Biosynthesis of fats from carbohydrates. Lipid synthesis as a reserve process for obtaining energy for the body. Structure of anisotropic and isotropic disks

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).

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).

1. 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.

1. Relationship between lipids and carbohydrates

   1. Synthesis of fats from carbohydrates

Figure 12 – Scheme of the conversion of carbohydrates into lipids

1. 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.

3.3. Fat synthesis

Fats are synthesized from glycerol and fatty acids. Glycerol in the body occurs during the breakdown of fat (food or own), and is also easily formed from carbohydrates. Fatty acids are synthesized from acetyl coenzyme A, a universal metabolite of the body. This synthesis also requires hydrogen (in the form of NADPH 2) and ATP energy. The body synthesizes only saturated and monounsaturated (those with one double bond) fatty acids. Acids containing two or more double bonds in their molecule (polyunsaturated) are not synthesized in the body and must be supplied with food. For fat synthesis, fatty acids - products of hydrolysis of food and body fats - can also be used.

All participants in fat synthesis must be in active form: glycerol in the form of glycerophosphate, and fatty acids in the form of acyl-enzyme A. Fat synthesis occurs in the cytoplasm of cells (mainly adipose tissue, liver, small intestine) and proceeds according to the following scheme

It should be emphasized that glycerol and fatty acids can be obtained from carbohydrates. Therefore, with excess consumption of carbohydrates against the background of a sedentary lifestyle, obesity develops.

Lecture 4. Protein metabolism

4.1. Protein catabolism

Proteins that make up the cells of the body are also subject to constant breakdown under the influence of intracellular proteolytic enzymes called intracellular proteinases or cathepsins. These enzymes are localized in special intracellular organelles - lysosomes. Under the influence of cathepsins, body proteins are also converted into amino acids. (It is important to note that the breakdown of both food and the body’s own proteins leads to the formation of the same 20 types of amino acids.) Approximately 200 g of body proteins are broken down per day. Therefore, about 300 g of free amino acids appear in the body during the day.

4.2. Protein synthesis

Most amino acids are used for protein synthesis. Protein synthesis occurs with the obligatory participation of nucleic acids.

The first stage of protein synthesis is transcription- carried out in the cell nucleus using DNA as a source of genetic information. Genetic information determines the order of amino acids in the polypeptide chains of the synthesized protein. This information is encoded by the sequence of nitrogenous bases in the DNA molecule. Each amino acid is coded for by a combination of three nitrogenous bases called codon, or triplet. The section of a DNA molecule containing information about a specific protein is called "gene". In this section of DNA, messenger RNA (mRNA) is synthesized during transcription according to the principle of complementarity. This nucleic acid is a copy of the corresponding gene. The resulting mRNA leaves the nucleus and enters the cytoplasm. In a similar way, the synthesis of ribosomal (rRNA) and transport (tRNA) occurs on DNA as a matrix.

During the second stage - recognition(recognition) occurring in the cytoplasm, amino acids selectively bind to their carriers - transport RNAs (tRNAs). Each tRNA molecule is a short polynucleotide chain containing approximately 80 nucleotides and partially twisted into a double helix, resulting in a “curved cloverleaf” configuration. At one end of the polynucleotide chain, all tRNAs have a nucleotide containing adenine. An amino acid is attached to this end of the tRNA molecule. The loop opposite the amino acid attachment site contains an anticodon, consisting of three nitrogenous bases and intended for subsequent binding to the complementary codon of the mRNA. One of the side loops of the tRNA molecule ensures the attachment of the tRNA to the enzyme involved in recognition, and the other, side loop is necessary for attaching tRNA to the ribosome at the next stage of protein synthesis.

At this stage, the ATP molecule is used as an energy source. As a result of recognition, an amino acid-tRNA complex is formed. In this regard, the second stage of protein synthesis is called amino acid activation.

The third stage of protein synthesis is broadcast- occurs on ribosomes. Each ribosome consists of two parts - a large and a small subunit. In terms of chemical composition, both subparticles consist of rRNA and proteins. Ribosomes are able to easily break down into subparticles, which can again combine with each other to form a ribosome. Translation begins with the dissociation of the ribosome into subparticles, which immediately attach to the initial part of the mRNA molecule coming from the nucleus. In this case, there remains a space between the subparticles (the so-called tunnel), where a small section of mRNA is located. Then, tRNAs bound to amino acids are added to the resulting ribosome-mRNA complex. The attachment of tRNA to this complex occurs by binding one of the side loops of tRNA to the ribosome and binding of the tRNA anticodon to its complementary mRNA codon located in the tunnel between the ribosomal subparticles. At the same time, only two tRNAs with amino acids can join the ribosome-mRNA complex.

Due to the specific binding of tRNA anticodons to mRNA codons, only tRNA molecules whose anticodons are complementary to the mRNA codons are attached to the portion of the mRNA molecule located in the tunnel. Therefore, these tRNAs deliver only strictly specific amino acids to the ribosomes. Next, the amino acids are connected to each other by a peptide bond and a dipeptide is formed, which is associated with one of the tRNAs. After this, the ribosome moves along the mRNA exactly one codon (this movement of the ribosome is called translocation).

As a result of translocation, free (without an amino acid) tRNA is split off from the ribosome, and a new codon appears in the tunnel zone, to which another tRNA with an amino acid corresponding to this codon is added according to the principle of complementarity. The delivered amino acid combines with the previously formed dipeptide, which leads to elongation of the peptide chain. This is followed by new translocations, the arrival of new tRNAs with amino acids on the ribosome and further elongation of the peptide chain.

Thus, the order of inclusion of amino acids in the synthesized protein is determined by the sequence of codons in the mRNA. The synthesis of the polypeptide chain is completed when a special codon enters the tunnel, which does not code for amino acids and to which no tRNA can join. Such codons are called stop codons.

As a result, due to the three stages described, polypeptides are synthesized, i.e., the primary structure of the protein is formed. Higher (spatial) structures (secondary, tertiary, quaternary) arise spontaneously.

Protein synthesis is an energy-intensive process. To include only one amino acid in a synthesized protein molecule, at least three ATP molecules are required.

4.3. Amino acid metabolism

In addition to protein synthesis, amino acids are also used for the synthesis of various non-protein compounds that have important biological significance. Some amino acids undergo decomposition and turn into the final products: C0 2, H 2 0 and NH 3 Decomposition begins with reactions common to most amino acids.

These include:

a) decarboxylation - removal of the carboxyl group from amino acids in the form of carbon dioxide:

All amino acids undergo transamination. This reaction involves a coenzyme - phosphopyridoxal, the formation of which requires vitamin B 6 - pyridoxine.

Transamination is the main transformation of amino acids in the body, since its rate is much higher than that of decarboxylation and deamination reactions.

Transamination performs two main functions:

a) due to transamination, some amino acids can be converted into others. In this case, the total number of amino acids does not change, but the ratio between them changes. With food, foreign proteins enter the body, in which amino acids are in different proportions compared to body proteins. By transamination, the amino acid composition of the body is adjusted.

b) is an integral part indirect (indirect) deamination amino acids - the process by which the breakdown of most amino acids begins.

In the first stage of this process, amino acids undergo a transamination reaction with α-ketoglutaric acid. Amino acids are converted into α-keto acids, and α-ketoglutaric acid is converted into glutamic acid (amino acid).

At the second stage, the resulting glutamic acid undergoes deamination, NH 3 is cleaved from it and α-ketoglutaric acid is formed again. The resulting α-keto acids then undergo deep decomposition and are converted into the final products C0 2 and H 2 0. Each of the 20 keto acids (there are as many of them formed as there are types of amino acids) has its own specific decomposition pathways. However, during the breakdown of some amino acids, pyruvic acid is formed as an intermediate product, from which glucose can be synthesized. Therefore, the amino acids from which such keto acids arise are called glucogenic. Other keto acids do not form pyruvate during their breakdown. Their intermediate product is acetyl coenzyme A, from which it is impossible to obtain glucose, but ketone bodies can be synthesized. Amino acids corresponding to such keto acids are called ketogenic.

The second product of indirect deamination of amino acids is ammonia. Ammonia is highly toxic to the body. Therefore, the body has molecular mechanisms for its neutralization. As NH 3 is formed, it binds to glutamic acid in all tissues to form glutamine. This temporary neutralization of ammonia. With the bloodstream, glutamine enters the liver, where it breaks down again into glutamic acid and NH3. The resulting glutamic acid is returned to the organs with the blood to neutralize new portions of ammonia. The released ammonia, as well as carbon dioxide in the liver, are used for the synthesis urea.

Urea synthesis is a cyclic, multi-stage process that consumes a large amount of energy. The amino acid ornithine plays a very important role in the synthesis of urea. This amino acid is not part of proteins. Ornithine is formed from another amino acid - arginine, which is present in proteins. Due to the important role of ornithine, urea synthesis is called ornithine cycle.

During the synthesis process, two molecules of ammonia and a molecule of carbon dioxide are added to ornithine, and ornithine is converted into arginine, from which urea is immediately split off, and ornithine is formed again. Along with ornithine and arginine, amino acids also participate in the formation of urea: glutamine And aspartic acid. Glutamine is a supplier of ammonia, and aspartic acid is its transporter.

Urea synthesis is final neutralization of ammonia. From the liver, urea enters the kidneys with the blood and is excreted in the urine. 20-35 g of urea are formed per day. The excretion of urea in urine characterizes the rate of breakdown of proteins in the body.

Section 3. Biochemistry of muscle tissue

Lecture 5. Biochemistry of muscles

5.1. Cellular structure of muscle fiber

Animals and humans have two main types of muscles: striated And smooth. Striated muscles are attached to the bones, i.e., to the skeleton, and therefore are also called skeletal. Striated muscle fibers also form the basis of the heart muscle - the myocardium, although there are certain differences in the structure of the myocardium and skeletal muscles. Smooth muscles form the muscles of the walls of blood vessels, intestines, and penetrate the tissues of internal organs and skin.

Each striated muscle consists of several thousand fibers, united by connective tissue layers and the same membrane - fascia. Muscle fibers (myocytes) are highly elongated multinucleated large cells up to 2-3 cm long, and in some muscles even more than 10 cm. The thickness of muscle cells is about 0.1-0.2 mm.

Like any cell, myocyte contains essential organelles such as nuclei, mitochondria, ribosomes, cytoplasmic reticulum and cell membrane. A feature of myocytes that distinguishes them from other cells is the presence of contractile elements - myofibrils

Cores are surrounded by a shell - the nucleolemma and consist mainly of nucleoproteins. The nucleus contains the genetic information for protein synthesis.

Ribosomes- intracellular formations that are nucleoproteins in chemical composition. Protein synthesis occurs on ribosomes.

Mitochondria- microscopic bubbles up to 2-3 microns in size, surrounded by a double membrane. In mitochondria, the oxidation of carbohydrates, fats and amino acids to carbon dioxide and water occurs using molecular oxygen (air oxygen). Due to the energy released during oxidation, ATP synthesis occurs in mitochondria. In trained muscles, mitochondria are numerous and located along the myofibrils.

Cytoplasmic reticulum(sarcoplasmic reticulum, sarcoplasmic reticulum) consists of tubes, tubules and vesicles formed by membranes and connected to each other. The sarcoplasmic reticulum, through special tubes called the T-system, is connected to the muscle cell membrane - the sarcolemma. Of particular note in the sarcoplasmic reticulum are vesicles called tankus and containing high concentrations of calcium ions. In the cisternae, the content of Ca 2+ ions is approximately a thousand times higher than in the cytosol. Such a high concentration gradient of calcium ions arises due to the functioning of the enzyme - calcium adenosine tri- phosphatases(calcium ATPase), built into the wall of the tank. This enzyme catalyzes the hydrolysis of ATP and, due to the energy released during this process, ensures the transfer of calcium ions inside the tanks. This mechanism of transport of calcium ions is figuratively called calciumpump, or calcium pump.

Cytoplasm(cytosol, sarcoplasm) occupies the internal space of myocytes and is a colloidal solution containing proteins, glycogen, fat droplets and other inclusions. Sarcoplasmic proteins account for 25-30% of all muscle proteins. Among the sarcoplasmic proteins there are active enzymes. These primarily include glycolytic enzymes, which break down glycogen or glucose into pyruvic or lactic acid. Another important sarcoplasmic enzyme is creatine kinase, involved in the energy supply of muscle work. The sarcoplasmic protein myoglobin, which is structurally identical to one of the subunits of the blood protein - hemoglobin, deserves special attention. Myoglobin consists of one polypeptide and one heme. The function of myoglobin is to bind molecular oxygen. Thanks to this protein, a certain supply of oxygen is created in muscle tissue. In recent years, another function of myoglobin has been established - the transfer of 0 2 from the sarcolemma to muscle mitochondria.

In addition to proteins, sarcoplasm contains non-protein nitrogen-containing substances. They are called, in contrast to proteins, extractives, since they are easily extracted with water. Among them are adenyl nucleotides ATP, ADP, AMP and other nucleotides, with ATP predominating. The resting ATP concentration is approximately 4-5 mmol/kg. Extractives also include creatine phosphate, its predecessor is creatine and the product of the irreversible breakdown of creatine phosphate - creatinine IN The resting concentration of creatine phosphate is usually 15-25 mmol/kg. Of the amino acids, glutamic acid and glutamic acid are found in large quantities. glutamine.

The main carbohydrate of muscle tissue is glycogen. Glycogen concentration ranges from 0.2-3%. Free glucose in the sarcoplasm is contained in very low concentrations - there are only traces of it. During muscle work, the products of carbohydrate metabolism - lactate and pyruvate - accumulate in the sarcoplasm.

Protoplasmic fat bound to proteins and available in a concentration of 1%. Spare fat accumulates in muscles trained for endurance.

5.2. Sarcolemma structure

Each muscle fiber is surrounded by a cell membrane - sarcolemma. The sarcolemma is a lyloprotein membrane about 10 nm thick. Outside, the sarcolemma is surrounded by a network of intertwined strands of collagen protein. During muscle contraction, elastic forces arise in the collagen shell, due to which, when relaxed, the muscle fiber stretches and returns to its original state. The endings of the motor nerves approach the sarcolemma. The point of contact between the nerve ending and the sarcolemma is called neuromuscular synapse, or end neural plate.

Contractile elements - myofibrils- occupy most of the volume of muscle cells, their diameter is about 1 micron. In untrained muscles, myofibrils are scattered, but in trained muscles they are grouped into bundles called fields of Conheim.

5.3. Structure of anisotropic and isotropic disks

Microscopic examination of the structure of myofibrils showed that they consist of alternating light and dark areas, or disks. In muscle cells, myofibrils are arranged in such a way that the light and dark areas of adjacent myofibrils coincide, which creates a transverse striation of the entire muscle fiber visible under a microscope. It was discovered that myofibrils are complex structures, built, in turn, from a large number of muscle threads (protofibrils, or filaments) of two types - fat And thin. Thick threads have a diameter of 15 nm, thin ones - 7 nm.

Myofibrils consist of alternating bundles of parallel thick and thin filaments, whose ends intersect each other. A section of the myofibril, consisting of thick filaments and the ends of thin filaments located between them, is birefringent. Under microscopy, this area blocks visible light or the flow of electrons (using an electron microscope) and therefore appears dark. Such areas are called anisotropic, or dark, discs (A-discs).

The light areas of myofibrils consist of central parts of thin filaments. They transmit light rays or a stream of electrons relatively easily, since they do not have birefringence and are called isotropic, or light, discs (I-disks). In the middle of the bundle of thin filaments, a thin plate of protein is located transversely, which fixes the position of the muscle filaments in space. This plate is clearly visible under a microscope in the form of a line running across the I-disc and is called Z- a record.

The section of myofibril between adjacent 2-lines is called sarcomere Its length is 2.5-3 microns. Each myofibril consists of several hundred sarcomeres (up to 1000).

5.4. Structure and properties of contractile proteins

A study of the chemical composition of myofibrils showed that thick and thin filaments consist only of proteins.

Thick filaments are made of protein myosin. Myosin is a protein with a molecular weight of about 500 kDa, containing two very long polypeptide chains. These chains form a double helix, but at one end these threads diverge and form a spherical formation - a globular head. Therefore, the myosin molecule has two parts - the globular head and the tail. The thick filament contains about 300 myosin molecules, and on a cross section of the thick filament, 18 myosin molecules are found. Myosin molecules in thick filaments are intertwined with their tails, and their heads protrude from the thick filament in a regular spiral. There are two important areas (centers) in the myosin heads. One of them catalyzes the hydrolytic cleavage of ATP, i.e., corresponds to the active center of the enzyme. The ATPase activity of myosin was first discovered by Russian biochemists Engelhardt and Lyubimova. The second section of the myosin head ensures the connection of thick filaments with the protein of thin filaments during muscle contraction - akmud.

The thin filaments are made up of three proteins: actin, troponin And tropomyosin.

The main protein of thin filaments is actin. Actin is a globular protein with a molecular weight of 42 kDa. This protein has two important properties. Firstly, it exhibits a high ability to polymerize with the formation of long chains called fibrillaractin(can be compared to a string of beads). Secondly, as already noted, actin can combine with myosin heads, which leads to the formation of cross bridges, or adhesions, between thin and thick filaments.

The basis of the thin filament is a double helix of two chains of fibrillar actin, containing about 300 molecules of globular actin (like two strands of beads twisted into a double helix, each bead corresponding to globular actin).

Another thin filament protein - tropomyosin– also has the shape of a double helix, but this helix is ​​formed by polypeptide chains and is much smaller in size than the actin double helix. Tropomyosin is located in the groove of the double helix of fibrillar actin.

Third thin filament protein - troponin- attaches to tropomyosin and fixes its position in the actin groove, which blocks the interaction of myosin heads with molecules of globular actin of thin filaments.

5.5. Mechanism of muscle contraction

Muscle contraction is a complex mechanochemical process during which the chemical energy of the hydrolytic breakdown of ATP is converted into mechanical work performed by the muscle.

At present, this mechanism has not yet been fully disclosed. But the following is certain:

The source of energy necessary for muscle work is ATP.

ATP hydrolysis, accompanied by the release of energy, is catalyzed by myosin, which, as already noted, has enzymatic activity.

The trigger mechanism for muscle contraction is an increase in the concentration of Ca ions in the sarcoplasm of myocytes, caused by a motor nerve impulse.

During muscle contraction, cross bridges, or adhesions, appear between the thick and thin filaments of myofibrils.

During muscle contraction, thin filaments slide along thick filaments, which leads to shortening of myofibrils and the entire muscle fiber as a whole.

There are many hypotheses trying to explain the molecular mechanism of muscle contraction. The most justified at present is rowing boat hypothesis", or the "rowing" hypothesis of X. Huxley. In a simplified form, its essence is as follows.

In a muscle at rest, thick and thin filaments of myofibrils are not connected to each other, since the binding sites on actin molecules are covered by tropomyosin molecules.

Muscle contraction occurs under the influence of a motor nerve impulse, which is a wave of increased membrane permeability propagating along the nerve fiber.

This wave of increased permeability is transmitted through the neuromuscular junction to the T-system of the sarcoplasmic reticulum and ultimately reaches cisterns containing high concentrations of calcium ions. As a result of a significant increase in the permeability of the tank wall, calcium ions leave the tanks and their concentration in the sarcoplasm increases 1000 times in a very short time (about 3 ms). Calcium ions, being in high concentration, attach to the protein of thin filaments - troponin - and change its spatial shape (conformation). A change in the conformation of troponin, in turn, leads to the fact that tropomyosin molecules are displaced along the groove of fibrillar actin, which forms the basis of thin filaments, and release that portion of actin molecules that is intended for binding to myosin heads. As a result, a cross bridge located at an angle of 90° appears between myosin and actin (i.e., between thick and thin filaments). Since thick and thin filaments contain a large number of myosin and actin molecules (about 300 each), a fairly large number of cross bridges, or adhesions, are formed between the muscle filaments. The formation of a bond between actin and myosin is accompanied by an increase in the ATPase activity of the latter, resulting in ATP hydrolysis:

ATP + H 2 0 ADP + H 3 P0 4 + energy

Due to the energy released during the breakdown of ATP, the myosin head, like a hinge or oar of a boat, rotates and the bridge between the thick and thin filaments is at an angle of 45°, which leads to the sliding of the muscle filaments towards each other. Having made a turn, the bridges between thick and thin threads are broken. As a result, the ATPase activity of myosin decreases sharply, and ATP hydrolysis stops. But if the motor nerve impulse continues to enter the muscle and a high concentration of calcium ions remains in the sarcoplasm, cross bridges are formed again, the ATPase activity of myosin increases and hydrolysis of new portions of ATP occurs again, providing energy for rotation of the cross bridges with their subsequent rupture. This leads to further movement of thick and thin filaments towards each other and shortening of myofibrils and muscle fiber.

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Departments) Full name author_____Rodina Elena Yurievna________________________________ Educational-methodicalcomplexBydiscipline MOLECULAR BIOLOGY (name) Specialty... with textbooks By molecular biology textbooks indicated Bybiochemistry. 2. Next...

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.

Option 2.
I. Describe organelles (mitochondria, cell center) according to plan.
a) Structure b) Functions
II.
Organoids
Characteristics
1.Plasma membrane
2. Core
3. Mitochondria
4. Plastids
5. Ribosomes
6. EPS
7. Cellular center
8. Golgi complex
9. Lysosomes

EPS
B) Ribosomal protein synthesis
B) Plastid photosynthesis
D) Storage of hereditary information core
D) Non-membrane cell center
E) Synthesis of fats and carbohydrates by the Golgi complex
G) Contains a DNA core
3) Providing the cell with mitochondria energy
I) Self-digestion of the cell and intracellular digestion of the lysosome
K) Nuclear fission control
M) Only plants have plastids
H) Only animals do not have plastids
III. Remove the excess.
Nucleus, mitochondria, Golgi complex, cytoplasm,
IV. Choose the correct answer.
1. Starch accumulation occurs:
A) in chloroplasts B) in vacuoles C) in leukoplasts yes D) in the cytoplasm
2. DNA formation occurs:
A) in the ER B) in the nucleus yes C) in the Golgi complex D) in the cytoplasm
3. Enzymes that break down proteins, fats, carbohydrates are synthesized:
A) on ribosomes yes B) on lysosomes C) on the cell center D) on the Golgi complex
4. Fats and carbohydrates are formed:
A) in ribosomes B) in the Golgi complex and C) in vacuoles D) in the cytoplasm
5. Proteins, fats and carbohydrates are stored in reserve:
A) in ribosomes B) in the Golgi complex C) in lysosomes D) in the cytoplasm yes
V. Determine whether this statement is correct (yes - no).
1. The Golgi complex is part of the EPS.net
2. Ribosomes are formed in the nucleus. yes
3. EPS is always covered with ribosomes. yes
4. Inclusions are permanent formations of the cell.
5. Only animals do not have a cell wall. yes
6. Plastids differ from mitochondria in the presence of DNA. no

Answer the questions please... 4. Fungi, animals and plants belong to... 12. Cell protection and selective

permeability (transport of substances into and out of the cell) is carried out...

18. Non-membrane organelles of movement, consisting of microtubules...

20. A non-membrane organelle located inside the nucleus and carrying out the synthesis of ribosomal subunits...

22. A single-membrane organelle located near the nucleus, carrying out intracellular transport, synthesis of fats and carbohydrates; packaging of substances into membrane vesicles....

24.Double-membrane organelles of a plant cell containing plant pigments of red, green or white...

26.Non-membrane organelle of the nucleus, consisting of DNA and responsible for the storage and transmission of hereditary information...

28.Plastids are red or orange.....

Distribute the characteristics according to the cell organelles (place the letters corresponding to the characteristics of the organelle opposite the name of the organelle).

Organoids

Characteristics

1.Plasma membrane

3. Mitochondria

4. Plastids

5. Ribosomes

7. Cellular center

8. Golgi complex

9. Lysosomes

A) Transport of substances throughout the cell, spatial separation of reactions in the cell

B) Protein synthesis

B) Photosynthesis

D) Movement of organelles throughout the cell

D) Storage of hereditary information

E) Non-membrane

G) Synthesis of fats and carbohydrates

3) Contains DNA

I) Single membrane

K) Providing the cell with energy

K) Cell self-digestion and intracellular digestion

M) Cell movement

N) Double membrane

PLEASE, HELP!!!

Distribute the characteristics according to the cell organelles (place the letters corresponding to the characteristics of the organelle opposite the name of the organelle).

Organoids:

1.Plasma membrane

3. Mitochondria

4. Plastids

5. Ribosomes

7. Cellular center

8. Golgi complex

9. Lysosomes

Characteristics:

A) Transport of substances throughout the cell, spatial separation of reactions in the cell

B) Protein synthesis

B) Photosynthesis

D) Storage of hereditary information

D) Non-membrane organelles

E) Synthesis of fats and carbohydrates

G) Contains DNA

3) Providing energy to the cell

I) Cell self-digestion and intracellular digestion

J) Communication of the cell with the external environment

K) Nuclear fission control

M) Only found in plants

N) Only found in animals

Help please 18. non-membrane organelles of movement, consisting of microtubules 19. single-membrane organelle, carrying out

transport of substances, synthesis of fats, carbohydrates and complex proteins 20. non-membrane organelle, located inside the nucleus and carrying out the synthesis of ribosomal subunits 21. liquid substance of real vacuoles 22. Single-membrane organelle, located near the nucleus, carrying out intracellular transport, synthesis of fats and carbohydrates, packaging of substances membrane vesicles 23. non-membrane organelle, consisting of microtubules and involved in the formation of the “spindle” 24. Double-membrane organelles of a plant cell, containing plant pigments of red green and white 25. outgrowths of the inner membrane of mitochondria 26. non-membrane organelle of the nucleus, consisting of DNA and responsible for the storage and transmission of hereditary information 27. organelle, which carries out the final stage of respiration and digestion 28. energy organelles of only plant cells 29. organelles of cells of all eukaryotes, carrying out the synthesis of ATP 30. Double-membrane organelle of restenia, accumulating starch 31. folds and stacks formed by the internal membrane chloroplast