Biosynthesis of saturated fatty acids. Biosynthesis of higher fatty acids During the synthesis of fatty acids, the correct sequence of reactions is

Previously, it was assumed that cleavage processes are the reversal of synthesis processes, including the synthesis of fatty acids was considered as a process reverse to their oxidation.

It has now been established that the mitochondrial system of fatty acid biosynthesis, including a slightly modified sequence of the β-oxidation reaction, only carries out the elongation of medium-chain fatty acids already existing in the body, while the complete biosynthesis of palmitic acid from acetyl-CoA actively proceeds outside mitochondria on a completely different path.

Let's look at some important features of the fatty acid biosynthesis pathway.

1. Synthesis occurs in the cytosol, in contrast to breakdown, which occurs in the mitochondrial matrix.

2. Fatty acid synthesis intermediates are covalently linked to the sulfhydryl groups of the acyl transfer protein (ATP), while fatty acid degradation intermediates are linked to coenzyme A.

3. Many enzymes for fatty acid synthesis in higher organisms are organized into a multienzyme complex called fatty acid synthetase. In contrast, enzymes that catalyze the breakdown of fatty acids do not appear to associate.

4. The growing fatty acid chain is lengthened by the sequential addition of two-carbon components derived from acetyl-CoA. The activated donor of two-carbon components at the elongation stage is malonyl-ACP. The elongation reaction is triggered by the release of CO 2 .

5. NADPH plays the role of a reducing agent in the synthesis of fatty acids.

6. Mn 2+ also participates in the reactions.

7. Elongation under the influence of the fatty acid synthetase complex stops at the stage of formation of palmitate (C 16). Further elongation and introduction of double bonds are carried out by other enzyme systems.

Formation of malonyl coenzyme A

Fatty acid synthesis begins with the carboxylation of acetyl-CoA to malonyl-CoA. This irreversible reaction represents a crucial step in the synthesis of fatty acids.

Malonyl-CoA synthesis is catalyzed acetyl-CoA carboxylase and is carried out using the energy of the APR. The source of CO 2 for the carboxylation of acetyl-CoA is bicarbonate.

Rice. Malonyl-CoA synthesis

Acetyl-CoA carboxylase contains as a prosthetic group biotin.

Rice. Biotin

The enzyme consists of a variable number of identical subunits, each of which contains biotin, biotin carboxylase, carboxybiotin transfer protein, transcarboxylase, as well as a regulatory allosteric center, i.e. represents multienzyme complex. The carboxyl group of biotin is covalently attached to the ε-amino group of the lysine residue of the carboxybiotin transfer protein. Carboxylation of the biotin component in the formed complex is catalyzed by the second subunit, biotin carboxylase. The third component of the system, transcarboxylase, catalyzes the transfer of activated CO 2 from carboxybiotin to acetyl-CoA.

Biotin enzyme + ATP + HCO 3 - ↔ CO 2 ~Biotin enzyme + ADP + Pi,

CO 2 ~Biotin-enzyme + Acetyl-CoA ↔ Molonyl-CoA + Biotin-enzyme.

The length and flexibility of the bond between biotin and its transport protein make it possible for the activated carboxyl group to move from one active site of the enzyme complex to another.

In eukaryotes, acetyl-CoA carboxylase exists as a protomer devoid of enzymatic activity (450 kDa) or as an active filamentous polymer. Their interconversion is regulated allosterically. The key allosteric activator is citrate, which shifts the equilibrium towards the active fibrous form of the enzyme. Optimal orientation of biotin in relation to substrates is achieved in fibrous form. In contrast to citrate, palmitoyl-CoA shifts the equilibrium towards the inactive protomer form. Thus, palmitoyl-CoA, the end product, inhibits the first critical step in fatty acid biosynthesis. The regulation of acetyl-CoA carboxylase in bacteria differs sharply from that in eukaryotes, since in them fatty acids are primarily precursors of phospholipids, and not reserve fuel. Here, citrate has no effect on bacterial acetyl-CoA carboxylase. The activity of the transcarboxylase component of the system is regulated by guanine nucleotides, which coordinate the synthesis of fatty acids with the growth and division of bacteria.

The building block for the synthesis of fatty acids in the cell cytosol is acetyl-CoA, which is formed in two ways: either as a result of oxidative decarboxylation of pyruvate. (see Fig. 11, Stage III), or as a result of b-oxidation of fatty acids (see Fig. 8).

Figure 11 – Scheme of the conversion of carbohydrates into lipids

Let us recall that the conversion of pyruvate formed during glycolysis into acetyl-CoA and its formation during b-oxidation of fatty acids occurs in mitochondria. The synthesis of fatty acids occurs in the cytoplasm. The inner mitochondrial membrane is impermeable to acetyl-CoA. Its entry into the cytoplasm is carried out by the type of facilitated diffusion in the form of citrate or acetylcarnitine, which in the cytoplasm are converted into acetyl-CoA, oxaloacetate or carnitine. However, the main pathway for the transfer of acetyl-CoA from the mitochondrion to the cytosol is the citrate route (see Fig. 12).

First, intramitochondrial acetyl-CoA reacts with oxaloacetate, resulting in the formation of citrate. The reaction is catalyzed by the enzyme citrate synthase. The resulting citrate is transported through the mitochondrial membrane into the cytosol using a special tricarboxylate transport system.

In the cytosol, citrate reacts with HS-CoA and ATP and again breaks down into acetyl-CoA and oxaloacetate. This reaction is catalyzed by ATP citrate lyase. Already in the cytosol, oxaloacetate, with the participation of the cytosolic dicarboxylate transport system, returns to the mitochondrial matrix, where it is oxidized to oxaloacetate, thereby completing the so-called shuttle cycle:

Figure 12 – Scheme of the transfer of acetyl-CoA from mitochondria to the cytosol

The biosynthesis of saturated fatty acids occurs in the direction opposite to their b-oxidation; the growth of hydrocarbon chains of fatty acids is carried out due to the sequential addition of a two-carbon fragment (C2) - acetyl-CoA - to their ends (see Fig. 11, stage IV.).

The first reaction in the biosynthesis of fatty acids is carboxylation of acetyl-CoA, which requires CO 2, ATP, and Mn ions. This reaction is catalyzed by the enzyme acetyl-CoA - carboxylase. The enzyme contains biotin (vitamin H) as a prosthetic group. The reaction occurs in two stages: 1 - carboxylation of biotin with the participation of ATP and II - transfer of the carboxyl group to acetyl-CoA, resulting in the formation of malonyl-CoA:

Malonyl-CoA is the first specific product of fatty acid biosynthesis. In the presence of the appropriate enzyme system, malonyl-CoA is rapidly converted into fatty acids.

It should be noted that the rate of fatty acid biosynthesis is determined by the sugar content in the cell. An increase in glucose concentration in adipose tissue of humans and animals and an increase in the rate of glycolysis stimulates the process of fatty acid synthesis. This indicates that fat and carbohydrate metabolism are closely related to each other. An important role here is played by the carboxylation reaction of acetyl-CoA with its conversion to malonyl-CoA, catalyzed by acetyl-CoA carboxylase. The activity of the latter depends on two factors: the presence of high molecular weight fatty acids and citrate in the cytoplasm.

The accumulation of fatty acids has an inhibitory effect on their biosynthesis, i.e. inhibit carboxylase activity.

A special role is given to citrate, which is an activator of acetyl-CoA carboxylase. Citrate at the same time plays the role of a link in carbohydrate and fat metabolism. In the cytoplasm, citrate has a dual effect in stimulating the synthesis of fatty acids: firstly, as an activator of acetyl-CoA carboxylase and, secondly, as a source of acetyl groups.

A very important feature of fatty acid synthesis is that all intermediate products of the synthesis are covalently linked to the acyl transfer protein (HS-ACP).

HS-ACP is a low-molecular protein that is thermostable, contains an active HS group and whose prosthetic group contains pantothenic acid (vitamin B 3). The function of HS-ACP is similar to the function of enzyme A (HS-CoA) in the b-oxidation of fatty acids.

In the process of building a chain of fatty acids, intermediate products form ester bonds with ABP (see Fig. 14):

The fatty acid chain elongation cycle includes four reactions: 1) condensation of acetyl-ACP (C 2) with malonyl-ACP (C 3); 2) restoration; 3) dehydration and 4) second reduction of fatty acids. In Fig. Figure 13 shows a diagram of the synthesis of fatty acids. One cycle of fatty acid chain elongation involves four sequential reactions.

Figure 13 – Scheme of fatty acid synthesis

In the first reaction (1) - the condensation reaction - the acetyl and malonyl groups interact with each other to form acetoacetyl-ABP with the simultaneous release of CO 2 (C 1). This reaction is catalyzed by the condensing enzyme b-ketoacyl-ABP synthetase. The CO 2 cleaved from malonyl-ACP is the same CO 2 that took part in the carboxylation reaction of acetyl-ACP. Thus, as a result of the condensation reaction, the formation of a four-carbon compound (C 4) occurs from two-carbon (C 2) and three-carbon (C 3) components.

In the second reaction (2), a reduction reaction catalyzed by b-ketoacyl-ACP reductase, acetoacetyl-ACP is converted to b-hydroxybutyryl-ACP. The reducing agent is NADPH + H +.

In the third reaction (3) of the dehydration cycle, a water molecule is split off from b-hydroxybutyryl-ACP to form crotonyl-ACP. The reaction is catalyzed by b-hydroxyacyl-ACP dehydratase.

The fourth (final) reaction (4) of the cycle is the reduction of crotonyl-ACP to butyryl-ACP. The reaction occurs under the action of enoyl-ACP reductase. The role of the reducing agent here is played by the second molecule NADPH + H +.

Then the cycle of reactions is repeated. Let us assume that palmitic acid (C 16) is being synthesized. In this case, the formation of butyryl-ACP is completed only by the first of 7 cycles, in each of which the beginning is the addition of a molonyl-ACP molecule (C 3) - reaction (5) to the carboxyl end of the growing fatty acid chain. In this case, the carboxyl group is split off in the form of CO 2 (C 1). This process can be represented as follows:

C 3 + C 2 ® C 4 + C 1 – 1 cycle

C 4 + C 3 ® C 6 + C 1 – 2 cycle

C 6 + C 3 ® C 8 + C 1–3 cycle

C 8 + C 3 ® C 10 + C 1 – 4 cycle

C 10 + C 3 ® C 12 + C 1 – 5 cycle

C 12 + C 3 ® C 14 + C 1 – 6 cycle

C 14 + C 3 ® C 16 + C 1 – 7 cycle

Not only higher saturated fatty acids can be synthesized, but also unsaturated ones. Monounsaturated fatty acids are formed from saturated fatty acids as a result of oxidation (desaturation) catalyzed by acyl-CoA oxygenase. Unlike plant tissues, animal tissues have a very limited ability to convert saturated fatty acids into unsaturated fatty acids. It has been established that the two most common monounsaturated fatty acids, palmitoleic and oleic, are synthesized from palmitic and stearic acids. In the body of mammals, including humans, linoleic (C 18:2) and linolenic (C 18:3) acids cannot be formed, for example, from stearic acid (C 18:0). These acids belong to the category of essential fatty acids. Essential fatty acids also include arachidic acid (C 20:4).

Along with the desaturation of fatty acids (formation of double bonds), their lengthening (elongation) also occurs. Moreover, both of these processes can be combined and repeated. Elongation of the fatty acid chain occurs by sequential addition of two-carbon fragments to the corresponding acyl-CoA with the participation of malonyl-CoA and NADPH + H +.

Figure 14 shows the pathways for the conversion of palmitic acid in desaturation and elongation reactions.

Figure 14 – Scheme of conversion of saturated fatty acids

to unsaturated

The synthesis of any fatty acid is completed by the cleavage of HS-ACP from acyl-ACP under the influence of the enzyme deacylase. For example:

The resulting acyl-CoA is the active form of the fatty acid.

The synthesis of fatty acids occurs in the cytoplasm of the cell. Mitochondria mainly involve elongation of existing fatty acid chains. It has been established that palmitic acid (16 carbon atoms) is synthesized in the cytoplasm of liver cells, and in the mitochondria of these cells from palmitic acid already synthesized in the cytoplasm of the cell or from fatty acids of exogenous origin, i.e. coming from the intestines, fatty acids containing 18, 20 and 22 carbon atoms are formed. The first reaction in fatty acid biosynthesis is carboxylation of acetyl-CoA, which requires bicarbonate, ATP, and manganese ions. This reaction is catalyzed by the enzyme acetyl-CoA carboxylase. The enzyme contains biotin as a prosthetic group. The reaction occurs in two stages: I - carboxylation of biotin with the participation of ATP and II - transfer of the carboxyl group to acetyl-CoA, resulting in the formation of malonyl-CoA. Malonyl-CoA is the first specific product of fatty acid biosynthesis. In the presence of the appropriate enzyme system, malonyl-CoA is rapidly converted into fatty acids. The sequence of reactions occurring during the synthesis of fatty acids:

Then the cycle of reactions is repeated. Compared with β-oxidation, the biosynthesis of fatty acids has a number of characteristic features: the synthesis of fatty acids is mainly carried out in the cytosol of the cell, and oxidation in the mitochondria; participation in the process of biosynthesis of malonyl-CoA fatty acids, which is formed by binding CO2 (in the presence of biotin enzyme and ATP) with acetyl-CoA; acyl-transfer protein (HS-ACP) is involved in all stages of fatty acid synthesis; during biosynthesis, the D(–)-isomer of 3-hydroxy acid is formed, and not the L(+)-isomer, as is the case in β-oxidation of fatty acids; necessary for the synthesis of fatty acids coenzyme NADPH.

50. Cholesterol - cholesterol is an organic compound, a natural fatty (lipophilic) alcohol contained in the cell membranes of all animal organisms with the exception of non-nuclear ones (prokaryotes). Insoluble in water, soluble in fats and organic solvents. Biological role. Cholesterol in the composition of the cell plasma membrane plays the role of a bilayer modifier, giving it a certain rigidity by increasing the density of “packing” of phospholipid molecules. Thus, cholesterol is a stabilizer of the fluidity of the plasma membrane. Cholesterol opens the chain of biosynthesis of steroid sex hormones and corticosteroids, serves as the basis for the formation of bile acids and vitamins D, participates in the regulation of cell permeability and protects red blood cells from the action of hemolytic poisons. Cholesterol exchange. Free cholesterol is subject to oxidation in the liver and organs that synthesize steroid hormones (adrenal glands, testes, ovaries, placenta). This is the only process of irreversible removal of cholesterol from membranes and lipoprotein complexes. Every day, 2-4% of cholesterol is consumed for the synthesis of steroid hormones. In hepatocytes, 60-80% of cholesterol is oxidized to bile acids, which, as part of bile, are released into the lumen of the small intestine and participate in digestion (emulsification of fats). Together with bile acids, a small amount of free cholesterol is released into the small intestine, which is partially removed with feces, and the rest of it dissolves and, together with bile acids and phospholipids, is absorbed by the walls of the small intestine. Bile acids ensure the decomposition of fats into their component parts (emulsification of fats). After performing this function, 70-80% of the remaining bile acids are absorbed in the final part of the small intestine (ileum) and enter the portal vein system into the liver. It is worth noting here that bile acids have another function: they are the most important stimulant for maintaining normal functioning (motility) of the intestines. In the liver, incompletely formed (nascent) high-density lipoproteins begin to be synthesized. Finally, HDL is formed in the blood from special proteins (apoproteins) of chylomicrons, VLDL and cholesterol coming from tissues, including from the arterial wall. More simply, the cholesterol cycle can be explained as follows: cholesterol in lipoproteins carries fat from the liver to various parts of your body, using blood vessels as a transport system. After the fat is delivered, cholesterol returns to the liver and repeats its work again. Primary bile acids. (cholic and chenodeoxycholic) are synthesized in liver hepatocytes from cholesterol. Secondary: deoxycholic acid (initially synthesized in the colon). Bile acids are formed in and outside the mitochondria of hepatocytes from cholesterol with the participation of ATP. Hydroxylation during the formation of acids occurs in the endoplasmic reticulum of the hepatocyte. The primary synthesis of bile acids is inhibited (inhibited) by bile acids present in the blood. However, if the absorption of bile acids into the blood is insufficient, for example, due to severe intestinal damage, then the liver, capable of producing no more than 5 g of bile acids per day, will not be able to replenish the amount of bile acids required by the body. Bile acids are the main participants in the enterohepatic circulation in humans. Secondary bile acids (deoxycholic, lithocholic, ursodeoxycholic, allocholic and others) are formed from primary bile acids in the colon under the influence of intestinal microflora. Their number is small. Deoxycholic acid is absorbed into the blood and secreted by the liver as part of bile. Lithocholic acid is absorbed much less well than deoxycholic acid.

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  Compared to β-oxidation biosynthesis fatty acids has a number of characteristic features: synthesis fatty acids mainly occurs in the cytosol of the cell, and oxidation...


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  Biosynthesis triglycerides (triacylglycerols). Biosynthesis fatty acids Fat can be synthesized both from fat breakdown products and from carbohydrates.


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  BIOSYNTHESIS TRIGLYCERIDES. Triglyceride synthesis occurs from glycerol and fatty acids(mainly stearic, pa.


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  Biosynthesis fatty acids. Synthesis fatty acids


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  Biosynthesis fatty acids. Synthesis fatty acids occurs in the cytoplasm of the cell. Most of the udli occurs in mitochondria.

Since the ability of animals and humans to store polysaccharides is quite limited, glucose received in quantities exceeding the immediate energy needs and the “storage capacity” of the body can be a “building material” for the synthesis of fatty acids and glycerol. In turn, fatty acids, with the participation of glycerol, are converted into triglycerides, which are deposited in adipose tissue.

An important process is also the biosynthesis of cholesterol and other sterols. Although in quantitative terms the pathway of cholesterol synthesis is not so important, it is of great importance due to the fact that numerous biologically active steroids are formed from cholesterol in the body.

Synthesis of higher fatty acids in the body

Currently, the mechanism of biosynthesis of fatty acids in animals and humans, as well as the enzyme systems catalyzing this process, has been sufficiently studied. The synthesis of fatty acids in tissues occurs in the cytoplasm of the cell. In mitochondria, elongation of existing fatty acid chains mainly occurs 1 .

1 In vitro experiments have shown that isolated mitochondria have a negligible ability to incorporate labeled acetic acid into long-chain fatty acids. For example, it has been established that palmitic acid is synthesized mainly in the cytoplasm of liver cells, and in the mitochondria of liver cells, on the basis of palmitic acid already synthesized in the cytoplasm of the cell or on the basis of fatty acids of exogenous origin, i.e., coming from the intestines, fatty acids containing 18, 20 and 22 carbon atoms. In this case, the reactions of fatty acid synthesis in mitochondria are essentially reverse reactions of fatty acid oxidation.

Extramitochondrial synthesis (basic, main) of fatty acids in its mechanism differs sharply from the process of their oxidation. The building block for the synthesis of fatty acids in the cell cytoplasm is acetyl-CoA, which is mainly derived from mitochondrial acetyl-CoA. It has also been established that the presence of carbon dioxide or bicarbonate ion in the cytoplasm is important for the synthesis of fatty acids. In addition, it was found that citrate stimulates the synthesis of fatty acids in the cell cytoplasm. It is known that acetyl-CoA formed in mitochondria during oxidative decarboxylation cannot diffuse into the cell cytoplasm, because the mitochondrial membrane is impermeable to this substrate. It has been shown that mitochondrial acetyl-CoA interacts with oxaloacetate, resulting in the formation of citrate, which freely penetrates into the cell cytoplasm, where it is cleaved to acetyl-CoA and oxaloacetate:

Therefore, in this case, citrate acts as a carrier of the acetyl radical.

There is another way of transferring intramitochondrial acetyl-CoA into the cell cytoplasm. This is the pathway involving carnitine. It was indicated above that carnitine plays the role of a carrier of acyl groups from the cytoplasm to mitochondria during the oxidation of fatty acids. Apparently, it can also perform this role in the reverse process, i.e., in the transfer of acyl radicals, including the acetyl radical, from mitochondria to the cell cytoplasm. However, when it comes to the synthesis of fatty acids, this acetyl-CoA transport pathway is not the main one.

The most important step in understanding the process of fatty acid synthesis was the discovery of the enzyme acetyl-CoA carboxylase. This complex enzyme containing biotin catalyzes the ATP-dependent synthesis of malonyl-CoA (HOOC-CH 2 -CO-S-CoA) from acetyl-CoA and CO 2.

This reaction occurs in two stages:

It has been established that citrate functions as an activator of the acetyl-CoA carboxylase reaction.

Malonyl-CoA is the first specific product of fatty acid biosynthesis. In the presence of an appropriate enzymatic system, malonyl-CoA (which in turn is formed from acetyl-CoA) is rapidly converted into fatty acids.

The enzyme system that synthesizes higher fatty acids consists of several enzymes interconnected in a certain way.

Currently, the process of fatty acid synthesis has been studied in detail in E. coli and some other microorganisms. In E. coli, a multienzyme complex called fatty acid synthetase consists of seven enzymes associated with the so-called acyl transfer protein (ATP). This protein is relatively thermostable, has free HS-rpynny and is involved in the process of synthesis of higher fatty acids at almost all its stages. The relative molecular weight of APB is about 10,000 daltons.

The following is the sequence of reactions that occur during the synthesis of fatty acids:

Then the cycle of reactions is repeated. Let's assume that palmitic acid (C 16) is being synthesized; in this case, only the first of seven cycles is completed with the formation of butyryl-ACP, each of which begins with the addition of a malonyl-ACP molecule to the carboxyl end of the growing fatty acid chain. In this case, the HS-ACP molecule and the distal carboxyl group of malonyl-ACP are split off in the form of CO 2 . For example, butyryl-ACP formed in the first cycle interacts with malonyl-ACP:

Fatty acid synthesis is completed by the cleavage of HS-ACP from acyl-ACP under the influence of the enzyme deacylase, for example:

The overall equation for the synthesis of palmitic acid can be written as follows:

Or, taking into account that the formation of one molecule of malonyl-CoA from acetyl-CoA requires one molecule of ATP and one molecule of CO 2, the overall equation can be presented as follows:

The main stages of fatty acid biosynthesis can be represented in the form of a diagram.

Compared with β-oxidation, the biosynthesis of fatty acids has a number of characteristic features:

• the synthesis of fatty acids is mainly carried out in the cytoplasm of the cell, and oxidation - in the mitochondria;
• participation in the process of biosynthesis of malonyl-CoA fatty acids, which is formed by binding CO 2 (in the presence of biotin enzyme and ATP) with acetyl-CoA;
• acyl-transfer protein (HS-ACP) is involved in all stages of fatty acid synthesis;
• necessity for the synthesis of fatty acids of the coenzyme NADPH 2. The latter in the body is formed partly (50%) in the reactions of the pentose cycle (hexose monophosphate “shunt”), partly as a result of the reduction of NADP with malate (malic acid + NADP-pyruvic acid + CO 2 + NADPH 2);
• restoration of the double bond in the enoyl-ACP reductase reaction occurs with the participation of NADPH 2 and an enzyme whose prosthetic group is flavin mononucleotide (FMN);
• During the synthesis of fatty acids, hydroxy derivatives are formed, which in their configuration belong to the D-series of fatty acids, and during the oxidation of fatty acids, hydroxy derivatives of the L-series are formed.

Formation of unsaturated fatty acids

Unsaturated fatty acids are present in mammalian tissues and can be classified into four families, differing in the length of the aliphatic chain between the terminal methyl group and the nearest double bond:

It has been established that the two most common monosaturated fatty acids, palmitoleic and oleic, are synthesized from palmitic and stearic acids. The double bond is introduced into the molecule of these acids in the microsomes of liver cells and adipose tissue with the participation of specific oxygenase and molecular oxygen. In this reaction, one oxygen molecule is used as an acceptor of two pairs of electrons, one pair of which belongs to the substrate (Acyl-CoA), and the other to NADPH 2:

At the same time, the tissues of humans and a number of animals are unable to synthesize linoleic and linolenic acids, but must receive them from food (the synthesis of these acids is carried out by plants). In this regard, linoleic and linolenic acids, containing two and three double bonds, respectively, are called essential fatty acids.

All other polyunsaturated acids found in mammals are formed from four precursors (palmitoleic acid, oleic acid, linoleic acid, and linolenic acid) by further chain extension and/or the introduction of new double bonds. This process occurs with the participation of mitochondrial and microsomal enzymes. For example, the synthesis of arachidonic acid occurs according to the following scheme:

The biological role of polyunsaturated fatty acids has become significantly clearer in connection with the discovery of a new class of physiologically active compounds - prostaglandins.

Triglyceride biosynthesis

There is reason to believe that the rate of biosynthesis of fatty acids is largely determined by the rate of formation of triglycerides and phospholipids, since free fatty acids are present in tissues and blood plasma in small quantities and do not normally accumulate.

Triglyceride synthesis occurs from glycerol and fatty acids (mainly stearic, palmitic and oleic). The biosynthesis pathway of triglycerides in tissues proceeds through the formation of glycerol-3-phosphate as an intermediate compound. In the kidneys, as well as in the intestinal wall, where the activity of the enzyme glycerol kinase is high, glycerol is phosphorylated by ATP to form glycerol-3-phosphate:

In adipose tissue and muscle, due to the very low activity of glycerol kinase, the formation of glycerol-3-phosphate is mainly due to glycolysis or glycogenolysis 1 . 1 In cases where the glucose content in adipose tissue is reduced (for example, during fasting), only a small amount of glycerol-3-phosphate is formed and the free fatty acids released during lipolysis cannot be used for the resynthesis of triglycerides, so fatty acids leave the adipose tissue . On the contrary, activation of glycolysis in adipose tissue promotes the accumulation of triglycerides in it, as well as their constituent fatty acids. It is known that dihydroxyacetone phosphate is formed during the glycolytic breakdown of glucose. The latter, in the presence of cytoplasmic NAD-dependent glycerol phosphate dehydrogenase, is capable of converting into glycerol-3-phosphate:

In the liver, both pathways for the formation of glycerol-3-phosphate are observed.

The resulting glycerol-3-phosphate is acylated in one way or another by two molecules of a CoA-derived fatty acid (i.e., “active” forms of the fatty acid) 2 . 2 In some microorganisms, for example, E. coli, the donor of the acyl group is not CoA-conductors, but ACP-derivatives of the fatty acid. As a result, phosphatidic acid is formed:

Note that although phosphatidic acid is present in cells in extremely small quantities, it is a very important intermediate product common to the biosynthesis of triglycerides and glycerophospholipids (see diagram).

If triglycerides are synthesized, then dephosphorylation of phosphatidic acid occurs using a specific phosphatase (phosphatidate phosphatase) and the formation of 1,2-diglyceride:

The biosynthesis of triglycerides is completed by the esterification of the resulting 1,2-diglyceride with a third acyl-CoA molecule:

Biosynthesis of glycerophospholipids

The synthesis of the most important glycerophospholipids is localized mainly in the endoplasmic reticulum of the cell. First, phosphatidic acid, as a result of a reversible reaction with cytidine triphosphate (CTP), is converted into cytidine diphosphate diglyceride (CDP-diglyceride):

Then, in subsequent reactions, each of which is catalyzed by the appropriate enzyme, cytidine monophosphate is displaced from the CDP-diglyceride molecule by one of two compounds - serine or inositol, forming phosphatidylserine or phosphatidylinositol, or 3-phosphatidyl-glycerol-1-phosphate. As an example, we give the formation of phosphatidylserine:

In turn, phosphatidylserine can be decarboxylated to form phosphatidylethanolamine:

Phosphatidemlethanolamine is a precursor to phosphatidylcholine. As a result of the sequential transfer of three methyl groups from three molecules of S-adenosylmethionine (methyl group donor) to the amino group of the ethanolamine residue, phosphatidylcholine is formed:

There is another route for the synthesis of phosphatidylethanolamine and phosphatidylcholine in animal cells. This pathway also uses CTP as a transporter, but not phosphatidic acid, but phosphorylcholine or phosphorylethanolamine (scheme).

Cholesterol biosynthesis

Back in the 60s of this century, Bloch et al. in experiments using acetate labeled with 14 C at the methyl and carboxyl groups, showed that both carbon atoms of acetic acid are included in liver cholesterol in approximately equal quantities. Additionally, it has been proven that all of the carbon atoms in cholesterol come from acetate.

Subsequently, thanks to the work of Linen, Redney, Polyak, Cornforth, A.N. Klimov and other researchers, the main details of the enzymatic synthesis of cholesterol, numbering more than 35 enzymatic reactions, were clarified. In the synthesis of cholesterol, three main stages can be distinguished: the first is the conversion of active acetate into mevalonic acid, the second is the formation of squalene from mevalonic acid, and the third is the cyclization of squalene into cholesterol.

First, let's consider the stage of conversion of active acetate to mevalonic acid. The initial step in the synthesis of mevalonic acid from acetyl-CoA is the formation of acetoacetyl-CoA through a reversible thiolase reaction:

Then the subsequent condensation of acetoacetyl-CoA with a third molecule of acetyl-CoA with the participation of hydroxymethylglutaryl-CoA synthase (HMG-CoA synthase) gives the formation of β-hydroxy-β-methylglutaryl-CoA:

Note that we have already considered these first stages of the synthesis of mevalonic acid when we were talking about the formation of ketone bodies. Next, β-hydroxy-β-methylglutaryl-CoA, under the influence of NADP-dependent hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase), as a result of the reduction of one of the carboxyl groups and the cleavage of HS-KoA, is converted into mevalonic acid:

The HMG-CoA reductase reaction is the first practically irreversible reaction in the cholesterol biosynthesis chain and it occurs with a significant loss of free energy (about 33.6 kJ). It has been established that this reaction limits the rate of cholesterol biosynthesis.

Along with the classical pathway of mevalonic acid biosynthesis, there is a second pathway in which not β-hydroxy-β-methylglutaryl-CoA, but β-hydroxy-β-methylglutaryl-S-ACP is formed as an intermediate substrate. The reactions of this pathway are apparently identical to the initial stages of fatty acid biosynthesis up to the formation of acetoacetyl-S-ACP. Acetyl-CoA carboxylase, an enzyme that converts acetyl-CoA into malonyl-CoA, takes part in the formation of mevalonic acid along this pathway. The optimal ratio of malonyl-CoA and acetyl-CoA for the synthesis of mevalonic acid is two molecules of acetyl-CoA per one molecule of malonyl-CoA.

The participation of malonyl-CoA, the main substrate of fatty acid biosynthesis, in the formation of mevalonic acid and various polyisoprenoids has been shown for a number of biological systems: pigeon and rat liver, rabbit mammary gland, cell-free yeast extracts. This pathway of mevalonic acid biosynthesis is observed predominantly in the cytoplasm of liver cells. A significant role in the formation of mevalonate in this case is played by hydroxymethylglutaryl-CoA reductase, found in the soluble fraction of rat liver and not identical to the microsomal enzyme in a number of kinetic and regulatory properties. It is known that microsomal hydroxymethylglutaryl-CoA reductase is the main link in the regulation of the biosynthesis pathway of mevalonic acid from acetyl-CoA with the participation of acetoacetyl-CoA thiolase and HMG-CoA synthase. Regulation of the second pathway of mevalonic acid biosynthesis under a number of influences (fasting, cholesterol feeding, administration of a surfactant - Triton WR-1339) differs from the regulation of the first pathway, in which microsomal reductase takes part. These data indicate the existence of two autonomous systems for the biosynthesis of mevalonic acid. The physiological role of the second pathway is incompletely understood. It is believed that it has a certain significance not only for the synthesis of substances of non-steroidal nature, such as the side chain of ubiquinone and the unique base N 6 (Δ 2 -isopentyl)-adenosine of some tRNAs, but also for the biosynthesis of steroids (A. N. Klimov, E . D. Polyakova).

In the second stage of cholesterol synthesis, mevalonic acid is converted to squalene. The second stage reactions begin with the phosphorylation of mevalonic acid with ATP. As a result, 5"-pyrophosphoric ester is formed, and then 5"-pyrophosphoric ester of mevalonic acid:

5"-pyrophosphomevalonic acid, as a result of subsequent phosphorylation of the tertiary hydroxyl group, forms an unstable intermediate product - 3"-phospho-5"-pyrophosphomevalonic acid, which, decarboxylated and losing phosphoric acid, is converted into isopentenyl pyrophosphate. The latter isomerizes into dimethylallyl pyrophosphate:

These two isomeric isopentenyl pyrophosphates (dimethylallyl pyrophosphate and isopentenyl pyrophosphate) are then condensed to release pyrophosphate and form geranyl pyrophosphate. Isopentenyl pyrophosphate is again added to geranyl pyrophosphate, resulting in farnesyl pyrophosphate.