I. Table 2. Classification of proteins according to their structure.
| Protein class | Characteristic | Function |
| fibrillar | The secondary structure is most important (the tertiary structure is almost not expressed) Insoluble in water Differ in high mechanical strength Long parallel polypeptide chains, fastened to each other by cross-links, form long fibers or layered structures | Perform structural functions. This group includes, for example, collagen (tendons, bones, connective tissue), myosin (muscles), fibroin (silk, cobweb), keratin (hair, horns, nails, feathers). |
| Globular | Tertiary structure is most important Polypeptide chains are coiled into compact globules Soluble | They act as enzymes, antibodies, and in some cases hormones (such as insulin) and a number of other important functions |
| Intermediate | Fibrillar in nature, but soluble | An example is fibrinogen, which is converted to insoluble fibrin during blood clotting. |
II. Classification of proteins according to their composition.
Simple Complex
Composed only of amino acids Consists of globular proteins and non-protein
material. The non-white part is called
prosthetic group.
Table 3. Complex proteins.
| Name | Prosthetic group | Example |
| Phosphoproteins | Phosphoric acid | Milk casein Egg yolk vitellin |
| Glycoproteins | Carbohydrate | Membrane components Mucin (component of saliva) |
| Nucleoproteins | Nucleic acid | Components of viruses Chromosomes Ribosomes |
| Chromoproteins | Pigment | Hemoglobin - heme (iron-containing pigment) Phytochrome (pigment of potent origin) Cytochrome (respiratory pigment) |
| Lipoproteins | Lipid | Membrane components Blood lipoproteins - the transport form of lipids |
| Metalloproteins | Metal | Nitrareductase is an enzyme that catalyzes the conversion of sodium to nitrite in plants. |
III. Table 4. Classification of proteins by function.
| Protein class | Examples | Localization/function |
| Structural proteins | Collagen Keratin Elastin | Component of connective tissue, bones, tendons, cartilage Skin, feathers, nails, hair, horns Ligaments |
| Enzymes | Trypsin Ribulose bisphosphate carboxylase | Catalyzes the hydrolysis of proteins Catalyzes (addition of CO 2) during photosynthesis |
| Hormones | Insulin Glucagon ACTH | Regulate glucose metabolism Stimulates the growth and activity of the adrenal cortex |
| Respiratory pigments | Hemoglobin Myoglobin | Carries O 2 in the blood of vertebrates Serves to store O 2 in muscles |
| Transport proteins | Albumen | Serves for the transport of fatty acids and lipids in the blood |
| Protective proteins | Antibodies Fibrinogen Thrombin | Form complexes with foreign proteins Fibrin precursor during blood clotting Involved in the process of blood clotting |
| Contractile proteins | myosin Actin | Movable muscle filaments Fixed muscle filaments |
| Spare proteins | Egg Albumin Casein | egg white milk protein |
| toxins | snake poison | Enzymes |
Enzymes(enzymes) - specific proteins that are present in all living organisms and play the role of biological catalysts.
Enzymes speed up reactions without changing its overall result.
Enzymes are highly specific: each enzyme catalyzes a specific type of chemical reaction in cells. This ensures fine regulation of all vital important processes(respiration, digestion, photosynthesis, etc.)
Example: the enzyme urease catalyzes the breakdown of urea only, without exerting catalytic pressure on structurally related compounds.
The activity of enzymes is limited by a rather narrow temperature range (35-45°C), beyond which the activity falls and disappears. Enzymes are active at physiological Ph values, i.e. in a slightly alkaline environment.
In terms of spatial organization, enzymes consist of several domains and usually have a quaternary structure.
Enzymes can also contain non-protein components. The protein part is called apoenzyme , and non-protein - cofactor (if it's simple inorganic matter, for example Zn 2+ , Mg 2+) or coenzyme (coenzyme) ) (if we are talking about organic compounds).
The precursors of many coenzymes are vitamins.
Example: pantathenic acid is a precursor of coenzyme A, which plays an important role in metabolism.
In enzyme molecules there is a so-called active center . It consists of two sections - sorption And catalytic . The former is responsible for the binding of enzymes to substrate molecules, while the latter is responsible for the actual act of catalysis.
The name of the enzymes contains the name of the substrate, which is affected by this enzyme, and the ending "-ase".
Cellulose - catalyzes the hydrolysis of cellulose to monosaccharides.
ü Protease - hydrolyzes proteins to amino acids.
According to this principle, all enzymes are divided into 6 classes.
Oxidoreductase catalyze redox reactions, carrying out the transfer of H and O atoms and electrons from one substance to another, while oxidizing the first and reducing the second. This group of enzymes is involved in all processes of biological oxidation.
Example: in the breath
AN + B ↔A + BH (oxidative)
A + O ↔ AO (reducing)
Transferases catalyze the transfer of a group of atoms (methyl, acyl, phosphate and amino groups) from one substance to another.
Example: under the pressure of phosphotransferases, residues are transferred phosphoric acid from ATP to glucose and fructose: ATP + glucose ↔ glucose - 6 - phosphate + ADP.
Hydrolases accelerate reactions split complex organic compounds into simpler ones by attaching water molecules at the point of rupture chemical bonds. Such splitting is called hydrolysis .
These include amylase (hydrolyzes starch), lipase (breaks down fats), etc.:
AB + H 2 O↔AOH + VN
Liase catalyze non-hydrolytic additions to the substrate and the elimination of a group of atoms from it. In this case, there may be a break in the connection C - C, C - N, C - O, C - S.
Example: removal of a carboxyl group by a decarboxylase
CH 3 - C - C ↔ CO 2 + CH 3 - C
Isomerases carry out intramolecular rearrangements, i.e. catalyze the transformation of one isomer into another:
glucose - 6 - phosphate ↔ glucose - 1 - phosphate
Lipases( synthetases) catalyze the reactions of joining two molecules with the formation of new bonds C - O, C - S, P - N, C - C, using the energy of ATP.
Lipases are a group of enzymes that catalyze the addition of amino acid residues to tRNA. These synthetases play an important role in the process of protein synthesis.
Example: the enzyme valine - t-RNA - synthetase under its action forms a valine-t-RNA complex:
ATP + valine + tRNA ↔ ADP + H 3 PO 4 + valine-tRNA
1. What is the name of the process of violation of the natural structure of the protein, in which its primary structure? The action of what factors can lead to a violation of the structure of protein molecules?
The process of violation of the natural structure of proteins under the influence of any factors without destroying the primary structure is called denaturation. Protein denaturation can be caused by various factors, for example, high temperature, concentrated acids and alkalis, heavy metals.
2. How do fibrillar proteins differ from globular ones? Give examples of fibrillar and globular proteins.
Molecules of fibrillar proteins have an elongated, filamentous shape. Globular proteins are characterized by a compact round shape of the molecules. Fibrillar proteins include, for example, keratin, collagen, myosin. Globular proteins are blood globulins and albumins, fibrinogen, hemoglobin, etc.
3. Name the main biological functions of proteins, give relevant examples.
● structural function. Proteins are part of all cells and intercellular substance, are components of various structures of living organisms. For example, in animals, collagen protein is part of cartilage and tendons, elastin is part of the ligaments and walls of blood vessels, keratin is the most important structural component of feathers, hair, nails, claws, horns, and hooves.
● Enzymatic (catalytic) function. Enzyme proteins are biological catalysts, accelerating the course of chemical reactions in living organisms. For example, the digestive enzymes amylase and maltase break down complex carbohydrates into simple ones, pepsin breaks down proteins into peptides, and under the action of lipases, fats are broken down into glycerol and carboxylic acids.
● Transport function. Many proteins are able to attach and carry various substances. For example, hemoglobin binds and transports oxygen and carbon dioxide. Blood albumins transport higher carboxylic acids, and globulins transport metal ions and hormones. Many proteins that make up the cytoplasmic membrane are involved in the transport of substances into and out of the cell.
● Contractile (motor) function. Contractile proteins provide the ability of cells, tissues, organs and whole organisms to change shape and move. For example, actin and myosin provide muscle work and non-muscle intracellular contractions, tubulin is part of the spindle microtubules, cilia and flagella of eukaryotic cells.
● Regulatory function. Some proteins and peptides are involved in the regulation of various physiological processes. For example, protein-peptide hormones insulin and glucagon regulate blood glucose levels, and somatotropin (growth hormone) regulates the processes of growth and physical development.
● The signaling function is that some proteins that make up the cytoplasmic membrane of cells, in response to the action of external factors, change their spatial configuration, thereby ensuring the reception of signals from the external environment and the transmission of information to the cell. For example, the opsin protein, which is part of the rhodopsin pigment, perceives light and ensures the appearance of visual excitation of the receptors (rods) of the retina.
● Protective function. Proteins protect the body from invading foreign objects and from damage. For example, immunoglobulins (antibodies) are involved in the immune response, interferon protects the body from a viral infection. Fibrinogen, thromboplastin and thrombin provide blood clotting, preventing blood loss.
● Toxic function. Many living organisms secrete proteins-toxins, which are poisons for other organisms.
● Energy function. After being broken down into amino acids, proteins can serve as a source of energy in the cell. At complete oxidation 1 g of protein releases 17.6 kJ of energy.
● Spare function. For example, special proteins are stored in plant seeds, which are used during germination by the embryo, and then by the seedling as a source of nitrogen.
4. What are enzymes? Why would most of the biochemical processes in the cell be impossible without their participation?
Enzymes are proteins that perform the function of biological catalysts, i.e., they accelerate the course of chemical reactions in living organisms. They catalyze the reactions of synthesis and splitting of various substances. Without the participation of enzymes, these processes would proceed too slowly or would not proceed at all. Almost all life processes of organisms are due to enzymatic reactions.
5. What is the specificity of enzymes? What is its reason? Why do enzymes function actively only in a certain range of temperature, pH and other factors?
The specificity of enzymes lies in the fact that each enzyme accelerates only one reaction or acts only on a certain type of bond. This feature is explained by the correspondence of the spatial configuration of the active center of the enzyme to one or another substrate (substrates).
Enzymes are proteins. Changes in pH, temperature, and other factors can cause enzymes to denature, causing them to lose their ability to bind to their substrates.
6. Why are proteins, as a rule, used as energy sources only in extreme cases, when carbohydrates and fats are exhausted in cells?
Proteins are the basis of life. They perform extremely important biological functions, many of which (enzymatic, transport, motor, etc.) are not capable of performing either carbohydrates or fats. Proteins used as an energy substrate provide as much energy as carbohydrates (1 g - 17.6 kJ) and 2.2 times less than fats (1 g - about 39 kJ). In addition, at complete splitting proteins (unlike carbohydrates and fats), not only CO 2 and H 2 O are formed, but also nitrogen and sulfur compounds, some of which are toxic to the body (for example, NH 3). Therefore, the energy function in living organisms is performed primarily by carbohydrates and fats.
7*. In many bacteria, para-aminobenzoic acid (PABA) is involved in the processes of synthesis of substances necessary for normal growth and reproduction. At the same time, sulfonamides, substances similar in structure to PABA, are used in medicine to treat a number of bacterial infections. What do you think the therapeutic effect of sulfonamides is based on?
With the help of an enzyme (dihydropteroate synthetase), bacteria convert PABA into a product (dihydropteroic acid), which is then used to synthesize the necessary growth factors. Due to their structural similarity to PABA, sulfonamides are also able to bind to the active center of this enzyme, blocking its work (i.e., competitive inhibition is observed). This leads to disruption of the synthesis of growth factors and nucleic acids in bacteria.
* Tasks marked with an asterisk require students to put forward various hypotheses. Therefore, when assigning a mark, the teacher should focus not only on the answer given here, but take into account each hypothesis, evaluating biological thinking students, the logic of their reasoning, the originality of ideas, etc. After that, it is advisable to familiarize students with the answer given.
Proteins and their functions.We will study the main substances that make up our organisms. One of the most important is proteins.
Squirrels(proteins, polypeptides) - carbon substances, consisting of chain-linked amino acids. are mandatory integral part all cells.
Amino acids- carbon compounds, the molecules of which simultaneously contain carboxyl (-COOH) and amine (NH2) groups.
A compound consisting of a large number of amino acids is called - polypeptide. Each protein in its chemical structure is a polypeptide. Some proteins are made up of several polypeptide chains. Most proteins contain an average of 300-500 amino acid residues. Several very short natural proteins, 3-8 amino acids long, and very long biopolymers, more than 1500 amino acids long, are known.
The properties of proteins determine their amino acid composition, in a strictly fixed sequence, and the amino acid composition, in turn, is determined genetic code. When creating proteins, 20 standard amino acids are used.
The structure of proteins.
There are several levels:
- Primary structure - determined by the order of alternation of amino acids in the polypeptide chain.
Twenty different amino acids can be likened to 20 letters of the chemical alphabet, which make up "words" 300-500 letters long. With 20 letters, you can write an unlimited number of such long words. If we consider that the replacement or rearrangement of at least one letter in a word gives it a new meaning, then the number of combinations in a word 500 letters long will be 20500.
It is known that the replacement of even one amino acid unit by another in a protein molecule changes its properties. Each cell contains several thousand different types of protein molecules, and each of them is characterized by a strictly defined sequence of amino acids. It is the order of alternation of amino acids in a given protein molecule that determines its special physicochemical and biological properties. Researchers are able to decipher the sequence of amino acids in long protein molecules and synthesize such molecules.
- secondary structure- protein molecules in the form of a spiral, with equal distances between the turns.
Between N-H groups and C=O, located on adjacent turns, hydrogen bonds arise. They are repeated many times, fasten the regular turns of the spiral.
- Tertiary structure - the formation of a spiral coil.
This tangle is formed by the regular interlacing of sections of the protein chain. Positively and negatively charged groups of amino acids attract and bring together even widely spaced parts of the protein chain. Other parts of the protein molecule, carrying, for example, “water-repellent” (hydrophobic) radicals, also approach each other.
Each type of protein is characterized by its own shape of a ball with bends and loops. The tertiary structure depends on the primary structure, that is, on the order of the amino acids in the chain.
- Quaternary structure- assembly protein, consisting of several chains that differ in primary structure.
Combining together, they create a complex protein that has not only a tertiary, but also a quaternary structure.protein denaturation.
Under the influence of ionizing radiation, high temperature, strong agitation, extreme pH values (concentration of hydrogen ions), as well as a number of organic solvents, such as alcohol or acetone, proteins change their natural state. Violation of the natural structure of the protein is called denaturation. The vast majority of proteins lose their biological activity, although their primary structure does not change after denaturation. The fact is that in the process of denaturation, secondary, tertiary and quaternary structures are violated, due to weak interactions between amino acid residues, and covalent peptide bonds(with the union of electrons) do not break. Irreversible denaturation can be observed when liquid and transparent chicken egg protein is heated: it becomes dense and opaque. Denaturation can also be reversible. After elimination of the denaturing factor, many proteins are able to return to their natural form, i.e. renature.
The ability of proteins to reversibly change the spatial structure in response to the action of physical or chemical factors underlies irritability, the most important property of all living beings.
Protein functions.
catalytic.
Hundreds of biochemical reactions take place continuously in every living cell. In the course of these reactions, the splitting and oxidation of nutrients coming from outside take place. The energy of nutrients obtained as a result of oxidation and the products of their breakdown are used by the cell to synthesize the various organic compounds it needs. The rapid occurrence of such reactions is provided by biological catalysts, or reaction accelerators - enzymes. More than a thousand different enzymes are known. They are all white.
Enzyme proteins - speed up reactions in the body. Enzymes are involved in the breakdown of complex molecules (catabolism) and their synthesis (anabolism), as well as the creation and repair of DNA and RNA template synthesis.Structural.
Structural proteins of the cytoskeleton, like a kind of armature, give shape to cells and many organelles and are involved in changing the shape of cells. Collagen and elastin are the main components of the intercellular substance of connective tissue (for example, cartilage), and hair, nails, bird feathers, and some shells are made up of another structural protein, keratin.
Protective.
- Physical protection.(example: collagen is a protein that forms the basis of the intercellular substance of connective tissues)
- Chemical protection. The binding of toxins to protein molecules ensures their detoxification. (example: liver enzymes that break down poisons or convert them into a soluble form, which contributes to their rapid removal from the body)
Regulatory.
- Immune protection. When bacteria or viruses enter the blood of animals and humans, the body reacts by producing special protective proteins - antibodies. These proteins bind to proteins of pathogens that are foreign to the body, which suppresses their vital activity. For each foreign protein, the body produces special "anti-proteins" - antibodies.
Hormones are carried in the blood. Most animal hormones are proteins or peptides. The binding of the hormone to the receptor is a signal that triggers a response in the cell. Hormones regulate the concentration of substances in the blood and cells, growth, reproduction and other processes. An example of such proteins is insulin which regulates the concentration of glucose in the blood.
Cells interact with each other using signal proteins transmitted through the intercellular substance. Such proteins include, for example, cytokines and growth factors.
Cytokines- small peptide information molecules. They regulate interactions between cells, determine their survival, stimulate or suppress growth, differentiation, functional activity and programmed cell death, ensure the coordination of actions of the immune, endocrine and nervous systems.
Transport.
Only proteins transport substances in the blood, for example, lipoproteins(fat transfer) hemoglobin(oxygen transport), transferrin(iron transport) or across membranes - Na +, K + -ATPase(opposite transmembrane transport of sodium and potassium ions), Ca2+-ATPase(pumping calcium ions out of the cell).
Receptor.
Protein receptors can either be located in the cytoplasm or integrated into cell membrane. One part of the receptor molecule receives a signal, most often a chemical substance, and in some cases, light, mechanical action (for example, stretching), and other stimuli.
Construction.
Animals in the process of evolution have lost the ability to synthesize ten particularly complex amino acids, called essential. They get them ready-made with plant and animal food. Such amino acids are found in the proteins of dairy products (milk, cheese, cottage cheese), in eggs, fish, meat, as well as in soybeans, beans and some other plants. In the digestive tract, proteins are broken down into amino acids, which are absorbed into the bloodstream and enter the cells. In cells, from ready-made amino acids, their own proteins are built, which are characteristic of a given organism. Proteins are an essential component of all cellular structures and this is their important building role.
Energy.
Proteins can serve as a source of energy for the cell. With a lack of carbohydrates or fats, amino acid molecules are oxidized. The energy released in this process is used to support the vital processes of the body. With prolonged fasting, proteins of muscles, lymphoid organs, epithelial tissues and liver are used.
Motor (motor).
A whole class of motor proteins provides for the movements of the body, for example, muscle contraction, including the movement of myosin bridges in the muscle, the movement of cells within the body (for example, the amoeboid movement of leukocytes).
Actually it is very short description functions of proteins, which can only clearly demonstrate their functions and significance in the body.
A little video for understanding about proteins:
Chapter 9 biological functions proteins
The functions of proteins are extremely diverse. Each given protein as a substance with a specific chemical structure performs one highly specialized function and only in a few separate cases - several interconnected ones. For example, the adrenal medulla hormone adrenaline, entering the bloodstream, increases consumptionoxygen and blood pressure, blood sugar, stimulates metabolism, and is also a mediator nervous system in cold-blooded animals.
1)
Catalytic (enzymatic) function:
Numerous biochemical reactions in living organisms proceed under mild conditions at temperatures close to 40 degrees C and pH values close to neutral. Under these conditions, the rates of most reactions are negligible, therefore, for their acceptable implementation, special biological catalysts are needed - enzymes. Even such a simple reaction as the dehydration of carbonic acid:
catalyzed by an enzyme carbonic anhydrase. In general, all reactions, with the exception of the photolysis of water, in living organisms are catalyzed by enzymes. As a rule, enzymes are either proteins or complexes of proteins with some cofactor- a metal ion or a special organic molecule. Enzymes have a high, sometimes unique, selectivity of action. For example, enzymes that catalyze the addition of α-amino acids to the corresponding tRNAs during protein biosynthesis catalyze the addition of L-amino acids only and do not catalyze the addition of D-amino acids.
2)
Transport function of proteins:
Numerous substances must enter the cell, providing it with building material and energy. At the same time, all biological membranes are built according to the same principle - a double layerlipids
, in which various proteins are immersed, and the hydrophilic regions of macromolecules are concentrated on the surface of the membranes, and the hydrophobic "tails" are in the thickness of the membrane. Such a structure is impermeable to such important components as sugars, amino acids, alkali metal ions. Their penetration into the cell is carried out with the help of special transport proteins embedded in the cell membrane. For example, bacteria have a special protein that transports milk sugar, lactose, through the outer membrane. According to the international nomenclature, lactose is designated -galatcoside, therefore the transport protein is called - galactosidepermease.
An important example of the transport of substances across biological membranes against a concentration gradient is the Na-K pump. In the course of its work, three positive ions are transferred from the cell for every two positive ions into the cell. This work is accompanied by the accumulation of electrical potential difference on the cell membrane. At the same time, ATP is broken down, giving energy. The molecular basis of the sodium-potassium pump was discovered recently, it turned out to be an enzyme that breaks down ATP, - sodium-potassium-dependent ATPase. The pump operates on the principle of opening and closing channels. The binding of molecules of the "channel" protein to the sodium ion leads to the disruption of the system of hydrogen bonds, as a result of which its conformation changes. The usual α-helix, in which there are 3.6 amino acid residues per turn, passes into a more “loose” α-helix (4.4 amino acid residues). As a result, an internal cavity is formed, sufficient for the passage of the sodium ion, but too narrow for the potassium ion. After passing through, the -helix passes into a tightly folded 310 -helix (3 amino acid residues per turn, and a hydrogen bond - at every 10th atom). In this case, the sodium channel closes, and the walls of the neighboring potassium channel expand, potassium ions pass through them into the cell. The sodium-potassium pump works on the principle of a peristaltic pump (reminiscent of the movement of a food bolus through the intestines), the principle of which is based on the variable compression and expansion of elastic tubes.
Multicellular organisms have a system for transporting substances from one organ to another. The first is hemoglobin. In addition, a transport protein is constantly in the blood plasma - serum albumin. This protein has the unique ability to form strong complexes with fatty acids formed during the digestion of fats, with some hydrophobic amino acids (for example, with tryptophan), with steroid hormones, as well as with many drugs, such as aspirin, sulfonamides, some penicillins. Another common example of a carrier protein is transferrin(provides the transfer of iron ions) and ceruplasmin(carrier of copper ions).
3)
Receptor function:
Of great importance, especially for the functioning of multicellular organisms, are receptor proteins, built into the plasma membrane of cells and serving for the perception and conversion of various signals entering the cell, both from the environment and from other cells. Acetylcholine receptors located on the cell membrane in a number of interneuronal contacts, including those in the cerebral cortex, and at neuromuscular junctions can be cited as the most studied. These proteins specifically interact with acetylcholine and respond to this by transmitting a signal into the cell. After receiving and converting the signal, the neurotransmitter must be removed in order for the cell to prepare for the perception of the next signal. For this, a special enzyme is used - acetylcholinesterase, which catalyzes the hydrolysis acetylcholine to acetate and choline.
Many hormones do not penetrate into target cells, but bind to specific receptors on the surface of these cells. Such binding is a signal that triggers physiological processes in the cell. An example is the action of the hormone insulin in adenylate cyclase system. The insulin receptor is a glycoprotein penetrating the plasmalemma. When the hormone binds to the receptor part of this complex protein, it activates the catalytic inner part, which represents the enzyme adenylate cyclase. This enzyme synthesizes cyclic adenosine monophosphoric acid (cAMP) from ATP, which in turn catalyzes the key stage of polysaccharide oxidation - the conversion of glycogen into the monomeric glucose derivative glucose-1-phosphate, which then undergoes oxidative degradation, accompanied by phosphorylation of a large amount of ADP.
4)
Protective function:
The immune system has the ability to respond to the appearance of foreign particles by producing a huge number of lymphocytes that can specifically damage precisely these particles, which can be foreign cells, such as pathogenic bacteria, cancer cells, supramolecular particles, such as viruses, macromolecules, including foreign proteins. One of the groups of lymphocytes - B- lymphocytes, produces special proteins released into the circulatory system that recognize foreign particles, while forming a highly specific complex at this stage of destruction. These proteins are called immunoglobulins. Foreign substances that trigger an immune response are called antigens, and their corresponding immunoglobulins - antibodies. If a large molecule, for example, a protein molecule, acts as an antigen, then the antibody does not recognize the entire molecule, but its specific section, called antigenic determinant. The fact that immunoglobulins interact with a relatively small part of the polymeric antigen allows the production of antibodies that specifically recognize some small molecules not found in nature. A classic example is the dinitrophenyl residue. When experimental animals are injected with a conjugate of dinitrophenol with some protein, antibodies begin to be produced that specifically recognize various derivatives of dinitrophenol. But with the introduction of pure dinitrophenol, there is no immune response. Such substances that can serve as antigenic determinants, but are not themselves capable of inducing an immune response, are called haptens.
Antibodies are built from four polypeptide chains linked by disulfide bridges. A simplified diagram of the structure of class G immunoglobulin is shown in the following figure.
Two polypeptide chains have a size of about 200 amino acid residues and are called light chains (L-chains). The other two are twice as large and are called heavy chains (H-chains). At the N-terminus of both chains there is a variable region of slightly more than 100 amino acid residues, which is different for immunoglobulins tuned to different antigens - it is this region that determines the specificity of a given population of lymphocytes.
Scheme of the structure of the immunoglobulin molecule: H-chain - heavy chain, L-chain - light chain, VH and VL - variable regions of the heavy and light chains.
The variable region forms a center that directly binds to a specific antigen or hapten; the rest, which is half the molecule in the light chain, and 3/4 in the heavy chain, does not depend on the type of immunoglobulin. This area is called constant.
According to modern concepts, each type of immunoglobulin is produced by a group of B-lymphocytes descended from one common precursor. This group of lymphocytes is called clone. The first successes in the study of the structure of immunoglobulins were associated with the study of immunoglobulins obtained from patients with myeloma (a pathology associated with overproduction of a certain type of immunoglobulin). In patients, from one malignantly overgrown clone of B-lymphocytes, a huge amount of individual immunoglobulin is produced, which is relatively easy to separate from the rest. Next, myeloma cells, as carriers of the ability for unlimited reproduction, were fused with normal B-lymphocytes, as carriers of a program for producing antibodies of a certain specificity set by the experimenter. The resulting cells hybridomas retain the ability to reproduce indefinitely and produce only certain antibodies. Since hybridomas originate from a single fused cell, they are a single clone; the resulting antibodies are therefore called monoclonal antibodies (MAT).
5)
Structural function:
Along with proteins that perform fine highly specialized functions, there are proteins that are mainly of structural importance. They provide mechanical strength and other mechanical properties of individual tissues of living organisms. First of all, this collagen- the main protein component of the extracellular matrix of connective tissue. In mammals, collagen makes up to 25% of the total mass of proteins. Collagen is synthesized in fibroblasts - the main cells of the connective tissue. Initially, it is formed in the form of procollagen, a precursor that undergoes a certain chemical processing in fibroblasts, consisting in the oxidation of proline residues to hydroxyproline and some lysine residues to α-hydroxylysine. Collagen is formed in the form of three polypeptide chains twisted into a spiral, which already outside the fibroblasts combine into collagen fibrils with a diameter of several hundred nanometers, and the latter into collagen filaments already visible under a microscope.
In elastic tissues - skin, walls of blood vessels, lungs - in addition to collagen, the extracellular matrix contains protein elastin, capable of stretching over a wide range and returning to its original state.
Another example of a structural protein is fibroin silk, secreted by silkworm caterpillars during the formation of the chrysalis and which is the main component of silk threads.
6)
motor proteins
Muscle contraction is a process during which the chemical energy stored in the form of high-energy pyrophosphate bonds in ATP molecules is converted into mechanical work. Direct participants in the contraction process are two proteins - actin and myosin.
Myosin is a protein of unusual structure, consisting of a long filamentous part (tail) and two globular heads. The total length of one molecule is about 1600 nm, of which about 200 nm are heads. Myosin is usually released as a hexamer, formed by two identical polypeptide chains with a molecular weight of 200,000 each ("heavy chains") and four "light chains" with a molecular weight of about 20,000. Heavy chains are twisted around one another, forming a tail, and carry at one end are globular heads associated with light chains. There are two important functional centers on the myosin heads - a catalytic center capable of hydrolytic cleavage of the ATP pyrophosphate bond under certain conditions, and a center that provides the ability to specifically bind to another muscle protein - actin.
actin is a globular protein molecular weight 42,000. In this form, it is called G-actin. However, it has the ability to polymerize to form a long structure called F-actin. In this form, actin is able to interact with the myosin head, and an important feature of this process is the dependence on the presence of ATP. At a sufficiently high concentration of ATP, the complex formed by actin and myosin is destroyed. After being under the influence myosin ATPase(enzyme) ATP hydrolysis occurs, the complex is restored again. This process is easy to observe in a solution containing both proteins. In the absence of ATP, the solution becomes viscous as a result of the formation of a high molecular weight complex. When ATP is added, the viscosity decreases sharply as a result of the destruction of the complex, and then begins to gradually recover as ATP is hydrolyzed. These interactions play an important role in the process of muscle contraction.
7)
Antibiotics:
A large and extremely important group of natural organic compounds in practical terms are antibiotics- substances of microbial origin, secreted by special types of microorganisms and inhibiting the growth of other, competing microorganisms. The discovery and use of antibiotics was made in the 40s. revolution in the treatment of infectious diseases caused by bacteria. It should be noted that in most cases antibiotics do not act on viruses and their use as antiviral drugs is ineffective.
Antibiotics of the group were the first to be introduced into practice. penicillin. Examples of them are benzylpenicillin And ampicillin:
Similar in structure to antibiotics of the group cephalosporins, an example of which is cefamycin C. These antibiotics have in common -lactam ring. Their mechanism of action is to inhibit one of the stages of the formation of murein - peptidoglycan, which forms the cell wall of bacteria.
Antibiotics are extremely diverse in their chemical nature and mechanism of action. Some of the widely used antibiotics interact with bacterial ribosomes, inhibiting protein synthesis in bacterial ribosomes, while at the same time practically do not interact with eukaryotic ribosomes. Therefore, they are detrimental to bacterial cells and slightly toxic to humans and animals. These include the well-known streptomycin, chloramphenicol (levomycetin):
Another well-known antibiotic is tetracycline:
One of the most effective anti-tuberculosis drugs antibiotic rifampicin- blocks the work of prokaryotic RNA polymerases of enzymes that catalyze RNA biosynthesis, - by binding with an enzyme, but at the same time does not have the ability to bind to eukaryotic RNA polymerases:
Antibiotics interacting with DNA and thereby disrupting the processes associated with the implementation of the DNA inherent in it are being intensively studied. hereditary information. Antibiotics with this mechanism of action are usually highly toxic and are used only in cancer chemotherapy. An example is actinomycin D:
8)
Toxins:
A number of living organisms, as a defense against potential enemies, produce highly toxic substances - toxins. Many of them are proteins, however, complex low molecular weight organic molecules are also found among them. An example of such a substance is the poisonous beginning of the pale toadstool - -amanitin:
This compound specifically blocks the synthesis of eukaryotic mRNAs. For humans, a lethal dose is a few mg of this toxin.
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