Chapter 2. The structure and functions of proteins. Proteins are high-molecular nitrogen-containing organic compounds consisting of amino acids linked into polypeptide chains using peptide bonds and having a complex structural organization. History of the study of proteins in 1728.

Protein Functioning Each individual protein having a unique primary structure and conformation has a unique function that distinguishes it from all other proteins. A set of individual proteins in the cell performs many diverse and complex

Post-translational changes in proteins Many proteins are synthesized in an inactive form (precursors) and, after convergence with the ribosome, undergo postsynthetic structural modifications. These conformational and structural changes in the polypeptide chains received

Levels of the study of metabolism Levels of the study of metabolism: 1. The whole organism. 2. Isolated organs (perfused). 3. Tissue sections. 4. Cell Culture. 5. Tissue homogenates. 6. Isolated cell organelles. 7. Molecular level (purified enzymes, receptors and

Digestion of proteins in the gastrointestinal tract Digestion of proteins begins in the stomach under the influence of enzymes of the gastric juice. Up to 2.5 liters are secreted per day and it differs from other digestive juices by a strongly acid reaction, due to the presence of

The breakdown of proteins in tissues is carried out using proteolytic lysosomal cathepsin enzymes. According to the structure of the active center, cysteine, serine, carboxyl and metalloprotein cathepsins are distinguished. The role of cathepsins: 1. creation of biologically active

The role of the liver in the exchange of amino acids and proteins The liver plays a central role in the exchange of proteins and other nitrogen-containing compounds. It performs the following functions: 1. synthesis of specific plasma proteins: - in the liver are synthesized: 100% albumin, 75 - 90%? -globulin, 50%

Characterization of serum proteins Proteins of the complement system - 20 proteins circulating in the blood in the form of inactive precursors belong to this system. Their activation occurs under the action of specific substances with proteolytic activity.

Proteins are organic substances. These macromolecular compounds are characterized by a specific composition and decompose into amino acids upon hydrolysis. Protein molecules can be of various forms, many of them consist of several polypeptide chains. Information about the structure of the protein is encoded in DNA, and the process of synthesis of protein molecules is called translation.

The chemical composition of proteins

The average protein contains:

• 52% carbon;
• 7% hydrogen;
• 12% nitrogen;
• 21% oxygen;
• 3% sulfur.

Protein molecules are polymers. In order to understand their structure, it is necessary to find out what their monomers - amino acids are.

Amino acids

They are usually divided into two categories: constantly meeting and sometimes meeting. The first includes 18 and 2 more amides: aspartic and glutamic acid. Sometimes there are only three acids found.

These acids can be classified in different ways: by the nature of the side chains or the charge of their radicals, and they can also be divided by the number of CN and COOH groups.

Primary protein structure

The order of alternation of amino acids in the protein chain determines its subsequent levels of organization, properties and functions. The main between the monomers is the peptide. It is formed by the removal of hydrogen from one amino acid and an OH group from another.

The first level of organization of a protein molecule is the sequence of amino acids in it, simply a chain that determines the structure of protein molecules. It consists of a "skeleton" having a regular structure. This is a repeating sequence of -NH-CH-CO-. Separate side chains are represented by amino acid (R) radicals; their properties determine the composition of the structure of proteins.

Even if the structure of the protein molecules is the same, they can differ in properties only from the fact that their monomers have a different sequence in the chain. The order of amino acids in a protein is determined by genes and dictates certain biological functions to the protein. The sequence of monomers in the molecules responsible for the same function is often close in different species. Such molecules are the same or similar in organization and perform the same functions in different species of organisms - homologous proteins. The structure, properties and functions of future molecules are laid already at the stage of synthesis of the amino acid chain.

Some common features

The structure of proteins has been studied for a long time, and analysis of their primary structure has allowed us to make some generalizations. A larger number of proteins is characterized by the presence of all twenty amino acids, of which especially a lot of glycine, alanine, glutamine and little tryptophan, arginine, methionine, histidine. Exceptions are only some groups of proteins, for example, histones. They are needed for packaging DNA and contain a lot of histidine.

Any kind of movement of organisms (muscle work, protoplasm movement in a cell, cilia flicker in protozoa, etc.) is carried out by proteins. The structure of proteins allows them to move, form fibers and rings.

The transport function is that many substances are transported through the cell membrane by special carrier proteins.

The hormonal role of these polymers is immediately understandable: a number of hormones in structure are proteins, for example, insulin, oxytocin.

The reserve function is determined by the fact that proteins are able to form deposits. For example, valgumin eggs, casein of milk, protein seeds of plants - they store a large amount of nutrients.

All tendons, articular joints, skeleton bones, hooves are formed by proteins, which brings us to their next function - supporting.

Protein molecules are receptors, providing selective recognition of certain substances. In this role, glycoproteins and lectins are especially known.

The most important factors of immunity are antibodies and are proteins by origin. For example, the blood coagulation process is based on changes in the fibrinogen protein. The inner walls of the esophagus and stomach are lined with a protective layer of mucous proteins - licins. Toxins are also proteins of origin. The basis of the skin protecting the body of animals is collagen. All of these protein functions are protective.

Well, the last function is regulatory. There are proteins that control the work of the genome. That is, they regulate transcription and translation.

No matter what important role proteins play, the structure of proteins has been unraveled by scientists for a long time. And now they open up new ways of using this knowledge.

Proteins and their functions.

We will study the basic substances that make up our organisms. Some of them are the most important proteins.

Squirrels  (proteins, polypeptides) - carbon substances consisting of chain-linked amino acids. They are an essential part of all cells.

  Amino acids  - carbon compounds in 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 composed of several polypeptide chains. Most proteins contain an average of 300-500 amino acid residues. Several very short natural proteins are known, with a length of 3-8 amino acids, and very long biopolymers with a length of more than 1,500 amino acids.

The properties of proteins are determined by their amino acid composition, in a strictly fixed sequence, and the amino acid composition, in turn, is determined by the genetic code. When creating proteins, 20 standard amino acids are used.

The structure of proteins.

There are several levels:

- Primary structure -determined by the alternation of amino acids in the polypeptide chain.

Twenty different amino acids can be likened to 20 letters of the chemical alphabet, of which “words” are made up of 300-500 letters in length. With 20 letters, you can write a limitless number of such long words. If we consider that replacing or rearranging 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 link 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 alternation order of amino acids in a given protein molecule that determines its special physicochemical and biological properties. Researchers can 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 the same distance between the turns.

Between the groups N — H and C \u003d 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 ball is formed by the regular interweaving of sections of the protein chain. Positive and negatively charged groups of amino acids attract and bring together even far-spaced sections of the protein chain. Other parts of the protein molecule, carrying, for example, “water-repellent” (hydrophobic) radicals, are also approaching.

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, i.e., on the order of amino acids in the chain.
- Quaternary structure  - a combined protein consisting of several chains that differ in their primary structure.
  Joining 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 shaking, extreme pH values \u200b\u200b(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 by heating a liquid and transparent chicken egg protein: it becomes dense and opaque. Denaturation may also be reversible. After eliminating 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 is the basis of irritability - the most important property of all living things.

The functions of proteins.

Catalytic.

Hundreds of biochemical reactions take place continuously in every living cell. In the course of these reactions, the breakdown and oxidation of nutrients coming from outside. The energy obtained from oxidation of nutrients and the products of their cleavage, the cell uses 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 squirrels.
  Protein-enzymes - accelerate the ongoing 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 matrix RNA synthesis.

Structural.

The structural proteins of the cytoskeleton, as a kind of armature, shape cells and many organoids and are involved in changing the shape of the 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 keratin structural protein.

Protective.

1. Physical protection.(example: collagen is a protein that forms the basis of the intercellular substance of connective tissues)

1. Chemical protection.  The binding of toxins to protein molecules provides their detoxification. (example: liver enzymes that break down poisons or convert them into a soluble form, which helps them quickly excrete from the body)

1. Immune defense.  The body reacts to the ingress of bacteria or viruses into the blood of animals and humans by the production of special protective proteins - antibodies. These proteins bind to disease-causing proteins that are alien to the body, thereby inhibiting their vital functions. For every foreign protein, the body produces special “anti-protein” antibodies.

Regulatory.

Hormones are carried by 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 signaling 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 inhibit growth, differentiation, functional activity and programmed cell death, ensure the coordination of the immune, endocrine and nervous systems.

Transport.

Only proteins carry out the transfer of substances in the blood, for example, lipoproteins  (fat transfer) hemoglobin  (oxygen transport) transferrin  (iron transport) or across membranes - Na +, K + -ATPase  (opposite transmembrane transfer of sodium and potassium ions), Ca2 + -ATPase  (pumping out calcium ions from the cell).

Receptor.

Protein receptors can either be in the cytoplasm or integrate into the cell membrane. One part of the receptor molecule receives a signal, which is most often served by a chemical substance, and in some cases - light, mechanical stress (for example, stretching) and other stimuli.

Construction.

Animals in the process of evolution have lost the ability to synthesize ten especially complex amino acids, called essential. They get them ready-made with plant and animal food. Such amino acids are found in 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 the cells of the finished amino acids are built their own proteins characteristic of this 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 body’s vital processes. With prolonged fasting, proteins of muscles, lymphoid organs, epithelial tissues and liver are used.

Motor (motor).

A whole class of motor proteins provides body movements, 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).

In fact, this is a very brief description of the functions of proteins, which can only clearly demonstrate their functions and significance in the body.

A little video for understanding about proteins:

Protein biosynthesis.

1. The structure of one protein is determined by:

1) a group of genes 2) one gene

3) one DNA molecule 4) a set of genes of the body

2. The gene encodes information about the sequence of monomers in the molecule:

1) t-RNA 2) AK 3) glycogen 4) DNA

3. Anticodons are called triplets:

1) DNA 2) t-RNA 3) i-RNA 4) r-RNA

4. Plastic metabolism consists mainly of reactions:

1) decay of organic substances 2) decay of inorganic substances

3) synthesis of organic substances 4) synthesis of inorganic substances

5. The synthesis of protein in a prokaryotic cell occurs:

1) on the ribosomes in the nucleus 2) on the ribosomes in the cytoplasm 3) in the cell wall

4) on the outer surface of the cytoplasmic membrane

6. The translation process takes place:

1) in the cytoplasm 2) in the nucleus 3) in mitochondria

4) on the membranes of the rough endoplasmic reticulum

7. On the membranes of the granular endoplasmic reticulum synthesis occurs:

1) ATP; 2) carbohydrates; 3) lipids; 4) proteins.

8. One triplet encodes:

1. one AK 2 one sign of an organism 3. several AK

9. Protein synthesis ends at the moment

1. Recognition of the codon by an anticodon 2. Appearance of a “punctuation mark” on the ribosome

3. arrival of i-RNA on the ribosome

10. The process by which information is read from a DNA molecule.

1. Broadcast 2. Transcription 3. Transformation

11. The properties of proteins are determined ...

1. Secondary protein structure 2. Primary protein structure

3. tertiary protein structure

12. The process by which an anti-codon recognizes a codon on i-RNA

13. Stages of protein biosynthesis.

1.transcription, translation 2.transformation, translation

3. transorganization, transcription

14. Anticodon t-RNA consists of nucleotides of UGH. Which triplet of DNA is complementary to him?

1. УУГ 2. TTTs 3. ТЦГ

15.   The number of t-RNAs involved in the translation is equal to the number:

1. Codons of i-RNA, encoding amino acids 2. Molecules of i-RNA

3 Genes that make up the DNA molecule 4. Proteins synthesized on ribosomes

16. Establish the sequence of the location of the nucleotides of i-RNA during transcription with one of the DNA chains: A-G-T-C-G

1) Y 2) D 3) C 4) A 5) C

17.   When replicating a DNA molecule, the following is formed:

1) a thread disintegrated into individual fragments of daughter molecules

2) a molecule consisting of two new DNA strands

3) a molecule, half of which consists of an mRNA strand

4) a daughter molecule consisting of one old and one new DNA strand

18. The matrix for the synthesis of mRNA molecules during transcription is:

1) the whole DNA molecule 2) completely one of the chains of the DNA molecule

3) a section of one of the DNA strands

4) in some cases, one of the chains of a DNA molecule, in others, the entire DNA molecule.

19. The process of DNA molecule self-doubling.

1. replication 2. reparation

3. reincarnation

20. During protein biosynthesis in a cell, ATP energy:

1) consumed 2) stockpiled

3) not spent and not allocated

21. In somatic cells of a multicellular organism:

1) a different set of genes and proteins 2) the same set of genes and proteins

3) the same set of genes, but a different set of proteins

4) the same set of proteins, but a different set of genes

22 .. One triplet of DNA carries information about:

1) amino acid sequences in a protein molecule

2) the sign of the organism 3) the amino acid in the molecule of the synthesized protein

4) the composition of the RNA molecule

23. Which of the processes does not occur in cells of any structure and function:

1) protein synthesis 2) metabolism 3) mitosis 4) meiosis

24. The term "transcription" refers to the process:

1) DNA doubling 2) synthesis of i-RNA on DNA

3) the transition of i-RNA to ribosomes 4) the creation of protein molecules on the polysome

25. A portion of a DNA molecule that carries information about one protein molecule is:

1) gene 2) phenotype 3) genome 4) genotype

26. Transcription in eukaryotes occurs in:

1) cytoplasm 2) endoplasmic membrane 3) lysosomes 4) nucleus

27. Protein synthesis occurs in:

1) granular endoplasmic reticulum

2) smooth endoplasmic reticulum 3) nucleus 4) lysosomes

28. One amino acid is encoded:

1) four nucleotides 2) two nucleotides

3) one nucleotide 4) three nucleotides

29. The triplet of ATC nucleotides in the DNA molecule will correspond to the codon of the i-RNA molecule:

1) TAG 2) UAG 3) UTC 4) TsAU

30. Punctuation marks  genetic code:

1. encode certain proteins 2. start protein synthesis

3. stop protein synthesis

31. The process of self-doubling of a DNA molecule.

1. replication 2. reparation 3. reincarnation

32. The function of i-RNA in the process of biosynthesis.

1. storage of hereditary information 2. AK transport to ribosomes

3.submission of information to ribosomes

33. The process when t-RNAs bring amino acids to ribosomes.

1. Transcription 2. Broadcast 3. Transformation

34. Ribosomes synthesizing the same protein molecule.

1.chromosome 2.polysome 3.mega-chromosome

35.   The process by which amino acids form a protein molecule.

1. Transcription 2. Broadcast 3. Transformation

36. Matrix synthesis reactions include ...

1. DNA replication 2. transcription, translation 3. both correct responses

37. One triplet of DNA carries information about:

1.Amino Acid Sequences in a Protein Molecule
2. The site of a certain AK in the protein chain
3. A sign of a specific organism
4. Amino acid included in the protein chain

38.   The gene encoded information about:

1) the structure of proteins, fats and carbohydrates 2) the primary structure of the protein

3) nucleotide sequences in DNA

4) amino acid sequences in 2 or more protein molecules

39.   MRNA synthesis begins with:

1) separation of DNA into two strands 2) the interaction of the RNA enzyme - polymerase and gene

3) gene duplication 4) gene decay into nucleotides

40. Transcription occurs:

1) in the nucleus 2) on the ribosomes 3) in the cytoplasm 4) on the channels of smooth EPS

41.   Protein synthesis does not occur on ribosomes in:

1) the causative agent of tuberculosis 2) bees 3) fly agaric 4) bacteriophage

42. When translating the matrix for the assembly of the protein polypeptide chain are:

1) both strands of DNA 2) one of the strands of a DNA molecule

3) mRNA molecule 4) in some cases, one of the DNA chains, in others - an mRNA molecule

The primary structure of proteins is called a linear polypeptide chain of amino acids linked by peptide bonds. The primary structure is the simplest level of structural organization of a protein molecule. High stability is given to it by covalent peptide bonds between the α-amino group of one amino acid and the α-carboxyl group of another amino acid

If the imino group of proline or hydroxyproline is involved in the formation of the peptide bond, then it has a different form

When peptide bonds are formed in cells, the carboxyl group of one amino acid is first activated, and then it combines with the amino group of another. The laboratory synthesis of polypeptides is carried out in approximately the same way.

A peptide bond is a repeating fragment of a polypeptide chain. It has a number of features that affect not only the shape of the primary structure, but also the higher levels of organization of the polypeptide chain:

· Coplanarity - all atoms included in the peptide group are in the same plane;

· The ability to exist in two resonant forms (keto or enol form);

· The trans position of the substituents with respect to the C-N bond;

· The ability to form hydrogen bonds, and each of the peptide groups can form two hydrogen bonds with other groups, including peptide.

The exception is peptide groups with the participation of the amino group of proline or hydroxyproline. They are capable of forming only one hydrogen bond (see above). This affects the formation of the secondary structure of the protein. The polypeptide chain at the site where the proline or hydroxyproline is located is easily bent, since it is not held, as usual, by a second hydrogen bond.

tripeptide formation scheme:

The levels of spatial organization of proteins: the secondary structure of proteins: the concept of α-helix and β-folded layer. Tertiary structure of proteins: the concept of native protein and protein denaturation. Quaternary structure of proteins as exemplified by the structure of hemoglobin.

The secondary structure of the protein.By secondary structure of a protein is meant a method of folding a polypeptide chain into an ordered structure. By configuration, the following elements of the secondary structure are distinguished: α spiral and β -fold layer.

Building model α-helix taking into account all the properties of the peptide bond, was developed by L. Pauling and R. Cory (1949 - 1951).

In figure 3, acircuit diagram α -helix, giving an idea of \u200b\u200bits main parameters. The polypeptide chain folds into α -spiral in such a way that the turns of the spiral are regular, therefore the spiral configuration has helical symmetry (Fig. 3, b) For every turn α -helixes account for 3.6 amino acid residues. The distance between the turns or the pitch of the spiral is 0.54 nm, the angle of rise of the turn is 26 °. Formation and maintenance α -helical configuration occurs due to hydrogen bonds formed between the peptide groups of each nth and ( p  + 3) th amino acid residues. Although the energy of hydrogen bonds is small, a large number of them leads to a significant energy effect, resulting in α The spiral configuration is quite stable. Lateral radicals of amino acid residues are not involved in maintaining α spiral configuration, therefore all amino acid residues in α -helixes are equivalent.

In natural proteins, there are only twisted α spirals.

β-folded layer- the second element of the secondary structure. Unlike α spirals β The folded layer has a linear, rather than a rod, shape (Fig. 4). The linear structure is retained due to the occurrence of hydrogen bonds between peptide groups standing on different parts of the polypeptide chain. These sites turn out to be closer to the distance of the hydrogen bond between the - C \u003d O and HN - groups (0.272 nm).

  Fig. 4. Schematic representation β -fold layer (arrows indicate

  about the direction of the polypeptide chain)

Fig. 3. Scheme ( a) and model ( b) α spirals

The secondary structure of the protein is determined by the primary. Amino acid residues are capable of forming hydrogen bonds to varying degrees, this affects the formation of α spirals or β -layer. Helix-forming amino acids include alanine, glutamic acid, glutamine, leucine, lysine, methionine and histidine. If a protein fragment consists mainly of the amino acid residues listed above, then this section will form α -spiral. Valine, isoleucine, threonine, tyrosine and phenylalanine contribute to the formation of β -layer polypeptide chain. Disordered structures arise in parts of the polypeptide chain where amino acid residues such as glycine, series, aspartic acid, asparagine, and proline are concentrated.

Many proteins also have α spirals and β layers. The proportion of the helical configuration for different proteins is different. So, the muscle protein paramyosin is almost 100% helical; a high proportion of the spiral configuration in myoglobin and hemoglobin (75%). In contrast, in trypsin and ribonuclease, a significant part of the polypeptide chain is laid in layered β structures. Proteins of supporting tissues - keratin (hair protein), collagen (protein of the skin and tendons) - have β -configuration of polypeptide chains.

Tertiary structure of the protein.The tertiary structure of a protein is a way of folding a polypeptide chain in space. In order for a protein to acquire its inherent functional properties, the polypeptide chain must curl in a certain way in space, forming a functionally active structure. This structure is called native. Despite the huge number of spatial structures theoretically possible for an individual polypeptide chain, protein folding leads to the formation of a single native configuration.

The tertiary structure of the interaction protein that arises between the side radicals of amino acid residues of different parts of the polypeptide chain is stabilized. These interactions can be divided into strong and weak.

Strong interactions include covalent bonds between sulfur atoms of cysteine \u200b\u200bresidues located in different parts of the polypeptide chain. Otherwise, such bonds are called disulfide bridges; the formation of a disulfide bridge can be represented as follows:

In addition to covalent bonds, the tertiary structure of the protein molecule is supported by weak interactions, which, in turn, are divided into polar and nonpolar.

The polar interactions include ionic and hydrogen bonds. Ionic interactions are formed upon the contact of positively charged groups of side radicals of lysine, arginine, histidine and a negatively charged COOH group of aspartic and glutamic acids. Hydrogen bonds arise between the functional groups of the side radicals of amino acid residues.

Nonpolar or van der Waals interactions between hydrocarbon radicals of amino acid residues contribute to the formation of hydrophobic core (oily drop) inside the protein globule, because hydrocarbon radicals tend to avoid contact with water. The more non-polar amino acids in the protein, the greater the role of van der Waals bonds in the formation of its tertiary structure.

Numerous bonds between the side radicals of amino acid residues determine the spatial configuration of the protein molecule (Fig. 5).

  Fig. 5. Types of bonds supporting the tertiary structure of the protein:
a- disulfide bridge; b -ionic bond; c, d -hydrogen bonds;
d -van der vaals ties

The tertiary structure of a single protein is unique, as is its primary structure. Only the correct spatial folding of the protein makes it active. Various violations of the tertiary structure lead to a change in the properties of the protein and the loss of biological activity.

Quaternary protein structure.Proteins with a molecular weight of more than 100 kDa 1, as a rule, consist of several polypeptide chains with a relatively small molecular weight. The structure consisting of a certain number of polypeptide chains occupying a strictly fixed position relative to each other, as a result of which the protein has one or another activity, is called the quaternary structure of the protein. A quaternary protein is called epimoleculeor multimer , and its constituent polypeptide chains, respectively subunits or protomers . A characteristic property of quaternary structure proteins is that a single subunit does not have biological activity.

The stabilization of the quaternary structure of the protein occurs due to polar interactions between the side radicals of amino acid residues localized on the surface of the subunits. Such interactions firmly hold the subunits in the form of an organized complex. Plots of subunits on which interactions occur are called contact pads.

A classic example of a quaternary protein is hemoglobin. A hemoglobin molecule with a molecular weight of 68,000 Da consists of four subunits of two different types - α   and β / α The subunit consists of 141 amino acid residues, a β   - from 146. Tertiary structure α - and β -subunits is similar, as is their molecular weight (17,000 Da). Each subunit contains a prosthetic group - heme . Since heme is also present in other proteins (cytochromes, myoglobin), which will be studied further, at least briefly discuss the structure of the topic (Fig. 6). The heme group is a complex coplanar cyclic system consisting of a central atom that forms coordination bonds with the four pyrrole residues connected by methane bridges (\u003d CH -). In hemoglobin, iron is usually in an oxidized state (2+).

Four subunits - two α   and two β   - are combined into a single structure in such a way that α -subunits contact only with β -subunits and vice versa (Fig. 7).

  Fig. 6. The structure of hemoglobin heme

  Fig. 7. Schematic representation of the Quaternary hemoglobin structure:
  Fe - hemoglobin heme

As can be seen from Figure 7, one hemoglobin molecule is capable of transporting 4 oxygen molecules. Both oxygen binding and release are accompanied by conformational structural changes. α - and β -subunits of hemoglobin and their relative position in the epimolecule. This fact indicates that the quaternary structure of the protein is not absolutely rigid.

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