the blood, e.g. hyperlipoproteinemia, hyperlipemia.

Structures of some common lipids. At the top are oleic acid^[1] and cholesterol.^[2] The middle structure is a triglyceride composed of oleoyl, stearoyl, and palmitoyl chains attached to a glycerol backbone. At the bottom is the common phospholipid, phosphatidylcholine.^[3]

Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.

Lipids may be broadly defined as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, liposomes, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building blocks": ketoacyl and isoprene groups.^[4] Using this approach, lipids may be divided into eight categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids and polyketides (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits).

Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as other sterol-containing metabolites such as cholesterol.^[5] Although humans and other mammals use various biosynthetic pathways to both break down and synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

Contents 1 Categories of lipids 1.1 Fatty acyls 1.2 Glycerolipids 1.3 Glycerophospholipids 1.4 Sphingolipids 1.5 Sterol lipids 1.6 Prenol lipids 1.7 Saccharolipids 1.8 Polyketides 2 Biological functions 2.1 Membranes 2.2 Energy storage 2.3 Signaling 2.4 Other functions 3 Metabolism 3.1 Biosynthesis 3.2 Degradation 4 Nutrition and health 5 See also 6 References 6.1 Bibliography 7 External links

Categories of lipids

Fatty acyls

Fatty acyls, a generic term for describing fatty acids, their conjugates and derivatives, are a diverse group of molecules synthesized by chain-elongation of an acetyl-CoA primer with malonyl-CoA or methylmalonyl-CoA groups in a process called fatty acid synthesis.^[6][7] They are made of a hydrocarbon chain that terminates with a carboxylic acid group; this arrangement confers the molecule with a polar, hydrophilic end, and a nonpolar, hydrophobic end that is insoluble in water. The fatty acid structure is one of the most fundamental categories of biological lipids, and is commonly used as a building block of more structurally complex lipids. The carbon chain, typically between four to 24 carbons long,^[8] may be saturated or unsaturated, and may be attached to functional groups containing oxygen, halogens, nitrogen and sulfur. Where a double bond exists, there is the possibility of either a cis or trans geometric isomerism, which significantly affects the molecule's molecular configuration. Cis-double bounds cause the fatty acid chain to bend, an effect that is more pronounced the more double bonds there are in a chain. This in turn plays an important role in the structure and function of cell membranes.^[9] Most naturally occurring fatty acids are of the cis configuration, although the trans form does exist in some natural and partially hydrogenated fats and oils.^[10]

Examples of biologically important fatty acids are the eicosanoids, derived primarily from arachidonic acid and eicosapentaenoic acid, which include prostaglandins, leukotrienes, and thromboxanes. Other major lipid classes in the fatty acid category are the fatty esters and fatty amides. Fatty esters include important biochemical intermediates such as wax esters, fatty acid thioester coenzyme A derivatives, fatty acid thioester ACP derivatives and fatty acid carnitines. The fatty amides include N-acyl ethanolamines, such as the cannabinoid neurotransmitter anandamide.^[11]

Glycerolipids

Glycerolipids are composed mainly of mono-, di- and tri-substituted glycerols,^[12] the most well-known being the fatty acid esters of glycerol (triacylglycerols), also known as triglycerides. In these compounds, the three hydroxyl groups of glycerol are each esterified, usually by different fatty acids. Because they function as a food store, these lipids comprise the bulk of storage fat in animal tissues. The hydrolysis of the ester bonds of triacylglycerols and the release of glycerol and fatty acids from adipose tissue is called fat mobilization.^[13]

Additional subclasses of glycerolipids are represented by glycosylglycerols, which are characterized by the presence of one or more sugar residues attached to glycerol via a glycosidic linkage. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes^[14] and seminolipid from mammalian sperm cells.^[15]

Glycerophospholipids

Glycerophospholipids, also referred to as phospholipids, are ubiquitous in nature and are key components of the lipid bilayer of cells, as well as being involved in metabolism and cell signaling. Neural tissue (including the brain) contains relatively high amounts of glycerophospholipids, and alterations in their composition has been implicated in various neurological disorders.^[16] Glycerophospholipids may be subdivided into distinct classes, based on the nature of the polar headgroup at the sn-3 position of the glycerol backbone in eukaryotes and eubacteria, or the sn-1 position in the case of archaebacteria.^[17]

Phosphatidylethanolamine^[3]

Examples of glycerophospholipids found in biological membranes are phosphatidylcholine (also known as PC, GPCho or lecithin), phosphatidylethanolamine (PE or GPEtn) and phosphatidylserine (PS or GPSer). In addition to serving as a primary component of cellular membranes and binding sites for intra- and intercellular proteins, some glycerophospholipids in eukaryotic cells, such as phosphatidylinositols and phosphatidic acids are either precursors of, or are themselves, membrane-derived second messengers.^[18] Typically, one or both of these hydroxyl groups are acylated with long-chain fatty acids, but there are also alkyl-linked and 1Z-alkenyl-linked (plasmalogen) glycerophospholipids, as well as dialkylether variants in archaebacteria.^[19]

Sphingolipids

Sphingolipids are a complex family of compounds^[20] that share a common structural feature, a sphingoid base backbone that is synthesized de novo from the amino acid serine and a long-chain fatty acyl CoA, then converted into ceramides, phosphosphingolipids, glycosphingolipids and other compounds. The major sphingoid base of mammals is commonly referred to as sphingosine. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an amide-linked fatty acid. The fatty acids are typically saturated or mono-unsaturated with chain lengths from 16 to 26 carbon atoms.^[21]

Sphingomyelin^[3]

The major phosphosphingolipids of mammals are sphingomyelins (ceramide phosphocholines),^[22] whereas insects contain mainly ceramide phosphoethanolamines^[23] and fungi have phytoceramide phosphoinositols and mannose-containing headgroups.^[24] The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a glycosidic bond to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as cerebrosides and gangliosides.

Sterol lipids

Sterol lipids, such as cholesterol and its derivatives, are an important component of membrane lipids,^[25] along with the glycerophospholipids and sphingomyelins. The steroids, all derived from the same fused four-ring core structure, have different biological roles as hormones and signaling molecules. The eighteen-carbon (C18) steroids include the estrogen family whereas the C19 steroids comprise the androgens such as testosterone and androsterone. The C21 subclass includes the progestogens as well as the glucocorticoids and mineralocorticoids.^[26] The secosteroids, comprising various forms of vitamin D, are characterized by cleavage of the B ring of the core structure.^[27] Other examples of sterols are the bile acids and their conjugates,^[28] which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the phytosterols, such as β-sitosterol, stigmasterol, and brassicasterol; the latter compound is also used as a biomarker for algal growth.^[29] The predominant sterol in fungal cell membranes is ergosterol.^[30]

Prenol lipids

Prenol lipids are synthesized from the 5-carbon precursors isopentenyl diphosphate and dimethylallyl diphosphate that are produced mainly via the mevalonic acid (MVA) pathway.^[31] The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed by the successive addition of C5 units, and are classified according to number of these terpene units. Structures containing greater than 40 carbons are known as polyterpenes. Carotenoids are important simple isoprenoids that function as antioxidants and as precursors of vitamin A.^[32] Another biologically important class of molecules is exemplified by the quinones and hydroquinones, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.^[33] Vitamin E and vitamin K, as well as the ubiquinones, are examples of this class. Prokaryotes synthesize polyprenols (called bactoprenols) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols (dolichols) the terminal isoprenoid is reduced.^[34]

Saccharolipids

Structure of the saccharolipid Kdo₂-Lipid A.^[35] Glucosamine residues in blue, Kdo residues in red, acyl chains in black and phosphate groups in green.

Saccharolipids describe compounds in which fatty acids are linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids. The most familiar saccharolipids are the acylated glucosamine precursors of the Lipid A component of the lipopolysaccharides in Gram-negative bacteria. Typical lipid A molecules are disaccharides of glucosamine, which are derivatized with as many as seven fatty-acyl chains. The minimal lipopolysaccharide required for growth in E. coli is Kdo₂-Lipid A, a hexa-acylated disaccharide of glucosamine that is glycosylated with two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.^[35]

Polyketides

Polyketides are synthesized by polymerization of acetyl and propionyl subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the fatty acid synthases. They comprise a large number of secondary metabolites and natural products from animal, plant, bacterial, fungal and marine sources, and have great structural diversity.^[36][37] Many polyketides are cyclic molecules whose backbones are often further modified by glycosylation, methylation, hydroxylation, oxidation, and/or other processes. Many commonly used anti-microbial, anti-parasitic, and anti-cancer agents are polyketides or polyketide derivatives, such as erythromycins, tetracyclines, avermectins, and antitumor epothilones.^[38]

Biological functions

Membranes

Eukaryotic cells are compartmentalized into membrane-bound organelles which carry out different biological functions. The glycerophospholipids are the main structural component of biological membranes, such as the cellular plasma membrane and the intracellular membranes of organelles; in animal cells the plasma membrane physically separates the intracellular components from the extracellular environment. The glycerophospholipids are amphipathic molecules (containing both hydrophobic and hydrophilic regions) that contain a glycerol core linked to two fatty acid-derived "tails" by ester linkages and to one "head" group by a phosphate ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as sphingomyelin and sterols (mainly cholesterol in animal cell membranes) are also found in biological membranes.^[39] In plants and algae, the galactosyldiacylglycerols,^[40] and sulfoquinovosyldiacylglycerol,^[14] which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are the most abundant lipids in photosynthetic tissues, including those of higher plants, algae and certain bacteria.

Bilayers have been found to exhibit high levels of birefringence which can be used to probe the degree of order (or disruption) within the bilayer using techniques such as dual polarisation interferometry

Self-organization of phospholipids: a spherical liposome, a micelle and a lipid bilayer.

A biological membrane is a form of lipid bilayer. The formation of lipid bilayers is an energetically-preferred process when the glycerophospholipids described above are in an aqueous environment.^[41] In an aqueous system, the polar heads of lipids align towards the polar, aqueous environment, while the hydrophobic tails minimize their contact with water and tend to cluster together, forming a vesicle; depending on the concentration of the lipid, this biophysical interaction may result in the formation of micelles, liposomes, or lipid bilayers. Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behavior. Phase behavior is an area of study within biophysics and is the subject of current academic research.^[42][43] Micelles and bilayers form in the polar medium by a process known as the hydrophobic effect.^[44] When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e., water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form hydrogen bonds to the lipophilic areas of the amphiphile. So in an aqueous environment, the water molecules form an ordered "clathrate" cage around the dissolved lipophilic molecule.^[45]

Energy storage

Triacylglycerols, stored in adipose tissue, are a major form of energy storage in animals. The adipocyte, or fat cell, is designed for continuous synthesis and breakdown of triacylglycerols, with breakdown controlled mainly by the activation of hormone-sensitive enzyme lipase.^[46] The complete oxidation of fatty acids provides high caloric content, about 9 kcal/g, compared with 4 kcal/g for the breakdown of carbohydrates and proteins. Migratory birds that must fly long distances without eating use stored energy of triacylglycerols to fuel their flights.^[47]

Signaling

In recent years, evidence has emerged showing that lipid signaling is a vital part of the cell signaling.^[48] Lipid signaling may occur via activation of G protein-coupled or nuclear receptors, and members of several different lipid categories have been identified as signaling molecules and cellular messengers.^[49] These include sphingosine-1-phosphate, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization,^[50] cell growth, and apoptosis;^[51] diacylglycerol (DAG) and the phosphatidylinositol phosphates (PIPs), involved in calcium-mediated activation of protein kinase C;^[52] the prostaglandins, which are one type of fatty-acid derived eicosanoid involved in inflammation and immunity;^[53] the steroid hormones such as estrogen, testosterone and cortisol, which modulate a host of functions such as reproduction, metabolism and blood pressure; and the oxysterols such as 25-hydroxy-cholesterol that are liver X receptor agonists.^[54]

Other functions

The "fat-soluble" vitamins (A, D, E and K) – which are isoprene-based lipids – are essential nutrients stored in the liver and fatty tissues, with a diverse range of functions. Acyl-carnitines are involved in the transport and metabolism of fatty acids in and out of mitochondria, where they undergo beta oxidation.^[55] Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of oligosaccharides across membranes. Polyprenol phosphate sugars and polyprenol diphosphate sugars function in extra-cytoplasmic glycosylation reactions, in extracellular polysaccharide biosynthesis (for instance, peptidoglycan polymerization in bacteria), and in eukaryotic protein N-glycosylation.^[56][57] Cardiolipins are a subclass of glycerophospholipids containing four acyl chains and three glycerol groups that are particularly abundant in the inner mitochondrial membrane.^[58] They are believed to activate enzymes involved with oxidative phosphorylation.^[59]

Metabolism

The major dietary lipids for humans and other animals are animal and plant triglycerides, sterols, and membrane phospholipids. The process of lipid metabolism synthesizes and degrades the lipid stores and produces the structural and functional lipids characteristic of individual tissues.

Biosynthesis

In animals, when there is an oversupply of dietary carbohydrate, the excess carbohydrate is converted to triacylglycerol. This involves the synthesis of fatty acids from acetyl-CoA and the esterification of fatty acids in the production of triacylglycerol, a process called lipogenesis.^[60] Fatty acids are made by fatty acid synthases that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acetyl group, reduce it to an alcohol, dehydrate it to an alkene group and then reduce it again to an alkane group. The enzymes of fatty acid biosynthesis are divided into two groups, in animals and fungi all these fatty acid synthase reactions are carried out by a single multifunctional protein,^[61] while in plant plastids and bacteria separate enzymes perform each step in the pathway.^[62][63] The fatty acids may be subsequently converted to triacylglycerols that are packaged in lipoproteins and secreted from the liver.

The synthesis of unsaturated fatty acids involves a desaturation reaction, whereby a double bond is introduced into the fatty acyl chain. For example, in humans, the desaturation of stearic acid by stearoyl-CoA desaturase-1 produces oleic acid. The doubly-unsaturated fatty acid linoleic acid as well as the triply-unsaturated linolenic acid cannot be synthesized in mammalian tissues, and are therefore essential fatty acids and must be obtained from the diet.^[64]

Triacylglycerol synthesis takes place in the endoplasmic reticulum by metabolic pathways in which acyl groups in fatty acyl-CoAs are transferred to the hydroxyl groups of glycerol-3-phosphate and diacylglycerol.^[65]

Terpenes and isoprenoids, including the carotenoids, are made by the assembly and modification of isoprene units donated from the reactive precursors isopentenyl pyrophosphate and dimethylallyl pyrophosphate.^[66] These precursors can be made in different ways. In animals and archaea, the mevalonate pathway produces these compounds from acetyl-CoA,^[67] while in plants and bacteria the non-mevalonate pathway uses pyruvate and glyceraldehyde 3-phosphate as substrates.^[66][68] One important reaction that uses these activated isoprene donors is steroid biosynthesis. Here, the isoprene units are joined together to make squalene and then folded up and formed into a set of rings to make lanosterol.^[69] Lanosterol can then be converted into other steroids such as cholesterol and ergosterol.^[69][70]

Degradation

Beta oxidation is the metabolic process by which fatty acids are broken down in the mitochondria and/or in peroxisomes to generate acetyl-CoA. For the most part, fatty acids are oxidized by a mechanism that is similar to, but not identical with, a reversal of the process of fatty acid synthesis. That is, two-carbon fragments are removed sequentially from the carboxyl end of the acid after steps of dehydrogenation, hydration, and oxidation to form a beta-keto acid, which is split by thiolysis. The acetyl-CoA is then ultimately converted into ATP, CO₂, and H₂O using the citric acid cycle and the electron transport chain. The energy yield of the complete oxidation of the fatty acid palmitate is 106 ATP.^[71] Unsaturated and odd-chain fatty acids require additional enzymatic steps for degradation.

Nutrition and health

Most of the lipid found in food is in the form of triacylglycerols, cholesterol and phospholipids. A minimum amount of dietary fat is necessary to facilitate absorption of fat-soluble vitamins (A, D, E and K) and carotenoids.^[72] Humans and other mammals have a dietary requirement for certain essential fatty acids, such as linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) because they cannot be synthesized from simple precursors in the diet.^[73] Both of these fatty acids are 18-carbon polyunsaturated fatty acids differing in the number and position of the double bonds. Most vegetable oils are rich in linoleic acid (safflower, sunflower, and corn oils). Alpha-linolenic acid is found in the green leaves of plants, and in selected seeds, nuts and legumes (particularly flax, rapeseed, walnut and soy).^[74] Fish oils are particularly rich in the longer-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).^[75] A large number of studies have shown positive health benefits associated with consumption of omega-3 fatty acids on infant development, cancer, cardiovascular diseases, and various mental illnesses, such as depression, attention-deficit hyperactivity disorder, and dementia.^[76][77] In contrast, it is now well-established that consumption of trans fats, such as those present in partially hydrogenated vegetable oils, are a risk factor for cardiovascular disease.^[78][79][80]

A few studies have suggested that total diary fat intake is linked to an increased risk of obesity^[81][82] and diabetes.^[83][84] However, a number of very large studies, including the Women's Health Initiative Dietary Modification Trial, an eight year study of 49,000 women, the Nurses' Health Study and the Health Professionals Follow-up Study, revealed no such links.^[85][86][87] None of these studies suggested any connection between percentage of calories from fat and risk of cancer, heart disease or weight gain. The Nutrition Source, a website maintained by the Department of Nutrition at the Harvard School of Public Health, summarizes the current evidence on the impact of dietary fat: "Detailed research—much of it done at Harvard—shows that the total amount of fat in the diet isn't really linked with weight or disease."^[88]

See also

Food portal

• Biochemistry
• Essential fatty acid
• Lipid microdomain
• Lipid signaling
• Protein-lipid interaction
• Saturation (chemistry)
• Vertical Auto Profile

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External links

Wikimedia Commons has media related to: Lipids

Introductory

• Nature Lipidomics Gateway - Round-up and summaries of recent lipid research
• Lipid Library - General reference on lipid chemistry and biochemistry
• European Lipidomics Initiative - European research resource for lipid biochemistry.
• Cyberlipid.org - Resources and history for lipids.
• Lipids and Proteins - UC Berkeley video lecture
• Science aid: Lipids - Resource for students
• Molecular Computer Simulations - Modeling of Lipid Membranes
• Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology

Classification

• LIPID MAPS - LIPID Metabolites And Pathways Strategy

Nomenclature

• IUPAC nomenclature of lipids
• IUPAC glossary entry for the lipid class of molecules

Databases

• LIPID MAPS - Comprehensive lipid and lipid-associated gene/protein databases.
• LipidBank - Japanese database of lipids and related properties, spectral data and references.
• LIPIDAT - Database composed mainly of phospholipids and associated thermodynamic data.

General

• ApolloLipids - Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs
• National Lipid Association - Professional medical education organization for health care professionals who seek to prevent morbidity and mortality stemming from dyslipidemias and other cholesterol-related disorders.

v • d • e Metabolism (Catabolism, Anabolism) General Metabolic pathway · Metabolic network · Cellular respiration (Anaerobic/Aerobic) Specific paths Protein metabolism Protein synthesis · Amino acid synthesis · Catabolism Nucleotide metabolism: Purine metabolism  · Nucleotide salvage · Pyrimidine metabolism Carbohydrate metabolism Anabolism Gluconeogenesis · Glycogenesis · Photosynthesis (Carbon fixation) Carbohydrate catabolism Glycolysis · Glycogenolysis · Fermentation (Ethanol, Lactic acid)  · Cellular respiration · Xylose metabolism Other Pentose phosphate pathway Lipid metabolism Fatty acid metabolism Fatty acid degradation (Lipolysis, Beta oxidation) · Fatty acid synthesis Other Eicosanoid metabolism · Sphingolipid metabolism · Steroid metabolism Other Metal metabolism (Iron metabolism) · Ethanol metabolism

v • d • e Food chemistry Additives · Carbohydrates · Coloring · Enzymes · Essential fatty acids · Flavors · Lipids · Minerals · Proteins · Vitamins · Water

v • d • e Types of Lipids General Saturated fat | Unsaturated fat | Monounsaturated fat | Polyunsaturated fat Geometry Trans fat | Omega-3 fatty acid | Omega-6 fatty acid | Omega-9 fatty acid Phospholipids Phosphatidylserine | Phosphatidylinositol | Phosphatidyl ethanolamine | Cardiolipin | Dipalmitoylphosphatidylcholine Eicosanoids Arachidonic acid | Prostaglandin | Prostacyclin | Thromboxane | Leukotriene Fatty acids Lauric acid | Palmitic acid | Myristic acid | Stearic acid | Caprylic acid Major families of biochemicals Saccharides/Carbohydrates/Glycosides · Amino acids/Peptides/Proteins/Glycoproteins · Lipids/Terpenes/Steroids/Carotenoids · Alkaloids/Nucleobases/Nucleic acids · Cofactors/Flavonoids/Polyketides/Tetrapyrroles

v • d • e Lipids: lipoprotein metabolism Apolipoproteins APOA (1, 2, 5) · APOB · APOC (1, 2, 3, 4) · APOD · APOE · APOH Lipoproteins Chylomicron · HDL · LDL · IDL · Very low-density lipoprotein · Lp(a) HDL Cholesterylester transfer protein · Acetyl-CoA C-acyltransferase · LCAT · ABCA1 LDL LDL receptor · Microsomal triglyceride transfer protein · APOB Other Lipoprotein lipase see also disorders

v • d • e Lipids: phospholipids Glycerol backbone (Glycerophospholipids/ Phosphoglycerides) Phosphatidyl-: -ethanolamine/cephalin - -choline/lechithin (dipalmitoyl-) - -serine - -glycerol - -inositol (glyco-) Phosphoinositides: PIP (PI(3)P, PI(4)P, PI(5)P) - PIP2 (PI(3,4)P2, PI(3,5)P2, PI(4,5)P2) - PIP3 Cardiolipin Ether lipids: Plasmalogen (Platelet-activating factor) Sphingosine backbone Sphingomyelin

v • d • e Glycoconjugates, lipids and glycolipids: sphingolipids and glycosphingolipids, and metabolic intermediates Ceramide Cerebroside (Galactocerebroside, Glucocerebroside) · Lactosylceramide Ganglioside pathway From ganglioside GM1 · GM2 · GD2 From globoside Globotriaosylceramide From sphingomyelin Glucocerebroside From sulfatide Galactocerebroside To sphingosine Ceramide see also enzymes, disorders

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