Peptide bond. structure and biological properties of peptides

Polypeptides are proteins that have a high degree of condensation. They are widespread among organisms of both plant and animal origin. That is, here we are talking about components that are mandatory. They are extremely diverse, and there is no clear line between such substances and ordinary proteins. If we talk about the diversity of such substances, then it should be noted that when they are formed, at least 20 amino acids of the protenogenic type are involved in this process, and if we talk about the number of isomers, then they can be indefinite.

This is why protein-type molecules have so many possibilities that are almost limitless when it comes to their multifunctionality. So, it’s clear why proteins are called the main ones of all living things on Earth. Proteins are also called one of the most complex substances that have ever been formed by nature, and they are also very unique. Just like protein, proteins contribute to the active development of living organisms.

To be as specific as possible, we are talking about substances that are biopolymers based on amino acids containing at least a hundred residues of the amino acid type. Moreover, there is also a division here - there are substances that belong to the low-molecular group, they include only a few dozen amino acid residues, there are also substances that belong to high-molecular groups, they contain significantly more such residues. A polypeptide is a substance that is truly distinguished by great diversity in its structure and organization.

Groups of polypeptides

All these substances are conventionally divided into two groups; this division takes into account the features of their structure, which have a direct impact on their functionality:

• The first group includes substances that differ in a typical protein structure, that is, this includes a linear chain and amino acids themselves. They are found in all living organisms, and substances with increased hormonal activity are of greatest interest here.
• As for the second group, here are those compounds whose structure does not have the most typical features for proteins.

What is a polypeptide chain

The polypeptide chain is a protein structure that includes amino acids, all of which are tightly connected by peptide-type compounds. If we talk about the primary structure, then we are talking about the simplest level of structure of a protein-type molecule. This organizational form is characterized by increased stability.

When peptide bonds begin to form in cells, the first thing that activates is the carboxyl group of one amino acid, and only then the active connection with another similar group begins. That is, polypeptide chains are characterized by constantly alternating fragments of such bonds. There are a number of specific factors that have a significant impact on the shape of the primary type structure, but their influence is not limited to this. There is active influence on those organizations of such a chain that have the highest level.

If we talk about the features of this organizational form, they are as follows:

• there is a regular alternation of structures belonging to the rigid type;
• There are areas that have relative mobility; they have the ability to rotate around bonds. It is features of this kind that influence how the polypeptide chain fits in space. Moreover, various types of organizational issues can occur with peptide chains under the influence of many factors. There may be a detachment of one of the structures, when the peptides form into a separate group and are separated from one chain.

Protein secondary structure

Here we are talking about a variant of chain laying in such a way that an ordered structure is organized; this becomes possible due to hydrogen bonds between groups of peptides of one chain with the same groups of another chain. If we take into account the configuration of such a structure, it can be:

1. Spiral type, this name comes from its unique shape.
2. Layered-fold type.

If we talk about a helical group, then this is a protein structure that is formed in the shape of a helix, which is formed without going beyond one polypeptide-type chain. If we talk about appearance, it is in many ways similar to a regular electric spiral, which is found in tiles that run on electricity.

As for the layered-fold structure, here the chain is distinguished by a curved configuration; its formation is carried out on the basis of hydrogen-type bonds, and here everything is limited to the limits of one section of a specific chain.

The peptide bond is covalent in its chemical nature and imparts high strength to the primary structure of the protein molecule. Being a repeating element of the polypeptide chain and having specific structural features, the peptide bond affects not only the shape of the primary structure, but also the higher levels of organization of the polypeptide chain.

L. Pauling and R. Corey made a great contribution to the study of the structure of the protein molecule. Noticing that the protein molecule contains the most peptide bonds, they were the first to conduct painstaking X-ray studies of this bond. We studied the bond lengths, the angles at which the atoms are located, and the direction of the atoms relative to the bond. Based on the research, the following main characteristics of the peptide bond were established.

1. Four atoms of the peptide bond (C, O, N, H) and two attached
a-carbon atoms lie in the same plane. The R and H groups of a-carbon atoms lie outside this plane.

2. The O and H atoms of the peptide bond and the two a-carbon atoms, as well as the R-groups, have a trans orientation relative to the peptide bond.

3. The C–N bond length, equal to 1.32 Å, is intermediate between the length of a double covalent bond (1.21 Å) and a single covalent bond (1.47 Å). It follows that the C–N bond is partially unsaturated. This creates the prerequisites for tautomeric rearrangements to occur at the double bond with the formation of the enol form, i.e. the peptide bond can exist in the keto-enol form.

Rotation around the –C=N– bond is difficult and all atoms included in the peptide group have a planar trans configuration. The cis configuration is energetically less favorable and is found only in some cyclic peptides. Each planar peptide fragment contains two bonds with a-carbon atoms capable of rotation.

There is a very close connection between the primary structure of a protein and its function in a given organism. In order for a protein to perform its inherent function, a very specific sequence of amino acids is required in the polypeptide chain of this protein. This specific sequence of amino acids, qualitative and quantitative composition is fixed genetically (DNA→RNA→protein). Each protein is characterized by a specific sequence of amino acids; replacing at least one amino acid in a protein leads not only to structural rearrangements, but also to changes in physicochemical properties and biological functions. The existing primary structure predetermines subsequent (secondary, tertiary, quaternary) structures. For example, the red blood cells of healthy people contain a protein called hemoglobin with a certain sequence of amino acids. A small proportion of people have a congenital abnormality in the structure of hemoglobin: their red blood cells contain hemoglobin, which in one position contains the amino acid valine (hydrophobic, non-polar) instead of glutamic acid (charged, polar). Such hemoglobin differs significantly in physicochemical and biological properties from normal. The appearance of a hydrophobic amino acid leads to the appearance of a “sticky” hydrophobic contact (red blood cells do not move well in blood vessels), to a change in the shape of the red blood cell (from biconcave to crescent-shaped), as well as to a deterioration in oxygen transfer, etc. Children born with this anomaly die in early childhood from sickle cell anemia.

Comprehensive evidence in favor of the statement that biological activity is determined by the amino acid sequence was obtained after the artificial synthesis of the enzyme ribonuclease (Merrifield). A synthesized polypeptide with the same amino acid sequence as the natural enzyme had the same enzymatic activity.

Research in recent decades has shown that the primary structure is fixed genetically, i.e. the sequence of amino acids in a polypeptide chain is determined by the genetic code of DNA, and, in turn, determines the secondary, tertiary and quaternary structures of the protein molecule and its general conformation. The first protein whose primary structure was established was the protein hormone insulin (contains 51 amino acids). This was done in 1953 by Frederick Sanger. To date, the primary structure of more than ten thousand proteins has been deciphered, but this is a very small number, considering that there are about 10 12 proteins in nature. As a result of free rotation, polypeptide chains are able to twist (fold) into various structures.

Secondary structure. The secondary structure of a protein molecule refers to the way the polypeptide chain is arranged in space. The secondary structure of a protein molecule is formed as a result of one or another type of free rotation around the bonds connecting a-carbon atoms in the polypeptide chain. As a result of this free rotation, polypeptide chains are able to twist (fold) in space into various structures.

Three main types of structure are found in natural polypeptide chains:

- a-helix;

- β-structure (folded sheet);

- statistical tangle.

The most probable type of structure of globular proteins is considered to be α-helix Twisting occurs clockwise (right-hand spiral), which is due to the L-amino acid composition of natural proteins. The driving force in the emergence α-helices is the ability of amino acids to form hydrogen bonds. Amino acid R groups point outward from the central axis a-helices. dipoles >C=O and >N–H of neighboring peptide bonds are oriented optimally for dipole interaction, thereby forming an extensive system of intramolecular cooperative hydrogen bonds that stabilize the a-helix.

The helix pitch (one full turn) of 5.4Å includes 3.6 amino acid residues.

Figure 2 – Structure and parameters of the a-helix of the protein

Each protein is characterized by a certain degree of helicity of its polypeptide chain

The spiral structure can be disrupted by two factors:

1) the presence of a proline residue in the chain, the cyclic structure of which introduces a break in the polypeptide chain - there is no –NH 2 group, therefore the formation of an intrachain hydrogen bond is impossible;

2) if in a polypeptide chain there are many amino acid residues in a row that have a positive charge (lysine, arginine) or a negative charge (glutamic, aspartic acids), in this case the strong mutual repulsion of similarly charged groups (–COO– or –NH 3 +) significantly exceeds stabilizing influence of hydrogen bonds in a-helices.

Another type of polypeptide chain configuration found in hair, silk, muscle and other fibrillar proteins is called β-structures or folded sheet. The folded sheet structure is also stabilized by hydrogen bonds between the same dipoles –NH...... O=C<. Однако в этом случае возникает совершенно иная структура, при которой остов полипептидной цепи вытянут таким образом, что имеет зигзагообразную структуру. Складчатые участки полипептидной цепи проявляют кооперативные свойства, т.е. стремятся расположиться рядом в белковой молекуле, и формируют параллельные

polypeptide chains that are identically directed or antiparallel,

which are strengthened due to hydrogen bonds between these chains. Such structures are called b-folded sheets (Figure 2).

Figure 3 – b-structure of polypeptide chains

a-Helix and folded sheets are ordered structures; they have a regular arrangement of amino acid residues in space. Some regions of the polypeptide chain do not have any regular periodic spatial organization; they are designated as disordered or statistical tangle.

All these structures arise spontaneously and automatically due to the fact that a given polypeptide has a certain amino acid sequence, which is genetically predetermined. a-helices and b-structures determine a certain ability of proteins to perform specific biological functions. Thus, the a-helical structure (a-keratin) is well adapted to form external protective structures - feathers, hair, horns, hooves. The b-structure promotes the formation of flexible and inextensible silk and web threads, and the collagen protein conformation provides the high tensile strength required for tendons. The presence of only a-helices or b-structures is characteristic of filamentous (fibrillar) proteins. In the composition of globular (spherical) proteins, the content of a-helices and b-structures and structureless regions varies greatly. For example: insulin is spiralized 60%, ribonuclease enzyme - 57%, chicken egg protein lysozyme - 40%.

Tertiary structure. Tertiary structure refers to the way a polypeptide chain is arranged in space in a certain volume.

The tertiary structure of proteins is formed by additional folding of the peptide chain containing an a-helix, b-structures and random coil regions. The tertiary structure of a protein is formed completely automatically, spontaneously and completely predetermined by the primary structure and is directly related to the shape of the protein molecule, which can be different: from spherical to filamentous. The shape of a protein molecule is characterized by such an indicator as the degree of asymmetry (the ratio of the long axis to the short one). U fibrillar or filamentous proteins, the degree of asymmetry is greater than 80. With a degree of asymmetry less than 80, proteins are classified as globular. Most of them have a degree of asymmetry of 3-5, i.e. the tertiary structure is characterized by a fairly dense packing of the polypeptide chain, approaching the shape of a ball.

During the formation of globular proteins, nonpolar hydrophobic amino acid radicals are grouped within the protein molecule, while polar radicals are oriented toward water. At some point, the thermodynamically most favorable stable conformation of the molecule, a globule, appears. In this form, the protein molecule is characterized by minimal free energy. The conformation of the resulting globule is influenced by factors such as the pH of the solution, the ionic strength of the solution, as well as the interaction of protein molecules with other substances.

The main driving force in the emergence of a three-dimensional structure is the interaction of amino acid radicals with water molecules.

Fibrillar proteins. During the formation of the tertiary structure, they do not form globules - their polypeptide chains do not fold, but remain elongated in the form of linear chains, grouping into fibril fibers.

Drawing – Structure of collagen fibril (fragment).

Recently, evidence has emerged that the process of tertiary structure formation is not automatic, but is regulated and controlled by special molecular mechanisms. This process involves specific proteins - chaperones. Their main functions are the ability to prevent the formation of nonspecific (chaotic) random coils from the polypeptide chain, and to ensure their delivery (transport) to subcellular targets, creating conditions for the completion of the folding of the protein molecule.

Stabilization of the tertiary structure is ensured due to non-covalent interactions between the atomic groups of side radicals.

Figure 4 - Types of bonds that stabilize the tertiary structure of a protein

A) electrostatic forces attraction between radicals carrying oppositely charged ionic groups (ion-ion interactions), for example, the negatively charged carboxyl group (– COO –) of aspartic acid and (NH 3 +) the positively charged e-amino group of the lysine residue.

b) hydrogen bonds between functional groups of side radicals. For example, between the OH group of tyrosine and the carboxylic oxygen of aspartic acid

V) hydrophobic interactions are caused by van der Waals forces between non-polar amino acid radicals. (For example, in groups
–CH 3 – alanine, valine, etc.

G) dipole-dipole interactions

d) disulfide bonds(–S–S–) between cysteine ​​residues. This bond is very strong and is not present in all proteins. This connection plays an important role in the protein substances of grain and flour, because influences the quality of gluten, the structural and mechanical properties of the dough and, accordingly, the quality of the finished product - bread, etc.

A protein globule is not an absolutely rigid structure: within certain limits, reversible movements of parts of the peptide chain relative to each other are possible with the breaking of a small number of weak bonds and the formation of new ones. The molecule seems to breathe, pulsate in its different parts. These pulsations do not disrupt the basic conformation plan of the molecule, just as thermal vibrations of atoms in a crystal do not change the structure of the crystal if the temperature is not so high that melting occurs.

Only after a protein molecule acquires a natural, native tertiary structure does it exhibit its specific functional activity: catalytic, hormonal, antigenic, etc. It is during the formation of the tertiary structure that the formation of active centers of enzymes occurs, centers responsible for the integration of proteins into the multienzyme complex, centers responsible for the self-assembly of supramolecular structures. Therefore, any effects (thermal, physical, mechanical, chemical) leading to the destruction of this native conformation of the protein (breaking bonds) are accompanied by partial or complete loss of the protein’s biological properties.

The study of the complete chemical structures of some proteins has shown that in their tertiary structure zones are identified where hydrophobic amino acid radicals are concentrated, and the polypeptide chain is actually wrapped around the hydrophobic core. Moreover, in some cases, two or even three hydrophobic nuclei are separated in a protein molecule, resulting in a 2- or 3-nuclear structure. This type of molecular structure is characteristic of many proteins that have a catalytic function (ribonuclease, lysozyme, etc.). A separate part or region of a protein molecule that has a certain degree of structural and functional autonomy is called a domain. A number of enzymes, for example, have separate substrate-binding and coenzyme-binding domains.

Biologically, fibrillar proteins play a very important role related to the anatomy and physiology of animals. In vertebrates, these proteins account for 1/3 of their total content. An example of fibrillar proteins is the silk protein fibroin, which consists of several antiparallel chains with a folded sheet structure. Protein a-keratin contains from 3-7 chains. Collagen has a complex structure in which 3 identical levorotatory chains are twisted together to form a dextrorotatory triple helix. This triple helix is ​​stabilized by numerous intermolecular hydrogen bonds. The presence of amino acids such as hydroxyproline and hydroxylysine also contributes to the formation of hydrogen bonds that stabilize the structure of the triple helix. All fibrillar proteins are poorly soluble or completely insoluble in water, since they contain many amino acids containing hydrophobic, water-insoluble R-groups isoleucine, phenylalanine, valine, alanine, methionine. After special processing, insoluble and indigestible collagen is converted into a gelatin-soluble polypeptide mixture, which is then used in the food industry.

Globular proteins. Perform a variety of biological functions. They perform a transport function, i.e. transport nutrients, inorganic ions, lipids, etc. Hormones, as well as components of membranes and ribosomes, belong to the same class of proteins. All enzymes are also globular proteins.

Quaternary structure. Proteins containing two or more polypeptide chains are called oligomeric proteins, they are characterized by the presence of a quaternary structure.

Figure - Schemes of tertiary (a) and quaternary (b) protein structures

In oligomeric proteins, each of the polypeptide chains is characterized by its primary, secondary and tertiary structure, and is called a subunit or protomer. The polypeptide chains (protomers) in such proteins can be either the same or different. Oligomeric proteins are called homogeneous if their protomers are the same and heterogeneous if their protomers are different. For example, the protein hemoglobin consists of 4 chains: two -a and two -b protomers. The enzyme a-amylase consists of 2 identical polypeptide chains. Quaternary structure refers to the arrangement of polypeptide chains (protomers) relative to each other, i.e. the method of their joint stacking and packaging. In this case, protomers interact with each other not with any part of their surface, but with a certain area (contact surface). Contact surfaces have such an arrangement of atomic groups between which hydrogen, ionic, and hydrophobic bonds arise. In addition, the geometry of the protomers also favors their connection. Protomers fit together like a key to a lock. Such surfaces are called complementary. Each protomer interacts with the other at multiple points, making connection with other polypeptide chains or proteins impossible. Such complementary interactions of molecules underlie all biochemical processes in the body.

α-Amino acids can be covalently linked to each other by peptide bonds. The carboxyl group of one amino acid is covalently bonded to the amino group of another amino acid. In this case, R- CO-NH-R bond, called peptide bond. In this case, the water molecule is split off.

With the help of peptide bonds, proteins and peptides are formed from amino acids. Peptides containing up to 10 amino acids are called oligopeptides. Often the name of such molecules indicates the number of amino acids included in the oligopeptide: tripeptide, pentapeptide, octapeptide, etc. Peptides containing more than 10 amino acids are called "polypeptides" and polypeptides consisting of more than 50 amino acid residues are usually called proteins. The monomers of amino acids that make up proteins are called "amino acid residues". An amino acid residue that has a free amino group is called N-terminal and is written on the left, and one that has a free C-carboxyl group is called C-terminal and is written on the right. Peptides are written and read from the N-terminus.

The bond between an α-carbon atom and an α-amino group or α-carboxyl group is freely rotatable (although limited by the size and nature of the radicals), allowing the polypeptide chain to adopt different configurations.

Peptide bonds are usually located in the trans configuration, i.e. α-carbon atoms are located on opposite sides of the peptide bond. As a result, the side radicals of amino acids are located at the furthest distance from each other in space. Peptide bonds are very strong and are covalent.

The human body produces many peptides that participate in the regulation of various biological processes and have high physiological activity. These are a number of hormones - oxytocin (9 amino acid residues), vasopressin (9), bradykinin (9) regulating vascular tone, thyroid hormones (3), antibiotics - gramicidin, peptides that have an analgesic effect (enkephalins (5) and endorphins and other opioids peptides). The analgesic effect of these peptides is hundreds of times greater than the analgesic effect of morphine;

Application of amino acids based on properties.

Amino acids, mainly α-amino acids, are necessary for the synthesis of proteins in living organisms. Humans and animals receive the amino acids necessary for this in the form of food containing various proteins. The latter undergo splitting into individual amino acids in the digestive tract, from which proteins characteristic of a given organism are then synthesized. Some amino acids are used for medical purposes. Many amino acids are used to feed animals.

Amino acid derivatives are used to synthesize fiber, such as nylon.

Questions for self-control

· Write the electronic structure of nitrogen and hydrogen.

· Write the electronic and structural formula of ammonia.

· What is a hydrocarbon radical?

· What hydrocarbon radicals do you know?

· Replace one hydrogen in the ammonia molecule with a methyl radical.

· What do you think this connection is and what is it called?

· What substance will you get if you replace the remaining hydrogen atoms with hydrocarbon radicals, for example, methyl radicals?

· How will the properties of the resulting compounds change?

· Determine the formula of an organic substance if it is known that its vapor density for hydrogen is 22.5, the mass fraction of carbon is 0.533, the mass fraction of hydrogen is 0.156 and the mass fraction of nitrogen is 0.311. (Answer: C 2 H 7 N.)

· Textbook by G.E.Rudzitis, F.G.Feldman. Page 173, No. 6, 7.

ü What is an acid?

ü What is a functional group?

ü What functional groups do you remember?

ü What is an amino group?

ü What properties does the amino group have?

ü What properties does acid have?

ü What reaction do you think a molecule containing an acidic and a basic group will give in the environment?

ü TEST

Option 1.

1) Amino acids include functional groups:

a) -NH2 and –OH

b) -NH2 and –SON

c) -NH2 and –COOH

d) -OH and –COOH

2. Amino acids can be considered as derivatives:

a) alkenes;

b) alcohols;

c) carboxylic acids;

d) carbohydrates.

3. Amino acids react

a) polymerization;

b) polycondensation;

c) neutralization.

4. Bonding between amino acids in a polymer:

a) hydrogen;

b) ionic;

c) peptide.

5. Essential amino acids are...

Option 2.

1.General formula of amino acids:

a) R-CH2 (NH2)-COOH;

2. In a solution of amino acids, the medium

a) alkaline;

b) neutral;

c) acidic.

3. Amino acids can interact with each other to form:

a) carbohydrates;

b) nucleic acids;

c) polypeptides;

d) starch.

4. Amino acids are...

a) organic bases;

b) acids

c) organic amphoteric compounds.

5. Amino acids are used in...

ü From what inorganic substances can aminoacetic acid be obtained? Write the corresponding reaction equations.

ü Task. Determine the amino acid formula if the mass fractions of carbon, hydrogen, oxygen and nitrogen are respectively equal: 48%, 9.34%, 42.67% and 18.67%. Write all possible structural formulas and name them.

LESSON PLAN No. 16

Discipline: Chemistry.

Subject: Squirrels.

Purpose of the lesson: Study the primary, secondary, tertiary structures of proteins. Chemical properties of proteins: combustion, denaturation, hydrolysis, color reactions. Biological functions of proteins.

Planned results

Subject: the formation of ideas about the place of chemistry in the modern scientific picture of the world; understanding the role of chemistry in shaping a person’s horizons and functional literacy for solving practical problems;

Metasubject: the use of various types of cognitive activity and basic intellectual operations (statement of a problem, formulation of hypotheses, analysis and synthesis, comparison, generalization, systematization, identification of cause-and-effect relationships, search for analogues, formulation of conclusions) to solve the problem;

Personal: a sense of pride and respect for the history and achievements of domestic chemical science; chemically competent behavior in professional activities and at home when handling chemicals, materials and processes;

Standard time: 2 hours

Type of lesson: Lecture.

Lesson plan:

Equipment: Textbook.

Literature:

1. Chemistry 10th grade: textbook. for general education organizations with adj. per electron Media (DVD) / G.E. Rudzitis, F.G. Feldman. – M.: Education, 2014. -208 p.: ill.

2. Chemistry for professions and technical specialties: a textbook for students. institutions prof. education / O.S. Gabrielyan, I.G. Ostroumov. – 5th ed., erased. – M.: Publishing Center “Academy”, 2017. – 272 pp., with colors. ill.

Teacher: Tubaltseva Yu.N.

Topic 16. PROTEINS.

1. Proteins. Primary, secondary, tertiary structures of proteins.

2. Chemical properties of proteins: combustion, denaturation, hydrolysis, color reactions.

3. Biological functions of proteins.

1) Squirrels. Primary, secondary, tertiary structures of proteins.

1 – Protein composition: C – 54%, O – 23%, H – 7%, N – 17%, S – 2% and others: Zn, P, Fe, Cu, Mg, Mn

In 1903, the German scientist E.G. Fischer proposed the peptide theory, which became the key to the secret of protein structure. Fischer proposed that proteins are polymers of amino acid residues linked by an NH–CO peptide bond. The idea that proteins are polymer formations was expressed back in 1888 by the Russian scientist A.Ya. Danilevsky.

2 - Proteins – IUD – proteins

“Protos” from the Greek means “primary, most important.” Proteins are natural polymers consisting of AA.

Mr (albumin)=36000

Mr (myosin)=150000

Mr (hemoglobin)=68000

Mr (collagen)=350000

Mr (fibrinogen)=450000

Milk protein formula - casein C 1894 H 3021 O 576 N 468 S 21

Proteins are natural high molecular weight natural compounds (biopolymers), built from alpha amino acids connected by a special peptide bond. Proteins contain 20 different amino acids, which means there is a huge variety of proteins with different combinations of amino acids. Just as we can form an infinite number of words from 33 letters of the alphabet, we can form an infinite number of proteins from 20 amino acids. There are up to 100,000 proteins in the human body.

The number of amino acid residues included in the molecules is different: insulin - 51, myoglobin - 140. Hence the M r of the protein ranges from 10,000 to several million.

Proteins are divided into proteins (simple proteins) and proteids (complex proteins).

4 - 20 AKs are the “building blocks” of a protein building; by combining them in different orders, you can build an innumerable variety of substances with very different properties. Chemists are trying to decipher the structure of giant protein molecules. This task is very difficult: nature carefully hides the “blueprints” according to which these particles are built.

In 1888, Russian biochemist A.Ya. Danilevsky pointed out that protein molecules contain repeating peptide groups of –C–N– atoms

At the beginning of the twentieth century, the German scientist E. Fischer and other researchers managed to synthesize compounds into molecules that included 18 residues of various AAs connected by peptide bonds.

5 - The primary structure of the protein is a sequential alternation of AAs (PPC polypeptide chain). The spatial configuration of a protein molecule, resembling a spiral, is formed due to numerous hydrogen bonds between groups.

– CO– and –NH–

This protein structure is called secondary. In space, the twisted helix of the PPC forms the tertiary structure of the protein, which is maintained by the interaction of different functional groups of the PPC.

–S–S– (disulfide bridge)

–COOH and –OH (ester bridge)

–COOH and –NH 2 (salt bridge)

Some protein macromolecules can combine with each other and form large molecules. Polymer formations of proteins are called quaternary structures (hemoglobin only with such a structure is able to attach and transport O 2 into the body)

2) Chemical properties of proteins: combustion, denaturation, hydrolysis, color reactions.

1. Proteins are characterized by reactions that result in precipitate appears. But in some cases, the resulting precipitate dissolves with excess water, and in others, irreversible protein coagulation occurs, i.e. denaturation.

Denaturation is a change in the tertiary and quaternary structures of a protein macromolecule under the influence of external factors (increase or decrease in temperature, pressure, mechanical stress, the action of chemical reagents, UV radiation, radiation, poisons, heavy metal salts (lead, mercury, etc.))

Every person is “built” of proteins. Regardless of gender, age or race. And the structural unit of all proteins are amino acids, connected to each other by a special type of bond. It is so important that it even received a separate name - peptide bond.

Amino acid associations can have different names depending on how many “building blocks” they contain. If no more than 10 amino acids come together, then these are peptides, if from 10 to 40, then we are talking about a polypeptide, and if there are more than forty amino acid bricks, then this is a protein, a structural unit of our body.

If we talk about theory, the structure of a peptide bond is a connection between the α-amino group (–NH 2) of one amino acid and the α-carboxyl (–COOH) group of another. Such compound reactions are accompanied by the release of water molecules. It is on this principle that all proteins, and therefore every person, are built.

If we talk about the whole of nature, then there are about 300 amino acids found in it. However, proteins consist of only 20 α-amino acids. And despite such a small number of them, there are different proteins, which is due to the different order of amino acids in them.

The properties of the amino acids themselves are determined by the R radical. It can be a fatty acid residue and include an aromatic ring or heterocycles. Depending on which amino acids with which radicals formed the protein, it will show certain physical properties, as well as chemical properties and physiological functions that it will perform in the human body.

Properties of a peptide bond

The properties of the peptide bond determine its uniqueness. Among them are:

It must be said that of all the amino acids we need for life, some are quite successfully synthesized by our body itself.

According to one classification, they are called nonessential amino acids. And there are also 8 others that cannot arise in the human body in any other way except through food. And the third group is very small, only 3 names: arginine, histidine and tyrosine. In principle, they are formed here, but the quantity is so small that it is impossible to do without outside help. They were called partially irreplaceable. An interesting fact is that plants produce all these amino acids themselves.

The role of proteins in the body

Whatever organ or tissue in your body you name, it will be made of protein. They are part of the heart, blood, muscles, and kidneys. People have about five million different types, and by mass this will be expressed in 15-20%.

None of the processes in humans takes place without the participation of proteins. These include metabolic processes, food digestion, and energy processes. With the help of a wide variety of proteins, the immune system will also be able to properly protect the body, and carbohydrates, fats, vitamins and microelements will be absorbed by the person as needed.

Proteins in our body are constantly “in motion”. Some of them break down into amino acid bricks, others are formed from the same bricks, forming the structure of organs and tissues. When eating food, it is worth considering that it is not only the fact of consumption that is important, but the quality characteristics of the products. Most of the amino acids, mainly coming from the “wrong” food, are simply excreted from us without being retained. And if many especially important proteins are lost in this way, such as, for example, insulin or hemoglobin, then the health losses can be irreparable.

Some choose fad diets based on insufficient protein intake. First of all, calcium begins to be poorly absorbed. This means that the bones become brittle and the process of muscle tissue atrophy will begin. Then, which is especially unpleasant for girls, the skin begins to peel, nails constantly break off, and hair falls out in clumps.

Quaternary structure

Tertiary structure

Different ways to depict the three-dimensional structure of a protein using triosephosphate isomerase as an example. On the left is a “core” model, depicting all the atoms and the bonds between them; The colors show the elements. In the middle is the styling motif. On the right is the contact surface of the protein, constructed taking into account the van der Waals radii of the atoms; The colors show the activity features of the areas

Tertiary structure is the spatial structure of the polypeptide chain. Structurally, it consists of secondary structure elements stabilized by various types of interactions, in which hydrophobic interactions play a critical role. The following take part in stabilizing the tertiary structure:

– covalent bonds (between two cysteine ​​residues – disulfide bridges);

– ionic bonds between oppositely charged side groups of amino acid residues;

– hydrogen bonds;

– hydrophobic interactions. When interacting with surrounding water molecules, the protein molecule folds so that the non-polar side groups of amino acids are isolated from the aqueous solution; polar hydrophilic side groups appear on the surface of the molecule.

Quaternary structure (or subunit, domain) - the relative arrangement of several polypeptide chains as part of a single protein complex. Protein molecules that make up a protein with a quaternary structure are formed separately on ribosomes and only after completion of synthesis form a common supramolecular structure. A protein with a quaternary structure can contain both identical and different polypeptide chains. The same types of interactions take part in the stabilization of the quaternary structure as in the stabilization of the tertiary structure. Supramolecular protein complexes can consist of dozens of molecules.

https://ru.wikipedia.org/wiki/Squirrels

Peptide bond - main parameters and features

A peptide bond is a type of amide bond that occurs during the formation of proteins and peptides as a result of the interaction of the α-amino group (– NH 2) of one amino acid with the α-carboxyl group (– COOH) of another amino acid.

From two amino acids (1) and (2) a dipeptide (a chain of two amino acids) and a water molecule are formed. According to the same scheme, the ribosome generates longer chains of amino acids: polypeptides and proteins. Different amino acids, which are the “building blocks” of protein, differ in the R radical.

As in the case of any amides, in a peptide bond, due to the resonance of canonical structures, the C-N bond between the carbon of the carbonyl group and the nitrogen atom is partially double in nature:

This is manifested, in particular, in a decrease in its length to 1.33 angstroms:

This results in the following properties:

– 4 bond atoms (C, N, O and H) and 2 α-carbons are in the same plane. The R-groups of amino acids and the hydrogens at α-carbons are outside this plane.

– H and O in the peptide bond, as well as the α-carbons of two amino acids are trans-oriented (the trans isomer is more stable). In the case of L-amino acids, which is the case in all natural proteins and peptides, the R-groups are also trans-oriented.

– Rotation around the C-N bond is difficult, rotation around the C-C bond is possible.

To detect proteins and peptides, as well as their quantitative determination in solution, the biuret reaction is used.

https://ru.wikipedia.org/wiki/Peptide bond

Literature:

1) Alberts B., Bray D., Lewis J. et al. Molecular biology of cells. In 3 volumes. – M.: Mir, 1994.

2) Leninger A. Fundamentals of biochemistry. In 3 volumes. – M.: Mir, 1985.

3) Strayer L. Biochemistry. In 3 volumes. – M.: Mir, 1984.

1.3. Amino acids are structural monomers of proteins. Structure, nomenclature, classification and properties of amino acids.

Amino acids(aminocarboxylic acids) are organic compounds whose molecule simultaneously contains carboxyl and amine groups. Amino acids can be considered as derivatives of carboxylic acids in which one or more hydrogen atoms are replaced by amine groups.