The process of synthesis of carbohydrates from fats can be represented by a general diagram:
Figure 7 - General scheme for the synthesis of carbohydrates from fats
One of the main products of lipid breakdown, glycerol, is easily used in the synthesis of carbohydrates through the formation of glyceraldehyde-3-phosphate and its entry into gluneogenesis. In plants and microorganisms, it is just as easily used for the synthesis of carbohydrates and another important product of lipid breakdown, fatty acids (acetyl-CoA), through the glyoxylate cycle.
But the general scheme does not reflect all the biochemical processes that occur as a result of the formation of carbohydrates from fats.
Therefore, we will consider all stages of this process.
The scheme for the synthesis of carbohydrates and fats is more fully presented in Figure 8 and occurs in a number of stages.
Stage 1. Hydrolytic breakdown of fat under the action of the lipase enzyme into glycerol and higher fatty acids (see paragraph 1.2). The hydrolysis products must, after going through a series of transformations, turn into glucose.
Figure 8 – Scheme of the biosynthesis of carbohydrates from fats
Stage 2. Conversion of higher fatty acids into glucose. Higher fatty acids, which were formed as a result of fat hydrolysis, are destroyed mainly by b-oxidation (this process was discussed earlier in section 1.2, paragraph 1.2.2). The final product of this process is acetyl-CoA.
Glyoxylate cycle
Plants, some bacteria and fungi can use acetyl-CoA not only in the Krebs cycle, but also in a cycle called the glyoxylate cycle. This cycle plays an important role as a link in the metabolism of fats and carbohydrates.
The glyoxylate cycle functions especially intensively in special cellular organelles—glyoxysomes—during the germination of oilseed seeds. In this case, fat is converted into carbohydrates necessary for the development of the seed sprout. This process continues until the seedling develops the ability to photosynthesize. When the storage fat is depleted at the end of germination, the glyoxysomes in the cell disappear.
The glyoxylate pathway is specific only to plants and bacteria; it is absent in animal organisms. The ability of the glyoxylate cycle to function is due to the fact that plants and bacteria are able to synthesize enzymes such as isocitrate lyase And malate synthase, which, together with some enzymes of the Krebs cycle, participate in the glyoxylate cycle.
The scheme of acetyl-CoA oxidation through the glyoxylate pathway is shown in Figure 9.
Figure 9 – Scheme of the glyoxylate cycle
The two initial reactions (1 and 2) of the glyoxylate cycle are identical to those of the tricarboxylic acid cycle. In the first reaction (1), acetyl-CoA is condensed with oxaloacetate by citrate synthase to form citrate. In the second reaction, citrate isomerizes to isocitrate with the participation of aconitate hydratase. The following reactions specific to the glyoxylate cycle are catalyzed by special enzymes. In the third reaction, isocitrate is cleaved by isocitrate lyase into glyoxylic acid and succinic acid:
In the fourth reaction, catalyzed by malate synthase, glyoxylate condenses with acetyl-CoA (the second acetyl-CoA molecule entering the glyoxylate cycle) to form malic acid (malate):
The fifth reaction then oxidizes the malate to oxaloacetate. This reaction is identical to the final reaction of the tricarboxylic acid cycle; it is also the final reaction of the glyoxylate cycle, because the resulting oxaloacetate condenses again with a new acetyl-CoA molecule, thereby starting a new turn of the cycle.
The succinic acid formed in the third reaction of the glyoxylate cycle is not used by this cycle, but undergoes further transformations.
Lipidsare very important in cell metabolism. All lipids are organic, water-insoluble compounds present in all living cells. According to their functions, lipids are divided into three groups:
- structural and receptor lipids of cell membranes
- energy “depot” of cells and organisms
- vitamins and hormones of the “lipid” group
The basis of lipids is fatty acid(saturated and unsaturated) and organic alcohol - glycerol. We get the bulk of fatty acids from food (animal and plant). Animal fats are a mixture of saturated (40-60%) and unsaturated (30-50%) fatty acids. Vegetable fats are the richest (75-90%) in unsaturated fatty acids and are the most beneficial for our body.
The bulk of fats is used for energy metabolism, broken down by special enzymes - lipases and phospholipases. The result is fatty acids and glycerol, which are subsequently used in the reactions of glycolysis and the Krebs cycle. From the point of view of the formation of ATP molecules - fats form the basis of the energy reserves of animals and humans.
The eukaryotic cell receives fats from food, although it can synthesize most fatty acids itself ( with the exception of two irreplaceable– linoleic and linolenic). Synthesis begins in the cytoplasm of cells with the help of a complex complex of enzymes and ends in mitochondria or the smooth endoplasmic reticulum.
The starting product for the synthesis of most lipids (fats, steroids, phospholipids) is a “universal” molecule – acetyl-Coenzyme A (activated acetic acid), which is an intermediate product of most catabolic reactions in the cell.
There are fats in any cell, but there are especially many of them in special fat cells - adipocytes forming adipose tissue. Fat metabolism in the body is controlled by special pituitary hormones, as well as insulin and adrenaline.
Carbohydrates(monosaccharides, disaccharides, polysaccharides) are the most important compounds for energy metabolism reactions. As a result of the breakdown of carbohydrates, the cell receives most of the energy and intermediate compounds for the synthesis of other organic compounds (proteins, fats, nucleic acids).
The cell and body receives the bulk of sugars from the outside - from food, but can synthesize glucose and glycogen from non-carbohydrate compounds. Substrates for various types of carbohydrate synthesis are molecules of lactic acid (lactate) and pyruvic acid (pyruvate), amino acids and glycerol. These reactions take place in the cytoplasm with the participation of a whole complex of enzymes - glucose-phosphatases. All synthesis reactions require energy - the synthesis of 1 molecule of glucose requires 6 molecules of ATP!
The bulk of your own glucose synthesis occurs in the cells of the liver and kidneys, but does not occur in the heart, brain and muscles (there are no necessary enzymes there). Therefore, carbohydrate metabolism disorders primarily affect the functioning of these organs. Carbohydrate metabolism is controlled by a group of hormones: pituitary hormones, glucocorticosteroid hormones of the adrenal glands, insulin and glucagon of the pancreas. Disturbances in the hormonal balance of carbohydrate metabolism lead to the development of diabetes.
We have briefly reviewed the main parts of plastic metabolism. You can make a row general conclusions:
Lipid biosynthesis reactions can occur in the smooth endoplasmic reticulum of cells of all organs. Substrate for fat synthesis de novo is glucose.
As is known, when glucose enters the cell, it is converted into glycogen, pentoses and oxidized to pyruvic acid. When the supply is high, glucose is used to synthesize glycogen, but this option is limited by cell volume. Therefore, glucose “falls through” into glycolysis and is converted to pyruvate either directly or through the pentose phosphate shunt. In the second case, NADPH is formed, which will subsequently be needed for the synthesis of fatty acids.
Pyruvate passes into mitochondria, is decarboxylated into acetyl-SCoA and enters the TCA cycle. However, able peace, at vacation, in the presence of excess quantity energy in the cell, TCA cycle reactions (in particular, the isocitrate dehydrogenase reaction) are blocked by excess ATP and NADH.
General scheme of biosynthesis of triacylglycerols and cholesterol from glucose
Oxaloacetate, also formed from citrate, is reduced by malate dehydrogenase to malic acid and returned to the mitochondria
- via a malate-aspartate shuttle mechanism (not shown in the figure),
- after decarboxylation of malate to pyruvate NADP-dependent malik enzyme. The resulting NADPH will be used in the synthesis of fatty acids or cholesterol.
In the human body, the starting materials for the biosynthesis of fats can be carbohydrates coming from food, in plants - sucrose coming from photosynthetic tissues. For example, the biosynthesis of fats (triacylglycerols) in ripening seeds of oilseeds is also closely related to carbohydrate metabolism. In the early stages of ripening, the cells of the main seed tissues - cotyledons and endosperm - are filled with starch grains. Only then, at later stages of ripening, starch grains are replaced by lipids, the main component of which is triacylglycerol.
The main stages of fat synthesis include the formation of glycerol-3-phosphate and fatty acids from carbohydrates, and then ester bonds between the alcohol groups of glycerol and the carboxyl groups of fatty acids:
Figure 11 – General scheme of fat synthesis from carbohydrates
Let's take a closer look at the main stages of fat synthesis from carbohydrates (see Fig. 12).
Synthesis of glycerol-3-phosphate
Stage I - under the action of the corresponding glycosidases, carbohydrates undergo hydrolysis with the formation of monosaccharides (see paragraph 1.1.), which in the cytoplasm of cells are included in the process of glycolysis (see Fig. 2). Intermediate products of glycolysis are phosphodioxyacetone and 3-phosphoglyceraldehyde.
Stage II Glycerol-3-phosphate is formed as a result of the reduction of phosphodioxyacetone, an intermediate product of glycolysis:
In addition, glycero-3-phosphate can be formed during the dark phase of photosynthesis.
Relationship between lipids and carbohydrates
Synthesis of fats from carbohydrates
Figure 12 – Scheme of the conversion of carbohydrates into lipids
Fatty acid synthesis
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. 12, Stage III), or as a result of -oxidation of fatty acids (see Fig. 5). Let us recall that the conversion of pyruvate formed during glycolysis into acetyl-CoA and its formation during β-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. 13).
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 13 – 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 -oxidation; the growth of hydrocarbon chains of fatty acids is carried out due to the sequential addition of a two-carbon fragment (C 2) - acetyl-CoA - to their ends (see Fig. 12, 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 -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 14 shows a diagram of the synthesis of fatty acids. One cycle of fatty acid chain elongation involves four sequential reactions.
Figure 14 – 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 -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 -ketoacyl-ACP reductase, acetoacetyl-ACP is converted to -hydroxybutyryl-ACP. The reducing agent is NADPH + H +.
In the third reaction (3) of the dehydration cycle, a water molecule is split off from -hydroxybutyryl-ACP to form crotonyl-ACP. The reaction is catalyzed by -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 (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
С 6 + С 3 С 8 + С 1 –3 cycle
С 8 + С 3 С 10 + С 1 – 4 cycle
С 10 + С 3 С 12 + С 1 – 5 cycle
C 12 + C 3 C 14 + C 1 – 6 cycle
С 14 + С 3 С 16 + С 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 15 shows the pathways for the conversion of palmitic acid in desaturation and elongation reactions.
Figure 15 – 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.
Fats are synthesized from glycerol and fatty acids.
Glycerol in the body occurs during the breakdown of fat (food and own), and is also easily formed from carbohydrates.
Fatty acids are synthesized from acetyl coenzyme A. Acetyl coenzyme A is a universal metabolite. Its synthesis requires hydrogen and ATP energy. Hydrogen is obtained from NADP.H2. The body synthesizes only saturated and monosaturated (having one double bond) fatty acids. Fatty acids that have two or more double bonds in a molecule, called polyunsaturated, are not synthesized in the body and must be supplied with food. For fat synthesis, fatty acids can be used - products of hydrolysis of food and body fats.
All participants in fat synthesis must be in active form: glycerol in the form glycerophosphate, and fatty acids are in the form acetyl coenzyme A. Fat synthesis occurs in the cytoplasm of cells (mainly adipose tissue, liver, small intestine). The pathways for fat synthesis are presented in the diagram.
It should be noted that glycerol and fatty acids can be obtained from carbohydrates. Therefore, with excessive consumption of them against the background of a sedentary lifestyle, obesity develops.
DAP – dihydroacetone phosphate,
DAG – diacylglycerol.
TAG – triacylglycerol.
General characteristics of lipoproteins. Lipids in the aquatic environment (and therefore in the blood) are insoluble, therefore, for the transport of lipids by blood, complexes of lipids with proteins are formed in the body - lipoproteins.
All types of lipoproteins have a similar structure - a hydrophobic core and a hydrophilic layer on the surface. The hydrophilic layer is formed by proteins called apoproteins and amphiphilic lipid molecules - phospholipids and cholesterol. The hydrophilic groups of these molecules face the aqueous phase, and the hydrophobic parts face the hydrophobic core of the lipoprotein, which contains the transported lipids.
Apoproteins perform several functions:
Form the structure of lipoproteins;
They interact with receptors on the surface of cells and thus determine which tissues will capture this type of lipoprotein;
Serve as enzymes or activators of enzymes acting on lipoproteins.
Lipoproteins. The following types of lipoproteins are synthesized in the body: chylomicrons (CM), very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low density lipoproteins (LDL) and high density lipoproteins (HDL). Each type of lipoprotein is formed in different tissues and transports certain lipids. For example, CMs transport exogenous (dietary fats) from the intestine to tissues, so triacylglycerols account for up to 85% of the mass of these particles.
Properties of lipoproteins. LPs are highly soluble in blood, non-opalescent, as they are small in size and have a negative charge on them.
surfaces. Some drugs easily pass through the walls of capillaries of blood vessels and deliver lipids to cells. The large size of CM does not allow them to penetrate the walls of capillaries, so from the intestinal cells they first enter the lymphatic system and then flow through the main thoracic duct into the blood along with the lymph. Fate of fatty acids, glycerol and residual chylomicrons. As a result of the action of LP lipase on CM fats, fatty acids and glycerol are formed. The bulk of fatty acids penetrate into tissues. In adipose tissue during the absorptive period, fatty acids are deposited in the form of triacylglycerols; in cardiac muscle and working skeletal muscles they are used as a source of energy. Another product of fat hydrolysis, glycerol, is soluble in the blood and is transported to the liver, where during the absorption period it can be used for the synthesis of fats.
Hyperchylomicronemia, hypertriglyceronemia. After eating food containing fats, physiological hypertriglyceronemia develops and, accordingly, hyperchylomicronemia, which can last up to several hours. The rate of removal of cholesterol from the bloodstream depends on:
LP lipase activity;
The presence of HDL, supplying apoproteins C-II and E for CM;
Activities of apoC-II and apoE transfer to CM.
Genetic defects in any of the proteins involved in the metabolism of cholesterol lead to the development of familial hyperchylomicronemia - hyperlipoproteinemia type I.
In plants of the same species, the composition and properties of fat may vary depending on the climatic conditions of growth. The content and quality of fats in animal raw materials also depends on the breed, age, degree of fatness, gender, season of the year, etc.
Fats are widely used in the production of many food products; they have high calorie content and nutritional value, causing a long-term feeling of satiety. Fats are important flavor and structural components in the food preparation process and have a significant impact on the appearance of food. When frying, fat acts as a medium that transfers heat.
| The product's name | The product's name | Approximate fat content in food products, % by wet weight | |
| Seeds: | Rye bread | 1,20 | |
| sunflower | 35-55 | Fresh vegetables | 0,1-0,5 |
| Hemp | 31-38 | Fresh fruits | 0,2-0,4 |
| poppy | Beef | 3,8-25,0 | |
| Cocoa beans | Pork | 6,3-41,3 | |
| Peanut nuts | 40-55 | Mutton | 5,8-33,6 |
| Walnuts (kernels) | 58-74 | Fish | 0,4-20 |
| Cereals: | Cow's milk | 3,2-4,5 | |
| Wheat | 2,3 | Butter | 61,5-82,5 |
| Rye | 2,0 | Margarine | 82,5 |
| Oats | 6,2 | Eggs | 12,1 |
In addition to glycerides, fats obtained from plant and animal tissues may contain free fatty acids, phosphatides, sterols, pigments, vitamins, flavoring and aromatic substances, enzymes, proteins, etc., which affect the quality and properties of fats. The taste and smell of fats are also influenced by substances formed in fats during storage (aldehydes, ketones, peroxides and other compounds).
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