Make lipids. Lipids - what are they? Classification. Lipid metabolism in the body and their biological role. Saturated and unsaturated fatty acids

Lipids- Substances very heterogeneous in their chemical structure, characterized by different solubility in organic solvents and, as a rule, insoluble in water. They play an important role in life processes. As one of the main components of biological membranes, lipids affect their permeability, participate in the transmission of nerve impulses, and the creation of intercellular contacts.

Other functions of lipids are the formation of an energy reserve, the creation of protective water-repellent and thermal insulating covers in animals and plants, the protection of organs and tissues from mechanical stress.

CLASSIFICATION OF LIPIDS

Depending on the chemical composition, lipids are divided into several classes.

1. Simple lipids include substances whose molecules consist only of residues of fatty acids (or aldehydes) and alcohols. These include
   • fats (triglycerides and other neutral glycerides)
   • waxes
2. Complex lipids
   • derivatives of phosphoric acid (phospholipids)
   • lipids containing sugar residues (glycolipids)
   • sterols
   • sterides

In this section, lipid chemistry will be considered only to the extent that is necessary for understanding lipid metabolism.

If an animal or plant tissue is treated with one or more (more often sequentially) organic solvents, for example chloroform, benzene or petroleum ether, then some of the material goes into solution. The components of this soluble fraction (extract) are called lipids. The lipid fraction contains substances of various types, most of which are shown in the diagram. Note that due to the heterogeneity of the components included in the lipid fraction, the term "lipid fraction" cannot be regarded as a structural characteristic; it is only a working laboratory name for the fraction obtained from the extraction of biological material with low-polarity solvents. Nevertheless, most lipids have some common structural features that determine their important biological properties and similar solubility.

Fatty acid

Fatty acids - aliphatic carboxylic acids - in the body can be in a free state (trace amounts in cells and tissues) or serve as building blocks for most classes of lipids. Over 70 different fatty acids have been isolated from the cells and tissues of living organisms.

Fatty acids found in natural lipids contain an even number of carbon atoms and have a predominantly unbranched carbon chain. Below are the formulas for the most commonly found natural fatty acids.

Natural fatty acids, although somewhat conditionally, can be divided into three groups:

• saturated fatty acids [show]
• monounsaturated fatty acids [show]

  Monounsaturated (with one double bond) fatty acids:

• polyunsaturated fatty acids [show]

  Polyunsaturated (with two or more double bonds) fatty acids:

In addition to these main three groups, there is also a group of so-called unusual natural fatty acids [show] .

The fatty acids that make up the lipids of animals and higher plants have many properties in common. As already noted, almost all natural fatty acids contain an even number of carbon atoms, most often 16 or 18. Unsaturated fatty acids from animals and humans, which are involved in the construction of lipids, usually contain a double bond between the 9th and 10th carbon, additional double bonds, such as usually occur between the 10th carbon and the methyl end of the chain. The count comes from the carboxyl group: the C-atom closest to the COOH group is designated as α, the adjacent one is β, and the terminal carbon atom in the hydrocarbon radical is ω.

The peculiarity of double bonds of natural unsaturated fatty acids lies in the fact that they are always separated by two simple bonds, that is, there is always at least one methylene group between them (-CH = CH-CH 2 -CH = CH-). Such double bonds are referred to as "isolated". Naturally occurring unsaturated fatty acids have a cis configuration and trans configurations are extremely rare. It is believed that in unsaturated fatty acids with several double bonds, the cis configuration gives the hydrocarbon chain a curved and shortened appearance, which makes a biological sense (especially when you consider that many lipids are part of membranes). In microbial cells, unsaturated fatty acids usually contain one double bond.

Long chain fatty acids are practically insoluble in water. Their sodium and potassium salts (soaps) form micelles in water. In the latter, negatively charged carboxyl groups of fatty acids face the aqueous phase, and non-polar hydrocarbon chains are hidden inside the micellar structure. Such micelles have a total negative charge and remain suspended in solution due to mutual repulsion (Fig. 95).

Neutral fats (or glycerides)

Neutral fats are esters of glycerol and fatty acids. If all three hydroxyl groups of glycerol are esterified with fatty acids, then such a compound is called triglyceride (triacylglycerol), if two are esterified with diglyceride (diacylglycerol) and, finally, if one group is esterified, it is called monoglyceride (monoacylglycerol).

Neutral fats are found in the body either in the form of protoplasmic fat, which is a structural component of cells, or in the form of reserve, reserve fat. The role of these two forms of fat in the body is not the same. Protoplasmic fat has a constant chemical composition and is contained in tissues in a certain amount, which does not change even with morbid obesity, while the amount of reserve fat is subject to large fluctuations.

The bulk of natural neutral fats are triglycerides. The fatty acids in triglycerides can be saturated or unsaturated. Palmitic, stearic and oleic acids are more common among fatty acids. If all three acid radicals belong to the same fatty acid, then such triglycerides are called simple (for example, tripalmitin, tristearin, triolein, etc.), but if they are different fatty acids, then they are called mixed. Mixed triglycerides are named from their constituent fatty acids; the numbers 1, 2 and 3 indicate the bond of the fatty acid residue with the corresponding alcohol group in the glycerol molecule (for example, 1-oleo-2-palmitostearin).

Fatty acids that make up triglycerides practically determine their physicochemical properties. Thus, the melting point of triglycerides increases with an increase in the number and length of saturated fatty acid residues. In contrast, the higher the content of unsaturated fatty acids or short-chain acids, the lower the melting point. Animal fats (lard) usually contain a significant amount of saturated fatty acids (palmitic, stearic, etc.), due to which they are solid at room temperature. Fats, which contain many mono- and polyunsaturated acids, are liquid at ordinary temperatures and are called oils. So, in hemp oil, 95% of all fatty acids are oleic, linoleic and linolenic acids, and only 5% are stearic and palmitic acids. Note that human fat melting at 15 ° C (it is liquid at body temperature) contains 70% oleic acid.

Glycerides are able to enter into all chemical reactions inherent in esters. Of greatest importance is the saponification reaction, as a result of which glycerol and fatty acids are formed from triglycerides. Saponification of fat can occur both by enzymatic hydrolysis and by the action of acids or alkalis.

Alkaline cleavage of fat by the action of caustic soda or caustic potash is carried out in the industrial production of soap. Recall that soap is sodium or potassium salts of higher fatty acids.

The following indicators are often used to characterize natural fats:

1. iodine number - the number of grams of iodine, which, under certain conditions, binds 100 g of fat; this number characterizes the degree of unsaturation of fatty acids present in fats, the iodine number of beef fat 32-47, lamb 35-46, pork 46-66;
2. acid number - the number of milligrams of caustic potassium required to neutralize 1 g of fat. This number indicates the amount of free fatty acids present in the fat;
3. saponification number - the number of milligrams of caustic potassium consumed to neutralize all fatty acids (both included in triglycerides and free) contained in 1 g of fat. This number depends on the relative molecular weight of the fatty acids that make up the fat. The saponification number for the main animal fats (beef, lamb, pork) is practically the same.

Waxes are esters of higher fatty acids and higher monohydric or dihydric alcohols with the number of carbon atoms from 20 to 70. Their general formulas are shown in the diagram, where R, R "and R" are possible radicals.

Waxes can be part of the fat that covers the skin, wool, feathers. In plants, 80% of all lipids that form a film on the surface of leaves and trunks are waxes. It is also known that waxes are normal metabolites of some microorganisms.

Natural waxes (for example, beeswax, spermaceti, lanolin) usually contain, in addition to the aforementioned esters, a certain amount of free higher fatty acids, alcohols and hydrocarbons with 21-35 carbon atoms.

Phospholipids

This class of complex lipids includes glycerophospholipids and sphingolipids.

Glycerophospholipids are derivatives of phosphatidic acid: they contain glycerol, fatty acids, phosphoric acid, and usually nitrogen-containing compounds. The general formula of glycerophospholipids is shown in the diagram, where R 1 and R 2 are radicals of higher fatty acids, and R 3 is a radical of a nitrogenous compound.

It is characteristic of all glycerophospholipids that one part of their molecule (radicals R 1 and R 2) exhibits pronounced hydrophobicity, while the other part is hydrophilic due to the negative charge of the phosphoric acid residue and the positive charge of the radical R 3.

Of all lipids, glycerophospholipids have the most pronounced polar properties. When glycerophospholipids are placed in water, only a small part of them passes into a true solution, while the bulk of the "dissolved" lipid is in aqueous systems in the form of micelles. There are several groups (subclasses) of glycerophospholipids.

[show] .



Unlike triglycerides in the phosphatidylcholine molecule, one of the three hydroxyl groups of glycerol is associated not with fatty acid, but with phosphoric acid. In addition, phosphoric acid, in turn, is linked with an ether bond with a nitrogenous base [HO-CH 2 -CH 2 -N + = (CH 3) 3] - choline. Thus, glycerol, higher fatty acids, phosphoric acid and choline are combined in the phosphatidylcholine molecule.

[show] .



The main difference between phosphatidylcholines and phosphatidylethanolamines is that the latter include the nitrogenous base ethanolamine (HO-CH 2 -CH 2 -NH 3 +) instead of choline.

Of the glycerophospholipids in the body of animals and higher plants, phosphatidylcholines and phosphatidylethanolamines are found in the greatest amount. These two groups of glycerophospholipids are metabolically linked to each other and are the main lipid components of cell membranes.

• Phosphatidylserines [show] .

  

  In the phosphatidylserine molecule, the nitrogenous compound is the serine amino acid residue.

  Phosphatidylserines are much less widespread than phosphatidylcholines and phosphatidylethanolamines, and their importance is mainly determined by the fact that they are involved in the synthesis of phosphatidylethanolamines.

• Plasmalogens (acetal phosphatides) [show] .

  

  They differ from the glycerophospholipids discussed above in that instead of one higher fatty acid residue, they contain a fatty acid aldehyde residue, which is linked to the hydroxyl group of glycerol by an unsaturated ester bond:

  Thus, during hydrolysis, plasmalogen decomposes into glycerol, higher fatty acid aldehyde, fatty acid, phosphoric acid, choline, or ethanolamine.

[show] .



The R 3 -radical in this group of glycerophospholipids is a six-carbon sugar alcohol - inositol:

Phosphatidylinositols are quite widespread in nature. They are found in animals, plants and microbes. In the animal body, they are found in the brain, liver and lungs.

[show] .



It should be noted that free phosphatidic acid is found in nature, although in comparison with other glycerophospholipids in relatively small amounts.

Cardiolilin belongs to glycerophospholipids, more precisely to polyglycerol phosphates. The backbone of the cardiolipin molecule includes three glycerol residues connected to each other by two phosphodiester bridges through positions 1 and 3; the hydroxyl groups of the two outer glycerol residues are esterified with fatty acids. Cardiolipin is part of the mitochondrial membranes. Table 29 summarizes the data on the structure of the main glycerophospholipids.

Among the fatty acids that make up glycerophospholipids, both saturated and unsaturated fatty acids (more often stearic, palmitic, oleic and linoleic) are found.

It was also found that most phosphatidylcholines and phosphatidylethanolamines contain one saturated higher fatty acid esterified at position 1 (at the 1st carbon atom of glycerol), and one unsaturated higher fatty acid esterified at position 2. Hydrolysis of phosphatidylcholines and phosphatidylethanolamines with the participation of special enzymes , for example, in cobra venom, which are phospholipases A 2, leads to the elimination of unsaturated fatty acids and the formation of lysophosphatidylcholines or lysophosphatidylethanolamines with a strong hemolytic effect.

Sphingolipids

Glycolipids

Complex lipids containing carbohydrate groups in the molecule (more often a D-galactose residue). Glycolipids play an essential role in the functioning of biological membranes. They are found predominantly in brain tissue, but they are also found in blood cells and other tissues. There are three main groups of glycolipids:

• cerebrosides
• sulfatides
• gangliosides

Cerebrosides contain neither phosphoric acid nor choline. They include hexose (usually D-galactose), which is linked by an ether bond to the hydroxyl group of the amino alcohol sphingosine. In addition, a fatty acid is a part of cerebroside. Among these fatty acids, the most common are lignoceric, nervous and cerebronic acids, i.e. fatty acids having 24 carbon atoms. The structure of cerebrosides can be represented by the diagram. Cerebrosides can also be classified as sphingolipids, since they contain the alcohol sphingosine.

The most studied representatives of cerebrosides are the nerve containing neurotic acid, the cerebron, which contains cerebronic acid, and kerazine, which contains lignocyric acid. The content of cerebrosides is especially high in the membranes of nerve cells (in the myelin sheath).

Sulfatides differ from cerebrosides in that they contain a sulfuric acid residue in the molecule. In other words, the sulfatide is a cerebroside sulfate in which the sulfate is esterified at the third carbon atom of the hexose. In the mammalian brain, sulfatides, like cerebrosides, are found in the white matter. However, their content in the brain is much lower than that of cerebrosides.

During the hydrolysis of gangliosides, one can find higher fatty acid, sphingosine alcohol, D-glucose and D-galactose, as well as derivatives of amino sugars: N-acetylglucosamine and N-acetylneuraminic acid. The latter is synthesized in the body from glucosamine.

Structurally, gangliosides are largely similar to cerebrosides, with the only difference that instead of one galactose residue they contain a complex oligosaccharide. One of the simplest gangliosides is hematoside, isolated from the stroma of erythrocytes (scheme)

Unlike cerebrosides and sulfatides, gangliosides are found mainly in the gray matter of the brain and are concentrated in the plasma membranes of nerve and glial cells.

All the lipids considered above are usually called saponifiable, since soaps are formed during their hydrolysis. However, there are lipids that are not hydrolyzed to release fatty acids. These lipids include steroids.

Steroids are naturally occurring compounds. They are derivatives of the core containing three fused cyclohexane and one cyclopentane ring. Steroids include numerous hormonal substances, as well as cholesterol, bile acids and other compounds.

In the human body, sterols occupy the first place among steroids. The most important representative of sterols is cholesterol:

It contains an alcoholic hydroxyl group at C 3 and a branched aliphatic chain of eight carbon atoms at C 17. The hydroxyl group at C 3 can be esterified with a higher fatty acid; in this case, cholesterol esters (cholesterides) are formed:

Cholesterol plays the role of a key intermediate in the synthesis of many other compounds. Plasma membranes of many animal cells are rich in cholesterol; in a significantly smaller amount, it is contained in the membranes of mitochondria and in the endoplasmic reticulum. Note that there is no cholesterol in plants. Plants have other sterols known collectively as phytosterols.

Lipids are the most important source of the body's energy reserves. The fact is obvious even at the nomenclature level: the Greek "lipos" is translated as fat. Accordingly, the category of lipids unites fat-like substances of biological origin. The functional of the compounds is quite diverse, which is due to the heterogeneity of the composition of this category of bio-objects.

What functions do lipids perform?

List the main functions of lipids in the body, which are the main ones. At the introductory stage, it is advisable to highlight the key roles of fat-like substances in the cells of the human body. The basic list is the five functions of lipids:

1. reserve energy;
2. structure-forming;
3. transport;
4. insulating;
5. signal.

The secondary tasks that lipids perform in combination with other compounds include a regulatory and enzymatic role.

Energy reserve of the body

This is not only one of the important, but the priority role of fat-like compounds. In fact, part of the lipids is the energy source of the entire cell mass. Indeed, fat for cells is analogous to fuel in a car's tank. The energy function is realized by lipids as follows. Fats and similar substances are oxidized in the mitochondria, breaking down to the level of water and carbon dioxide. The process is accompanied by the release of a significant amount of ATP - high-energy metabolites. Their supply allows the cell to participate in energy-dependent reactions.

Structural blocks

At the same time, lipids carry out a building function: with their help, the cell membrane is formed. The process involves the following groups of fat-like substances:

1. cholesterol - lipophilic alcohol;
2. glycolipids - compounds of lipids with carbohydrates;
3. phospholipids are esters of complex alcohols and higher carboxylic acids.

It should be noted that the formed membrane does not contain fats directly. The formed wall between the cell and the external environment turns out to be two-layer. This is achieved due to the biphilicity. A similar characteristic of lipids indicates that one part of the molecule is hydrophobic, that is, insoluble in water, while the other, on the contrary, is hydrophilic. As a result, a cell wall bilayer is formed due to the ordered arrangement of simple lipids. Molecules unfold in hydrophobic regions towards each other, while hydrophilic tails are directed inward and outward of the cell.

This determines the protective functions of membrane lipids. First, the membrane gives the cell its shape and even preserves it. Secondly, the double wall is a kind of passport control point that does not allow unwanted visitors to pass through.

Autonomous heating system

Of course, this name is rather arbitrary, but it is quite applicable if we consider what functions lipids perform. The compounds do not so much heat the body as they keep the heat inside. A similar role is assigned to fatty deposits that form around various organs and in the subcutaneous tissue. This class of lipids is characterized by high heat-insulating properties, which protects vital organs from hypothermia.

Did you order a taxi?

The transport role of lipids is attributed to a secondary function. Indeed, the transfer of substances (mainly triglycerides and cholesterol) is carried out by separate structures. These are bound complexes of lipids and proteins called lipoproteins. As you know, fat-like substances are insoluble in water, respectively, in blood plasma. In contrast, the functions of proteins include hydrophilicity. As a result, the lipoprotein core is an accumulation of triglycerides and cholesterol esters, while the shell is a mixture of protein and free cholesterol molecules. In this form, lipids are delivered to the tissues or back to the liver for elimination from the body.

Secondary factors

The list of already listed 5 functions of lipids complements a number of equally important roles:

• enzymatic;
• signal;
• regulatory

Signal function

Some complex lipids, in particular their structure, allow the transmission of nerve impulses between cells. Glycolipids act as mediators in this process. No less important is the ability to recognize intracellular impulses, which is also realized by fat-like structures. This allows for the selection of substances necessary for the cell from the blood.

Enzymatic function

Lipids, regardless of their location in the membrane or outside it, are not part of the enzymes. However, their biosynthesis occurs with the presence of fat-like compounds. Additionally, lipids are involved in protecting the intestinal wall from pancreatic enzymes. The excess of the latter is neutralized by bile, where cholesterol and phospholipids are included in significant quantities.

Fat is considered to be the culprit of many troubles. Doctors and scientists advise cutting down on fat or eliminating it altogether. Of course, for those who are obese or have chronic diseases, it is better to heed this advice. However, the rest would be foolish to give up fat. Let's find out more about them from the facts below.

1. Consumption of fats does not necessarily lead to their storage in the body
Many people think that fat consumption will definitely affect the figure in the form of deposits on the waist, hips and abdomen. If you eat more than your body requires, then yes, such a problem may arise. For example, if you consume an unlimited amount of starchy carbohydrates, then you can expect an increase in insulin levels, and then fat will be deposited. But if you eat, consuming fat and protein evenly, then this problem can be avoided. In everything you need to know when to stop.

2. No need to avoid eating nuts
Nuts contain healthy forms of fat, monounsaturated fats, which help you feel full faster, but also raise your good cholesterol. Nuts do not affect weight gain in any way, because you cannot eat a lot of them due to their satiety, and besides, they are poorly digested by the body. Consequently, the cell walls of nuts are not easily destroyed when chewed. This means that they pass through the body in transit and do not excrete all of their fat.

3. It is not necessary to completely eliminate saturated fat from the body.
Saturated fats have always been thought to be the enemy of health, so they were advised to be eliminated from the diet. But today it has become clear that moderate consumption of saturated fat does no harm. And some of them even need to be included in a healthy eating program.

Extra-virgin coconut oil is one of the healthy sources of saturated fat. It contains lauric acid which is found nowhere else except in breast milk. It is a powerful immune stimulant. It is advised to fry food in coconut oil.

4. If the product label says "no trans fats" does not mean that they are not there.
Many manufacturers believe that if a product contains a very small amount of an ingredient, then it is not necessary to indicate it on the label. It happens that a product contains only 0.5 g of trans fat, but you will not find it among the ingredients on the package. After eating several servings of such a product, you will not even know that you have eaten enough of this harmful ingredient.

5. Nutrients from vegetables without fat are absorbed worse
Studies have shown that lettuce seasoned with fat or a sauce with fats is significantly better absorbed by the body and receives more of the necessary nutrients - carotenoids. If you constantly eat salads without fats, then carotenoids will not be absorbed by the body at all. They are responsible for the red, yellow, orange and green colors and are important in the prevention of many diseases. To help your body absorb all the nutrients from vegetables, consume them with healthy fats.

6. Extra virgin olive oil is not suitable for frying.
Although it contains healthy monounsaturated fats, it loses its properties at high temperatures. Better to use it for dressing salads or marinating meat. Olive oil is very delicate and quickly deteriorates, so it should be stored in a dark glass container with a tightly closed lid to avoid oxidation and retain all its beneficial properties.

7. Fats have many functions in the body
Without fat, our body and our body cannot live. There are several reasons for this:

The brain needs fats. About 60% of the dry weight of the human brain is fat. Healthy nerve cells contain fats - docosahexanoic acid;

Sex hormones are formed with the help of fats;

Fatty acids are essential for healthy skin and hair;

Fats are involved in metabolism, functions of the immune system, and help stabilize blood sugar.

Lipids - what are they? Translated from Greek, the word "lipids" means "small particles of fat". They are groups of natural organic compounds of an extensive nature, including fats themselves, as well as fat-like substances. They are part of all living cells without exception and are divided into simple and complex categories. The composition of simple lipids includes alcohol and fatty acids, while complex lipids contain high molecular weight components. Both are associated with biological membranes, have an effect on active enzymes, and also participate in the formation of nerve impulses that stimulate muscle contractions.

Fats and hydrophobia

One of them is the creation of the body's energy reserve and the provision of the water-repellent properties of the skin, coupled with thermal insulation protection. Some fatty acid-free substances are also classified as lipids, such as terpenes. Lipids are not susceptible to the action of an aqueous medium, but they easily dissolve in organic liquids such as chloroform, benzene, acetone.

Lipids, which are periodically presented at international seminars in connection with new discoveries, are an inexhaustible topic for research and scientific research. The question "Lipids - what are they?" never loses its relevance. However, scientific progress does not stand still. Recently, several new fatty acids have been identified that are biosynthetically related to lipids. The classification of organic compounds can be difficult due to the similarity in certain characteristics, but with a significant difference in other parameters. Most often, a separate group is created, after which the general picture of the harmonious interaction of related substances is restored.

Cell membranes

Lipids - what is it in terms of functional purpose? First of all, they are an essential component of living cells and tissues of vertebrates. Most of the processes in the body occur with the participation of lipids, the formation of cell membranes, interconnection and exchange of signals in the intercellular environment are not complete without fatty acids.

Lipids - what are they when viewed from the perspective of spontaneously emerging steroid hormones, phosphoinositides and prostaglandins? This is, first of all, the presence in blood plasma, which, by definition, are separate components of lipid structures. Because of the latter, the body is forced to develop the most complex systems for their transportation. Fatty acids of lipids are mainly transported in a complex with albumin, while lipoproteins, soluble in water, are transported in the usual manner.

Lipid classification

The categorization of biological compounds is a process that has some controversial issues. Lipids, due to their biochemical and structural properties, can be equally assigned to different categories. The main classes of lipids include simple and complex compounds.

Simple ones include:

• Glycerides are esters of glycerol alcohol and fatty acids of the highest category.
• Waxes are an ester of a higher fatty acid and a 2-atom alcohol.

Complex lipids:

• Phospholipid compounds - with the inclusion of nitrogenous components, glycerophospholipids, ophingolipids.
• Glycolipids are located in the outer biological layers of the body.
• Steroids are highly active substances of the animal spectrum.
• Complex fats - sterols, lipoproteins, sulfolipids, aminolipids, glycerol, hydrocarbons.

Functioning

Lipid fats act as a material for cell membranes. Participate in the transport of various substances along the periphery of the body. Fatty layers based on lipid structures help protect the body from hypothermia. They have the function of energy storage "in reserve".

Fat reserves are concentrated in the cytoplasm of cells in the form of droplets. Vertebrates, including humans, have special cells - adipocytes, which are capable of containing a lot of fat. The placement of fat accumulations in adipocytes is due to lipoid enzymes.

Biological functions

Fat is not only a reliable source of energy, it also has thermal insulating properties, aided by biology. At the same time, lipids allow achieving several useful functions, such as natural cooling of the body or, conversely, its thermal insulation. In the northern regions, characterized by low temperatures, all animals accumulate fat, which is deposited evenly throughout the body, and thus a natural protective layer is created that performs the function of heat protection. This is especially important for large marine animals: whales, walruses, seals.

Animals living in hot countries also accumulate fat deposits, but they are not distributed throughout the body, but are concentrated in certain places. For example, in camels, fat is collected in humps, in desert animals - in thick, short tails. Nature carefully monitors the correct placement of both fat and water in living organisms.

Structural function of lipids

All processes associated with the vital activity of the organism are subject to certain laws. Phospholipids are the basis of the biological layer of cell membranes, and cholesterol regulates the fluidity of these membranes. Thus, most living cells are surrounded by plasma membranes with a double layer of lipids. This concentration is essential for normal cellular activity. One microparticle of a biomembrane contains more than a million lipid molecules that have dual characteristics: they are simultaneously hydrophobic and hydrophilic. As a rule, these mutually exclusive properties are of a non-equilibrium nature, and therefore their functional purpose looks quite logical. Cell lipids are an effective natural regulator. The hydrophobic layer usually dominates and protects the cell membrane from the penetration of harmful ions.

Glycerophospholipids, phosphatidylethanolamine, phosphatidylcholine, cholesterol also contribute to cell impermeability. Other membrane lipids are located in tissue structures, these are sphingomyelin and sphingoglycolipid. Each substance has a specific function.

Lipids in the human diet

Triglycerides - nature, are an effective source of energy. acids are found in meat and dairy products. And fatty acids, but unsaturated, are found in nuts, sunflower and olive oil, seeds and corn grains. To prevent an increase in cholesterol in the body, it is recommended to limit the daily intake of animal fat to 10 percent.

Lipids and carbohydrates

Many organisms of animal origin "store" fats at certain points, subcutaneous tissue, folds of the skin, and other places. The oxidation of lipids of such fat deposits is slow, and therefore the process of their transition to carbon dioxide and water allows you to get a significant amount of energy, almost twice as much as carbohydrates can provide. In addition, the hydrophobic properties of fats eliminate the need for large amounts of water to stimulate hydration. The transition of fats to the energy phase occurs "dry". However, fats act much more slowly in terms of energy release, and are more suitable for hibernating animals. Lipids and carbohydrates, as it were, complement each other in the process of the body's vital activity.

Lipids constitute a large and quite heterogeneous in chemical composition group of organic substances that make up living cells, soluble in low-polar organic solvents (ether, benzene, chloroform, etc.) and insoluble in water. In general, they are regarded as derivatives of fatty acids.

The peculiarity of the structure of lipids is the presence in their molecules of simultaneously polar (hydrophilic) and non-polar (hydrophobic) structural fragments, which gives lipids an affinity for both water and the non-aqueous phase. Lipids are biphilic substances, which allows them to carry out their functions at the interface.

10.1. Classification

Lipids are divided into simple(two-component), if the products of their hydrolysis are alcohols and carboxylic acids, and complex(multicomponent), when, as a result of their hydrolysis, other substances are also formed, for example, phosphoric acid and carbohydrates. Simple lipids include waxes, fats and oils, as well as ceramides, complex ones - phospholipids, sphingolipids, and glycolipids (Scheme 10.1).

Scheme 10.1.General classification of lipids

10.2. Structural components of lipids

All lipid groups have two essential structural components - higher carboxylic acids and alcohols.

Higher fatty acids (HFA). Many higher carboxylic acids were first isolated from fats, hence the name fatty. Biologically important fatty acids can be saturated(Table 10.1) and unsaturated(Table 10.2). Their common structural features are:

Are monocarboxylic;

Includes an even number of carbon atoms in the chain;

Have the cis configuration of double bonds (if present).

Table 10.1.Essential Saturated Fatty Acids Lipids

In natural acids, the number of carbon atoms ranges from 4 to 22, but acids with 16 or 18 carbon atoms are more common. Unsaturated acids contain one or more double bonds with a cis configuration. The double bond closest to the carboxyl group is usually located between the C-9 and C-10 atoms. If there are several double bonds, then they are separated from each other by the methylene group CH 2.

The IUPAC rules for DRCs allow the use of their trivial names (see Tables 10.1 and 10.2).

Currently, its own nomenclature of unsaturated HFAs is also used. In it, the terminal carbon atom, regardless of the chain length, is denoted by the last letter of the Greek alphabet ω (omega). The position of the double bonds is counted not, as usual, from the carboxyl group, but from the methyl group. So, linolenic acid is designated as 18: 3 ω-3 (omega-3).

Linoleic acid itself and unsaturated acids with a different number of carbon atoms, but with the arrangement of double bonds also at the third carbon atom, counting from the methyl group, constitute the omega-3 HFA family. Other types of acids form similar families of linoleic (omega-6) and oleic (omega-9) acids. For normal human life, the correct balance of lipids of three types of acids is of great importance: omega-3 (flaxseed oil, fish oil), omega-6 (sunflower, corn oil) and omega-9 (olive oil) in the diet.

Of the saturated acids in the lipids of the human body, the most important are palmitic C 16 and stearic C 18 (see Table 10.1), and of the unsaturated ones, oleic C18: 1, linoleic C18: 2, linolenic and arachidonic C 20: 4 (see table 10.2).

It should be emphasized the role of polyunsaturated linoleic and linolenic acids as compounds, irreplaceable for humans ("vitamin F"). They are not synthesized in the body and must be supplied with food in an amount of about 5 g per day. In nature, these acids are found mainly in vegetable oils. They promote

Table 10 .2. Essential lipid unsaturated fatty acids

* Included for comparison. ** For cis isomers.

normalization of the lipid profile of blood plasma. Linetol, which is a mixture of ethyl esters of higher fatty unsaturated acids, is used as a hypolipidemic herbal medicine. Alcohols. Lipids may include:

Higher monohydric alcohols;

Polyhydric alcohols;

Amino alcohols.

In natural lipids, saturated and less often unsaturated long-chain alcohols (C 16 and more) are most often found, mainly with an even number of carbon atoms. As an example of higher alcohols, cetyl CH 3 (CH 2 ) 15 OH and melissilic CH 3 (CH 2) 29 OH alcohols that are part of the waxes.

Polyhydric alcohols in most natural lipids are represented by the trihydric alcohol glycerol. Other polyhydric alcohols are found, such as the dihydric alcohols ethylene glycol and propanediol-1,2, as well as myo-inositol (see 7.2.2).

The most important amino alcohols that are part of natural lipids are 2-aminoethanol (colamine), choline, also related to the α-amino acids serine and sphingosine.

Sphingosine is an unsaturated long chain dihydric amino alcohol. The double bond in sphingosine has trance-configuration, and asymmetric atoms C-2 and C-3 - D-configuration.

The alcohols in lipids are acylated with higher carboxylic acids at the corresponding hydroxyl groups or amino groups. In glycerol and sphingosine, one of the alcoholic hydroxyls can be esterified with substituted phosphoric acid.

10.3. Simple lipids

10.3.1. Waxes

Waxes are esters of higher fatty acids and higher monohydric alcohols.

Waxes form a protective lubricant on human and animal skin and prevent plants from drying out. They are used in the pharmaceutical and perfumery industries in the manufacture of creams and ointments. An example is cetyl palmitic acid ester(cetin) - the main component spermacet. Spermaceti is secreted from the fat contained in the cranial cavities of sperm whales. Another example is melissil ester of palmitic acid- a component of beeswax.

10.3.2. Fats and oils

Fats and oils are the most abundant group of lipids. Most of them belong to triacylglycerols - complete esters of glycerol and HFA, although mono- and diacylglycerols are also found and are involved in the metabolism.

Fats and oils (triacylglycerols) are esters of glycerol and higher fatty acids.

In the human body, triacylglycerols play the role of a structural component of cells or a storage substance ("fat depot"). Their energy value is about twice that of proteins.

or carbohydrates. However, an increased level of triacylglycerols in the blood is one of the additional risk factors for the development of coronary heart disease.

Solid triacylglycerols are called fats, liquid ones are called oils. Simple triacylglycerols contain residues of the same acids, mixed ones - different.

In the composition of triacylglycerols of animal origin, residues of saturated acids usually predominate. Such triacylglycerols are generally solids. On the contrary, vegetable oils contain mainly residues of unsaturated acids and have a liquid consistency.

Below are examples of neutral triacylglycerols and their systematic and (in parentheses) commonly used trivial names based on the names of their constituent fatty acids are indicated.

10.3.3. Ceramides

Ceramides are N-acylated derivatives of sphingosine alcohol.

Ceramides are present in small amounts in the tissues of plants and animals. Much more often they are part of complex lipids - sphingomyelins, cerebrosides, gangliosides, etc.

(see 10.4).

10.4. Complex lipids

Some complex lipids are difficult to classify unambiguously, since they contain groupings that allow them to be simultaneously assigned to different groups. According to the general classification of lipids (see Figure 10.1), complex lipids are usually divided into three large groups: phospholipids, sphingolipids, and glycolipids.

10.4.1. Phospholipids

The phospholipid group includes substances that cleave phosphoric acid during hydrolysis, for example, glycerophospholipids and some sphingolipids (Scheme 10.2). In general, phospholipids are characterized by a fairly high content of unsaturated acids.

Scheme 10.2.Classification of phospholipids

Glycerophospholipids. These compounds are the main lipid components of cell membranes.

According to the chemical structure, glycerophospholipids are derivatives l -glycero-3-phosphate.

l-Glycero-3-phosphate contains an asymmetric carbon atom and therefore can exist as two stereoisomers.

Natural glycerophospholipids have the same configuration, being derivatives of l-glycero-3-phosphate, which is formed during metabolism from dihydroxyacetone phosphate.

Phosphatides. Among glycerophospholipids, the most common are phosphatides - ester derivatives of l-phosphatidic acids.

Phosphatidic acids are derivatives l -glycero-3-phosphate esterified with fatty acids at alcoholic hydroxyl groups.

As a rule, in natural phosphatides in position 1 of the glycerol chain there is a saturated residue, in position 2 - an unsaturated acid, and one of the hydroxyls of phosphoric acid is esterified with a polyhydric alcohol or amino alcohol (X is the residue of this alcohol). In the body (pH ~ 7.4) the remaining free hydroxyl of phosphoric acid and other ionogenic groups in phosphatides are ionized.

Examples of phosphatides include compounds in which phosphatidic acids esterified for phosphate hydroxyl with the corresponding alcohols:

Phosphatidylserines, the esterifying agent is serine;

Phosphatidylethanolamines, the esterifying agent is 2-aminoethanol (in the biochemical literature it is often, but not quite correctly, called ethanolamine);

Phosphatidylcholines, esterifying agent - choline.

These esterifying agents are interrelated because ethanolamine and choline fragments can be metabolized from a serine fragment by decarboxylation and subsequent methylation with S-adenosylmethionine (SAM) (see 9.2.1).

A number of phosphatides, instead of an amine-containing esterifying agent, contain residues of polyhydric alcohols - glycerol, myoinositol, etc. The phosphatidylglycerols and phosphatidylinositols given below as an example refer to acidic glycerophospholipids, since in their structures there are no fragments of aminoalcohols imparting neutral and rhodylethanolamines.

Plasmalogens. Less common in comparison with ester glycerophospholipids are ether lipids, in particular plasmalogens. They contain the remainder of the unsaturated

* For convenience, the way of writing the configuration formula of the myo-inositol residue in phosphatidylinositols has been changed from the one given above (see 7.2.2).

alcohol linked by an ether bond to the C-1 atom of glycero-3-phosphate, such as, for example, plasmalogens with an ethanolamine moiety - L-phosphatidal ethanolamines. Plasmalogens make up 10% of all lipids in the central nervous system.

10.4.2. Sphingolipids

Sphingolipids are structural analogs of glycerophospholipids in which sphingosine is used instead of glycerol. Another example of sphingolipids is the ceramides discussed above (see 10.3.3).

An important group of sphingolipids are sphingomyelins, first discovered in nervous tissue. In sphingomyelins, the hydroxyl group at C-1 of ceramide is esterified, as a rule, with choline phosphate (less often with colamine phosphate); therefore, they can also be attributed to phospholipids.

10.4.3. Glycolipids

As the name suggests, the compounds of this group include carbohydrate residues (more often D-galactose, less often D-glucose) and do not contain a phosphoric acid residue. Typical representatives of glycolipids - cerebrosides and gangliosides - are sphingosine-containing lipids (therefore, they can also be considered sphingolipids).

V cerebrosides the ceramide residue is linked to D-galactose or D-glucose by a β-glycosidic bond. Cerebrosides (galactocerebrosides, glucocerebrosides) are part of the membranes of nerve cells.

Gangliosides- carbohydrate-rich complex lipids - were first isolated from the gray matter of the brain. Structurally, gangliosides are similar to cerebrosides, differing in that instead of a monosaccharide they contain a complex oligosaccharide containing at least one residue V-acetylneuraminic acid (see Appendix 11-2).

10.5. Lipid properties

and their structural components

A feature of complex lipids is their biphilicity, due to non-polar hydrophobic and highly polar ionized hydrophilic groups. In phosphatidylcholines, for example, the hydrocarbon radicals of fatty acids form two non-polar "tails", and the carboxyl, phosphate and choline groups form the polar part.

At the interface, these compounds act as excellent emulsifiers. In the composition of cell membranes, lipid components provide a high electrical resistance of the membrane, its impermeability to ions and polar molecules, and permeability to non-polar substances. In particular, most anesthetic drugs dissolve well in lipids, which allows them to penetrate the membranes of nerve cells.

Fatty acids are weak electrolytes( p K a~ 4.8). They are dissociated to a small extent in aqueous solutions. At pH< p K a non-ionized form predominates, at pH> p K a, that is, under physiological conditions, the ionized form RCOO - prevails. Soluble salts of higher fatty acids are called soaps. Sodium salts of higher fatty acids are solid, potassium salts are liquid. As salts of weak acids and strong bases of soaps are partially hydrolyzed in water, their solutions are alkaline.

Natural unsaturated fatty acids having cis-configuration of a double bond, have a large supply of internal energy and, therefore, in comparison with trance-isomers are thermodynamically less stable. Their cis-trans -isomerization proceeds easily upon heating, especially in the presence of radical initiators. Under laboratory conditions, this transformation can be carried out by the action of nitrogen oxides formed during the decomposition of nitric acid upon heating.

Higher fatty acids exhibit the general chemical properties of carboxylic acids. In particular, they readily form the corresponding functional derivatives. Fatty acids with double bonds exhibit the properties of unsaturated compounds - they add hydrogen, hydrogen halides and other reagents to the double bond.

10.5.1. Hydrolysis

With the help of the hydrolysis reaction, the structure of lipids is established, and also valuable products (soaps) are obtained. Hydrolysis is the first stage in the utilization and metabolism of dietary fats in the body.

Hydrolysis of triacylglycerols is carried out either by exposure to superheated steam (in industry), or by heating with water in the presence of mineral acids or alkalis (saponification). In the body, lipid hydrolysis takes place under the action of lipase enzymes. Some examples of hydrolysis reactions are shown below.

In plasmalogens, as in ordinary vinyl ethers, the ether bond is cleaved in an acidic, but not in an alkaline environment.

10.5.2. Addition reactions

Lipids containing residues of unsaturated acids in the structure are attached via double bonds with hydrogen, halogens, hydrogen halides, and water in an acidic medium. Iodine number is a measure of the unsaturation of triacylglycerols. It corresponds to the number of grams of iodine that can be added to 100 g of the substance. The composition of natural fats and oils and their iodine numbers vary within a fairly wide range. As an example, we give the interaction of 1-oleoyl-distearoylglycerol with iodine (the iodine number of this triacylglycerol is 30).

The catalytic hydrogenation (hydrogenation) of unsaturated vegetable oils is an important industrial process. In this case, hydrogen saturates the double bonds and liquid oils turn into solid fats.

10.5.3. Oxidation reactions

Oxidative processes involving lipids and their structural components are quite diverse. In particular, the oxidation of unsaturated triacylglycerols by oxygen in the air during storage (autoxidation, see 3.2.1), accompanied by hydrolysis, is part of a process known as oil rancidity.

The primary products of the interaction of lipids with molecular oxygen are hydroperoxides, which are formed as a result of a free radical chain process (see 3.2.1).

Lipid peroxidation - one of the most important oxidative processes in the body. It is the main cause of damage to cell membranes (for example, in radiation sickness).

Structural fragments of unsaturated higher fatty acids in phospholipids serve as targets for attack active forms of oxygen(ROS, see Appendix 03-1).

When attacked, in particular, by the hydroxyl radical HO ", the most active of the ROS, of the LH lipid molecule, a homolytic cleavage of the C-H bond in the allyl position occurs, as shown by the example of the model of lipid peroxidation (Scheme 10.3). The resulting allyl type radical L" instantly reacts with molecular oxygen in the oxidation medium to form the lipid-peroxyl radical LOO ". From this moment, a chain cascade of lipid peroxidation reactions begins, since there is a constant formation of allyl lipid radicals L", which resume this process.

Lipid peroxides LOOH are unstable compounds and can decompose spontaneously or with the participation of variable valence metal ions (see 3.2.1) with the formation of lipidoxyl radicals LO ", which can initiate further oxidation of the lipid substrate. Such an avalanche-like process of lipid peroxidation poses a risk of destruction of membrane structures cells.

The intermediate formed radical of the allyl type has a mesomeric structure and can further undergo transformations in two directions (see Scheme 10.3, paths a and b), leading to intermediate hydroperoxides. Hydroperoxides are unstable and decompose even at ordinary temperatures with the formation of aldehydes, which are further oxidized into acids - the end products of the reaction. The result is generally two monocarboxylic and two dicarboxylic acids with shorter carbon chains.

Unsaturated acids and lipids with residues of unsaturated acids under mild conditions are oxidized with an aqueous solution of potassium permanganate, forming glycols, and in more rigid ones (with rupture of carbon-carbon bonds), the corresponding acids.