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... -
Biosynthesis triglycerides (triacylglycerols). Biosynthesis fatty acids Fat can be synthesized both from fat breakdown products and from carbohydrates. -
BIOSYNTHESIS TRIGLYCERIDES. Triglyceride synthesis occurs from glycerol and fatty acids(mainly stearic, pa. -
Biosynthesis fatty acids. Synthesis fatty acids -
Biosynthesis fatty acids. Synthesis fatty acids occurs in the cytoplasm of the cell. Most of the udli occurs in mitochondria.
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.
Formation of acetyl-CoA and its transport into the cytosol
The synthesis of fatty acids occurs during the absorption period. Active glycolysis and subsequent oxidative decarboxylation of pyruvate contribute to an increase in the concentration of acetyl-CoA in the mitochondrial matrix. Since fatty acid synthesis occurs in the cytosol of cells, acetyl-CoA must be transported across the inner mitochondrial membrane into the cytosol. However, the inner membrane of mitochondria is impermeable to acetyl-CoA, therefore, in the mitochondrial matrix, acetyl-CoA condenses with oxaloacetate to form citrate with the participation of citrate synthase:
Acetyl-CoA + Oxaloacetate -> Citrate + HS-CoA.
The translocase then transports the citrate into the cytoplasm (Figure 8-35).
The transfer of citrate into the cytoplasm occurs only when the amount of citrate in the mitochondria increases, when isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are inhibited by high concentrations of NADH and ATP. This situation is created in the absorption period, when the liver cell receives a sufficient amount of energy sources. In the cytoplasm, citrate is broken down by the enzyme citrate lyase:
Citrate + HSKoA + ATP → Acetyl-CoA + ADP + Pi + Oxaloacetate.
Acetyl-CoA in the cytoplasm serves as the initial substrate for the synthesis of fatty acids, and oxaloacetate in the cytosol undergoes the following transformations (see diagram below).
Pyruvate is transported back into the mitochondrial matrix. NADPH, reduced as a result of the action of the malik enzyme, is used as a hydrogen donor for subsequent reactions of fatty acid synthesis. Another source of NADPH is the oxidative steps of the pentose phosphate pathway of glucose catabolism.
Formation of malonyl-CoA from acetyl-CoA - a regulatory reaction in the biosynthesis of fatty acids.
The first reaction in fatty acid synthesis is the conversion of acetyl-CoA to malonyl-CoA. The enzyme that catalyzes this reaction (acetyl-CoA carboxylase) is classified as a ligase. It contains covalently bound biotin (Figure 8-36). In the first stage of the reaction, CO 2 covalently binds to biotin due to the energy of ATP, in the second stage, COO is transferred to acetyl-CoA to form malonyl-CoA. The activity of the enzyme acetyl-CoA carboxylase determines the rate of all subsequent reactions of fatty acid synthesis.
Reactions catalyzed by fatty acid synthase- an enzyme complex that catalyzes the synthesis of palmitic acid, is described below.
After the formation of malonyl-CoA, the synthesis of fatty acids continues in the multienzyme complex - fatty acid synthase (palmitoyl synthetase). This enzyme consists of 2 identical protomers, each of which has a domain structure and, accordingly, 7 centers with different catalytic activities (Fig. 8-37). This complex sequentially extends the fatty acid radical by 2 carbon atoms, the donor of which is malonyl-CoA. The final product of this complex is palmitic acid, which is why the former name of this enzyme is palmitoyl synthetase.
The first reaction is the transfer of the acetyl group of acetyl-CoA to the thiol group of cysteine by the acetyltransacylase center (Fig. 8-38). The malonyl residue from malonyl-CoA is then transferred to the sulfhydryl group of the acyl-transfer protein by the malonyl transacylase site. After this, the complex is ready for the first cycle of synthesis.
The acetyl group condenses with the malonyl residue at the site of the separated CO 2 . The reaction is catalyzed by the ketoacyl synthase center. The resulting acetoacetyl radical
Scheme
Rice. 8-35. Transfer of acetyl residues from mitochondria to the cytosol. Active enzymes: 1 - citrate synthase; 2 - translocase; 3 - citrate lyase; 4 - malate dehydrogenase; 5 - malik enzyme.
Rice. 8-36. The role of biotin in the carboxylation reaction of acetyl-CoA.
Rice. 8-37. The structure of the multienzyme complex - fatty acid synthesis. The complex is a dimer of two identical polypeptide chains, each of which has 7 active centers and an acyl transfer protein (ATP). The SH groups of protomers belong to different radicals. One SH group belongs to cysteine, the other to a phosphopanthetheic acid residue. The cysteine SH group of one monomer is located next to the 4-phosphopantetheinate SH group of the other protomer. Thus, the enzyme protomers are arranged head to tail. Although each monomer contains all the catalytic sites, a complex of 2 protomers is functionally active. Therefore, 2 fatty acids are actually synthesized simultaneously. To simplify, diagrams usually depict the sequence of reactions during the synthesis of one acid molecule.
is sequentially reduced by ketoacyl reductase, then dehydrated and again reduced by enoyl reductase, the active centers of the complex. The first cycle of reactions produces a butyryl radical bound to a fatty acid synthase subunit.
Before the second cycle, the butyryl radical is transferred from position 2 to position 1 (where acetyl was located at the beginning of the first cycle of reactions). The butyryl residue then undergoes the same transformations and is extended by 2 carbon atoms derived from malonyl-CoA.
Similar cycles of reactions are repeated until a palmitic acid radical is formed, which, under the action of the thioesterase center, is hydrolytically separated from the enzyme complex, turning into free palmitic acid (palmitate, Fig. 8-38, 8-39).
The overall equation for the synthesis of palmitic acid from acetyl-CoA and malonyl-CoA is as follows:
CH 3 -CO-SKoA + 7 HOOC-CH 2 -CO-SKoA + 14 (NADPH + H +) → C 15 H 31 COOH + 7 CO 2 + 6 H 2 O + 8 HSKoA + 14 NADP +.
Main sources of hydrogen for fatty acid synthesis
In each cycle of palmitic acid biosynthesis, 2 reduction reactions take place,
Rice. 8-38. Synthesis of palmitic acid. Fatty acid synthase: in the first protomer the SH group belongs to cysteine, in the second to phosphopantetheine. After the end of the first cycle, the butyryl radical is transferred to the SH group of the first protomer. Then the same sequence of reactions is repeated as in the first cycle. Palmitoyl-E is a palmitic acid residue associated with fatty acid synthase. In the synthesized fatty acid, only the 2 distal carbon atoms, designated *, come from acetyl-CoA, the rest from malonyl-CoA.
Rice. 8-39. General scheme of reactions for the synthesis of palmitic acid.
the hydrogen donor in which is the coenzyme NADPH. NADP+ reduction occurs in the reactions:
dehydrogenation in the oxidative stages of the pentose phosphate pathway of glucose catabolism;
dehydrogenation of malate with malic enzyme;
dehydrogenation of isocitrate by cytosolic NADP-dependent dehydrogenase.
2. Regulation of fatty acid synthesis
The regulatory enzyme for fatty acid synthesis is acetyl-CoA carboxylase. This enzyme is regulated in several ways.
Association/dissociation of enzyme subunit complexes. In its inactive form, acetyl-CoA carboxylase is a separate complex, each of which consists of 4 subunits. Enzyme activator - citrate; it stimulates the association of complexes, as a result of which enzyme activity increases. Inhibitor - palmitoyl-CoA; it causes dissociation of the complex and a decrease in enzyme activity (Fig. 8-40).
Phosphorylation/dephosphorylation of acetyl-CoA carboxylase. In the postabsorptive state or during physical activity, glucagon or epinephrine activate protein kinase A through the adenylate cyclase system and stimulate phosphorylation of acetyl-CoA carboxylase subunits. The phosphorylated enzyme is inactive and fatty acid synthesis stops. During the absorptive period, insulin activates phosphatase, and acetyl-CoA carboxylase enters a dephosphorylated state (Fig. 8-41). Then, under the influence of citrate, polymerization of the enzyme protomers occurs, and it becomes active. In addition to activating the enzyme, citrate has another function in the synthesis of fatty acids. During the absorptive period, citrate accumulates in the mitochondria of liver cells, in which the acetyl residue is transported to the cytosol.
Induction of enzyme synthesis. Long-term consumption of foods rich in carbohydrates and low in fat leads to an increase in the secretion of insulin, which stimulates the induction of the synthesis of enzymes: acetyl-CoA carboxylase, fatty acid synthase, citrate lyase,
Rice. 8-40. Association/dissociation of acetyl-CoA carboxylase complexes.
Rice. 8-41. Regulation of acetyl-CoA carboxylase.
Rice. 8-42. Elongation of palmitic acid in the ER. The palmitic acid radical is extended by 2 carbon atoms, the donor of which is malonyl-CoA.
isocitrate dehydrogenase. Consequently, excess consumption of carbohydrates leads to an acceleration of the conversion of glucose catabolic products into fats. Fasting or eating foods rich in fat leads to a decrease in the synthesis of enzymes and, accordingly, fats.
3. Synthesis of fatty acids from palmitic acid
Elongation of fatty acids. In the ER, palmitic acid is elongated with the participation of malonyl-CoA. The sequence of reactions is similar to that which occurs during the synthesis of palmitic acid, but in this case the fatty acids are associated not with fatty acid synthase, but with CoA. Enzymes involved in elongation can use not only palmitic acid, but also other fatty acids as substrates (Fig. 8-42), therefore, not only stearic acid, but also fatty acids with a large number of carbon atoms can be synthesized in the body.
The main product of elongation in the liver is stearic acid (C 18:0), but in brain tissue a large amount of fatty acids with a longer chain are formed - from C 20 to C 24, which are necessary for the formation of sphingolipids and glycolipids.
Synthesis of other fatty acids, α-hydroxy acids, also occurs in nervous tissue. Mixed-function oxidases hydroxylate C22 and C24 acids to form lignoceric and cerebronic acids, found only in brain lipids.
Formation of double bonds in fatty acid radicals. The incorporation of double bonds into fatty acid radicals is called desaturation. The main fatty acids formed in the human body as a result of desaturation (Fig. 8-43) are palmitoo-leic (C16:1Δ9) and oleic (C18:1Δ9).
The formation of double bonds in fatty acid radicals occurs in the ER in reactions involving molecular oxygen, NADH and cytochrome b 5. Fatty acid desaturase enzymes found in humans cannot form double bonds in fatty acid radicals distal to the ninth carbon atom, i.e. between ninth and
Rice. 8-43. Formation of unsaturated fatty acids.
methyl carbon atoms. Therefore, fatty acids of the ω-3 and ω-6 families are not synthesized in the body, are essential and must be supplied with food, as they perform important regulatory functions.
The formation of a double bond in a fatty acid radical requires molecular oxygen, NADH, cytochrome b 5 and FAD-dependent cytochrome b 5 reductase. Hydrogen atoms removed from the saturated acid are released as water. One atom of molecular oxygen is included in a water molecule, and the other is also reduced to water with the participation of NADH electrons, which are transferred through FADH 2 and cytochrome b 5.
Eicosanoids are biologically active substances synthesized by most cells from polyene fatty acids containing 20 carbon atoms (the word "eicosis" in Greek means 20).
Synthesis of palmitic acid (C16) from Acetyl-CoA.
1) Occurs in the cytoplasm of liver cells and adipose tissue.
2) Significance: for the synthesis of fats and phospholipids.
3) Occurs after eating (during the absorption period).
4) Formed from acetyl-CoA obtained from glucose (glycolysis → ODPVK → Acetyl-CoA).
5) In the process, 4 reactions are repeated sequentially:
condensation → reduction → dehydration → reduction.
At the end of each LCD cycle lengthens by 2 carbon atoms.
Donor 2C is malonyl-CoA.
6) NADPH + H + takes part in two reduction reactions (50% comes from PPP, 50% from the MALIC enzyme).
7) Only the first reaction occurs directly in the cytoplasm (regulatory).
The remaining 4 cyclic ones are based on a special palmitate synthase complex (synthesis of only palmitic acid)
8) The regulatory enzyme functions in the cytoplasm - Acetyl-CoA carboxylase (ATP, vitamin H, biotin, class IV).
Structure of the palmitate synthase complex
Palmitate synthase is an enzyme consisting of 2 subunits.
Each consists of one PPC, on which there are 7 active centers.
Each active site catalyzes its own reaction.
Each PPC contains an acyl transfer protein (ATP), on which synthesis takes place (contains phosphopantetonate).
Each subunit has an HS group. In one, the HS group belongs to cysteine, in the other, to phosphopantothenic acid.
Mechanism
1) Acetyl-Coa, obtained from carbohydrates, cannot enter the cytoplasm, where FA synthesis occurs. It exits through the first reaction of the TCA cycle - the formation of citrate.
2) In the cytoplasm, citrate breaks down into Acetyl-Coa and oxaloacetate.
3) Oxaloacetate → malate (TCA cycle reaction in the opposite direction).
4) Malate → pyruvate, which is used in ODPVC.
5) Acetyl-CoA → synthesis of FA.
6) Acetyl-CoA is converted to malonyl-CoA by acetyl-CoA carboxylase.
Activation of the enzyme acetyl-CoA carboxylase:
a) by enhancing the synthesis of subunits under the influence of insulin - three tetramers are synthesized separately
b) under the influence of citrate, three tetramers combine and the enzyme is activated
c) during fasting, glucagon inhibits the enzyme (by phosphorylation), fat synthesis does not occur
7) one acetyl CoA from the cytoplasm is moved to the HS group (from cysteine) of palmitate synthase; one malonyl-CoA per HS group of the second subunit. Further on palmitate synthase occurs:
8) their condensation (acetyl CoA and malonyl-CoA)
9) reduction (donor – NADPH+H + from PPP)
10) dehydration
11) reduction (donor – NADPH + H + from MALIC enzyme).
As a result, the acyl radical increases by 2 carbon atoms.
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Fat mobilization
During fasting or prolonged physical activity, glucagon or adrenaline is released. They activate TAG lipase in adipose tissue, which is located in adipocytes and is called tissue lipase(hormone sensitive). It breaks down fats in adipose tissue into glycerol and fatty acids. Glycerol goes to the liver for gluconeogenesis. FAs enter the blood, bind to albumin and travel to organs and tissues, used as a source of energy (by all organs, except the brain, which uses glucose and ketone bodies during fasting or prolonged exercise).
For the heart muscle, FA is the main source of energy.
β-oxidation
β-oxidation– the process of splitting FAs in order to extract energy.
1) Specific pathway of FA catabolism to acetyl-CoA.
2) Occurs in mitochondria.
3) Includes 4 repeating reactions (i.e. conditionally cyclic):
oxidation → hydration → oxidation → splitting.
4) At the end of each cycle, FA is shortened by 2 carbon atoms in the form of acetyl-CoA (entering the TCA cycle).
5) Reactions 1 and 3 are oxidation reactions and are associated with CPE.
6) Vit. B 2 – coenzyme FAD, vit. PP – NAD, pantothenic acid – HS-KoA.
The mechanism of FA transfer from the cytoplasm to the mitochondrion.
1. FAs must be activated before entering the mitochondria.
Only activated FA = acyl-CoA can be transported across the lipid double membrane.
The carrier is L-carnitine.
The regulatory enzyme of β-oxidation is carnitine acyltransferase-I (KAT-I).
2. CAT-I transports FAs into the intermembrane space.
3. Under the influence of CAT-I, acyl-CoA is transferred to the L-carnitine transporter.
Acylcarnitine is formed.
4. With the help of a translocase built into the inner membrane, acylcarnitine moves into the mitochondrion.
5. In the matrix, under the influence of CAT-II, FA is cleaved from carnitine and enters into β-oxidation.
Carnitine returns back to the intermembrane space.
β-oxidation reactions
1. Oxidation: FA is oxidized with the participation of FAD (acyl-CoA-DG enzyme) → enoyl.
FAD arrives at the Center for Ethics (r/o=2)
2. Hydration: enoyl → β-hydroxyacyl-CoA (enzyme enoyl hydratase)
3. Oxidation: β-hydroxyacyl-CoA → β-ketoacyl-CoA (with the participation of NAD, which enters the CPE and has a p/o = 3).
4. Cleavage: β-ketoacyl-CoA → acetyl-CoA (thiolase enzyme, involving HS-KoA).
Acetyl-CoA → TCA cycle → 12 ATP.
Acyl-CoA (C-2) → next β-oxidation cycle.
Energy calculation for β-oxidation
Using meristic acid (14C) as an example.
· Calculate how much acetyl-CoA breaks down into fatty acids
½ n = 7 → TCA cycle (12ATP) → 84 ATP.
· We count how many cycles it takes for them to decay
(1/2 n)-1=6 5(2 ATP for 1 reaction and 3 ATP for 3 reaction) = 30 ATP
· Subtract 1 ATP spent on FA activation in the cytoplasm.
Total – 113 ATP.
Synthesis of ketone bodies
Almost all acetyl-CoA enters the TCA cycle. A small part is used for the synthesis of ketone bodies = acetone bodies.
Ketone bodies– acetoacetate, β-hydroxybutyrate, acetone (for pathology).
Normal concentration is 0.03-0.05 mmol/l.
Are synthesized only in the liver from acetyl-CoA produced by β-oxidation.
Used as a source of energy by all organs except the liver (no enzyme).
With prolonged fasting or diabetes, the concentration of ketone bodies can increase tens of times, because under these conditions, FAs are the main source of energy. Under these conditions, intense β-oxidation occurs, and all acetyl-CoA does not have time to be utilized in the TCA cycle, because:
lack of oxaloacetate (used in gluconeogenesis)
· as a result of β-oxidation, a lot of NADH+H+ is formed (in 3 reactions), which inhibits isocitrate-DH.
Consequently, acetyl-CoA goes to the synthesis of ketone bodies.
Because Ketone bodies are acids; they cause a shift in the acid-base balance. Acidosis occurs (due to ketonemia).
They do not have time to be utilized and appear in the urine as a pathological component → ketouria. There is also an odor of acetone from the mouth. This condition is called ketosis.
Cholesterol metabolism
Cholesterol(Xc) is a monohydric alcohol based on a ring.
27 carbon atoms.
The normal cholesterol concentration is 3.6-6.4 mmol/l, no higher than 5 is allowed.
· for the construction of membranes (phospholipids: Xc = 1:1)
· synthesis of bile acid
· synthesis of steroid hormones (cortisol, progesterone, aldosterone, calcitriol, estrogen)
· in the skin under the influence of UV it is used for the synthesis of vitamin D3 - cholecalciferol.
The body contains about 140 g of cholesterol (mainly in the liver and brain).
Daily requirement – 0.5-1 g.
Contained only in products of animal origin (eggs, butter, cheese, liver).
Chc is not used as a source of energy, because its ring does not split into CO 2 and H 2 O and does not release ATP (no enzyme).
Excess cholesterol is not excreted, is not deposited, and is deposited in the wall of large blood vessels in the form of plaques.
The body synthesizes 0.5-1 g of cholesterol. The more it is consumed in food, the less it is synthesized in the body (normally).
Cholesterol in the body is synthesized in the liver (80%), intestines (10%), skin (5%), adrenal glands, and gonads.
Even vegetarians can have elevated cholesterol levels because... Only carbohydrates are needed for its synthesis.
Biosynthesis of cholesterol
Occurs in 3 stages:
1) in the cytoplasm - before the formation of mevalonic acid (similar to the synthesis of ketone bodies)
2) in the EPR – to squalene
3) in the ER - to cholesterol
About 100 reactions.
The regulatory enzyme is β-hydroxymethylglutaryl-CoA reductase (HMG reductase). Cholesterol-lowering statins inhibit this enzyme).
Regulation of HMG reductase:
a) Inhibited by the principle of negative feedback by excess dietary cholesterol
b) Enzyme synthesis may increase (estrogen) or decrease (cholesterol and gallstones)
c) The enzyme is activated by insulin by dephosphorylation
d) If there is a lot of enzyme, then the excess can be broken down by proteolysis
Cholesterol is synthesized from acetyl-CoA, derived from carbohydrates(glycolysis → ODPVC).
The resulting cholesterol in the liver is packaged together with fat into non-VLDL. VLDL has apoprotein B100, enters the blood and, after the addition of apoproteins C-II and E, turns into mature VLDL, which goes to LP lipase. LP lipase removes fats from VLDL (50%), leaving LDL, consisting of 50-70% cholesterol esters.
· supplies cholesterol to all organs and tissues
· cells have receptors in B100, by which they recognize LDL and absorb it. Cells regulate cholesterol intake by increasing or decreasing the number of B100 receptors.
In diabetes mellitus, glycosylation of B100 (addition of glucose) can occur. Consequently, the cells do not recognize LDL and hypercholesterolemia occurs.
LDL can penetrate into blood vessels (atherogenic particle).
More than 50% of LDL returns to the liver, where cholesterol is used to synthesize bile acids and inhibit its own cholesterol synthesis.
There is a mechanism of protection against hypercholesterolemia:
· regulation of the synthesis of own cholesterol according to the principle of negative feedback
cells regulate cholesterol intake by increasing or decreasing the number of B100 receptors
· functioning of HDL
HDL is synthesized in the liver. It is disc-shaped and contains little cholesterol.
LVP functions:
removes excess cholesterol from cells and other lipoproteins
· supplies C-II and E to other lipoproteins
The mechanism of HDL functioning:
HDL has apoprotein A1 and LCAT (lecithin cholesterol acyltransferase enzyme).
HDL goes into the blood and LDL comes up to it.
A1 recognizes that LDL contains a lot of cholesterol and activates LCAT.
LCAT cleaves FAs from HDL phospholipids and transfers them to cholesterol. Cholesterol esters are formed.
Cholesterol esters are hydrophobic, so they move inside the lipoprotein.
TOPIC 8
METABOLISM: PROTEIN METABOLISM
Squirrels - These are high-molecular compounds consisting of α-amino acid residues that are connected to each other by peptide bonds.
Peptide bonds are located between the α-carboxyl group of one amino acid and the amino group of the next α-amino acid.
Functions of proteins (amino acids):
1) plastic (main function) - proteins of muscles, tissues, gems, carnitine, creatine, some hormones and enzymes are synthesized from amino acids;
2) energy
a) in case of excess intake from food (>100 g)
b) during prolonged fasting
Peculiarity:
Amino acids, unlike fats and carbohydrates, are not deposited .
The amount of free amino acids in the body is about 35 g.
Sources of protein for the body:
food proteins (main source)
tissue proteins
· synthesized from carbohydrates.
Nitrogen balance
Because 95% of the total nitrogen in the body belongs to amino acids, then their metabolism can be judged by nitrogen balance – the ratio of incoming nitrogen to that excreted in the urine.
ü Positive – less is excreted than is ingested (in children, pregnant women, during the period of recovery after illness);
ü Negative – more is excreted than is supplied (old age, period of long-term illness);
ü Nitrogen balance - in healthy people.
Because food proteins are the main source of amino acids, then they talk about “ completeness of protein nutrition ».
All amino acids are divided into:
· interchangeable (8) – Ala, Gli, Ser, Pro, Glu, Gln, Asp, Asn;
· partially replaceable (2) – Arg, Gis (synthesized slowly);
· conditionally replaceable (2) – Cis, Tyr (can be synthesized given that receipts of irreplaceables – Met → Cis, Fen → Tyr);
· irreplaceable (8) – Val, Ile, Lei, Liz, Met, Tre, Fen, Tpf.
In this regard, proteins are released:
ü Complete – contain all essential amino acids
ü Incomplete – do not contain Met and Tpf.
Digestion of proteins
Peculiarities:
1) Proteins are digested in the stomach and small intestine
2) Enzymes – peptidases (cleave peptide bonds):
a) exopeptidases – along the edges from the C-N ends
b) endopeptidases - inside the protein
3) Enzymes of the stomach and pancreas are produced in an inactive form - proenzymes(since they would digest their own tissues)
4) Enzymes are activated by partial proteolysis (cleavage of part of the PPC)
5) Some amino acids undergo decay in the large intestine
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1. They are not digested in the oral cavity.
2. In the stomach it acts on proteins pepsin(endopeptidase). It breaks down bonds formed by amino groups of aromatic amino acids (Tyr, Fen, Tpf).
Pepsin is produced by chief cells as inactive pepsinogen.
Parietal cells produce hydrochloric acid.
Functions of HCl:
ü Creates an optimal pH for pepsin (1.5 – 2.0)
ü Activates pepsinogen
ü Denatures proteins (facilitates the action of the enzyme)
ü Bactericidal effect
Pepsinogen activation
Pepsinogen under the influence of HCl is converted into active pepsin by the cleavage of 42 amino acids slowly. Active pepsin then rapidly activates pepsinogen ( autocatalytically).
Thus, in the stomach, proteins are broken down into short peptides, which enter the intestines.
3. In the intestines, pancreatic enzymes act on peptides.
Activation of trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase
In the intestine, under the influence of enteropeptidase, it is activated trypsinogen. Then activated from it trypsin activates all other enzymes through partial proteolysis (chymotrypsinogen → chymotrypsin, proelastase → elastase, procarboxypeptidase → carboxypeptidase).
Trypsin cleaves bonds formed by carboxyl groups Lys or Arg.
Chymotrypsin– between carboxyl groups of aromatic amino acids.
Elastase- bonds formed by carboxyl groups of Ala or Gly.
Carboxypeptidase cleaves carboxyl bonds from the C-terminus.
Thus, short di- and tripeptides are formed in the intestine.
4. Under the action of intestinal enzymes, they are broken down into free amino acids.
Enzymes – di-, tri-, aminopeptidases. They are not species specific.
The resulting free amino acids are absorbed by secondary active transport with Na + (against the concentration gradient).
5. Some amino acids undergo decay.
Rotting – an enzymatic process of breakdown of amino acids into low-toxic products with the release of gases (NH 3, CH 4, CO 2, mercaptan).
Value: to maintain the vital activity of intestinal microflora (when rotting, Tyr forms toxic products phenol and cresol, Tpf - indole and skatole). Toxic products enter the liver and are neutralized.
Amino acid catabolism
Main path – deamination – the enzymatic process of elimination of the amino group in the form of ammonia and the formation of a nitrogen-free keto acid.
Oxidative deamination
· Non-oxidizing (Ser, Tre)
Intramolecular (His)
· Hydrolytic
Oxidative deamination (main)
A) Direct – only for Glu, because for everyone else, the enzymes are inactive.
Occurs in 2 stages:
1) Enzymatic
2) Spontaneous
As a result, ammonia and α-ketoglutarate are formed.
Transamination functions:
ü Because the reaction is reversible, serves for the synthesis of non-essential amino acids;
ü The initial stage of catabolism (transamination is not catabolism, since the number of amino acids does not change);
ü For the redistribution of nitrogen in the body;
ü Participates in the malate-aspartate shuttle mechanism of hydrogen transfer in glycolysis (reaction 6).
To determine the activity of ALT and AST In the clinic, the de Ritis coefficient is measured to diagnose heart and liver diseases:
At 0.6 – hepatitis,
1 – cirrhosis,
10 – myocardial infarction.
Decarboxylation amino acids - the enzymatic process of removing the carboxyl group in the form of CO 2 from amino acids.
As a result, biologically active substances are formed - biogenic amines.
Enzymes – decarboxylases.
Coenzyme – pyridoxal phosphate ← vit. AT 6.
After exerting their effect, biogenic amines are neutralized in 2 ways:
1) Methylation (adding CH 3; donor - SAM);
2) Oxidation with elimination of the amino group in the form of NH 3 (MAO enzyme - monoamine oxidase).
The biosynthesis of fatty acids most actively occurs in the cytosol of cells of the liver, intestines, and adipose tissue in the state peace or after meal.
Conventionally, 4 stages of biosynthesis can be distinguished:
1. Formation of acetyl-SCoA from glucose, other monosaccharides or ketogenic amino acids.
2. Transfer of acetyl-SCoA from mitochondria to the cytosol:
- may be in combination with carnitine, just as higher fatty acids are transported into the mitochondria, but here the transport goes in a different direction,
- usually included citric acid, formed in the first TCA reaction.
Citrate coming from mitochondria is broken down in the cytosol ATP citrate lyase to oxaloacetate and acetyl-SCoA.
Formation of acetyl-SCoA from citric acid
Oxaloacetate is further reduced to malate, and the latter either passes into the mitochondria (malate-aspartate shuttle) or is decarboxylated into pyruvate by the malic enzyme ("malic" enzyme).
3. Formation of malonyl-SCoA from acetyl-SCoA.
Carboxylation of acetyl-SCoA is catalyzed acetyl-SCoA carboxylase, a multienzyme complex of three enzymes.
Formation of malonyl-SCoA from acetyl-SCoA
4. Synthesis of palmitic acid.
Implemented multienzyme complex " fatty acid synthase" (synonym palmitate synthase) which includes 6 enzymes and acyl transfer protein (APP).
Acyl transfer protein includes a derivative of pantothenic acid – 6-phosphopantetheine(FP), which has an HS group, like HS-CoA. One of the enzyme complexes, 3-ketoacyl synthase, also has an HS group in cysteine. The interaction of these groups determines the beginning and continuation of the biosynthesis of fatty acids, namely palmitic acid. Synthesis reactions require NADPH.
Active groups of fatty acid synthase
In the first two reactions, malonyl-SCoA is sequentially added to the phosphopantetheine of the acyl-transfer protein and acetyl-SCoA to the cysteine of the 3-ketoacyl synthase.
3-Ketoacyl synthase catalyzes the third reaction - the transfer of an acetyl group to C 2 malonyl with the elimination of a carboxyl group.
Next, the keto group in reduction reactions ( 3-ketoacyl reductase), dehydration (dehydratase) and again restoration (enoyl reductase) is converted to methylene to form a saturated acyl, related to phosphopantetheine.
Acyltransferase transfers the resulting acyl to cysteine 3-ketoacyl synthase, malonyl-SCoA is added to phosphopantetheine and the cycle is repeated 7 times until a palmitic acid residue is formed. Palmitic acid is then cleaved off by the sixth enzyme of the complex, thioesterase.
Fatty acid synthesis reactions
Fatty acid chain elongation
Synthesized palmitic acid, if necessary, enters the endoplasmic reticulum. Here with participation malonyl-S-CoA And NADPH the chain lengthens to C 18 or C 20.
Unsaturated fatty acids (oleic, linoleic, linolenic) can also be lengthened to form derivatives of eicosanoic acid (C 20). But the double bond is introduced by animal cells no further than 9 carbon atoms, therefore, ω3- and ω6-polyunsaturated fatty acids are synthesized only from the corresponding precursors.
For example, arachidonic acid can be formed in a cell only in the presence of linolenic or linoleic acids. In this case, linoleic acid (18:2) is dehydrogenated to γ-linolenic acid (18:3) and extended to eicosotrienoic acid (20:3), the latter is then dehydrogenated again to arachidonic acid (20:4). This is how ω6-series fatty acids are formed
For the formation of ω3-series fatty acids, for example, timnodonic acid (20:5), the presence of α-linolenic acid (18:3) is necessary, which is dehydrogenated (18:4), lengthened (20:4) and dehydrogenated again (20:5 ).
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