General properties of protein molecules. Chemical properties of proteins. What are amino acids

Squirrels are biopolymers consisting of α-amino acid residues connected to each other by peptide bonds (-CO-NH-). Proteins are part of the cells and tissues of all living organisms. Protein molecules contain 20 residues of various amino acids.

Protein structure

Proteins have an inexhaustible variety of structures.

Primary protein structure is a sequence of amino acid units in a linear polypeptide chain.

Secondary structure- this is the spatial configuration of a protein molecule, resembling a helix, which is formed as a result of twisting of the polypeptide chain due to hydrogen bonds between groups: CO and NH.

Tertiary structure- this is the spatial configuration that a polypeptide chain twisted into a spiral takes on.

Quaternary structure- These are polymer formations from several protein macromolecules.

Physical properties

The properties that proteins perform are very diverse. Some proteins dissolve in water, usually forming colloidal solutions (for example, egg white); others dissolve in dilute salt solutions; still others are insoluble (for example, proteins of integumentary tissues).

Chemical properties

Denaturation– destruction of the secondary, tertiary structure of the protein under the influence of various factors: temperature, the action of acids, salts of heavy metals, alcohols, etc.

During denaturation under the influence of external factors (temperature, mechanical stress, the action of chemical agents and other factors), a change occurs in the secondary, tertiary and quaternary structures of the protein macromolecule, that is, its native spatial structure. The primary structure, and therefore chemical composition proteins do not change. Physical properties change: solubility and ability to hydrate decrease, biological activity is lost. The shape of the protein macromolecule changes and aggregation occurs. At the same time, the activity of some groups increases, the effect of proteolytic enzymes on proteins is facilitated, and, therefore, it is more easily hydrolyzed.

In food technology, thermal denaturation of proteins is of particular practical importance, the degree of which depends on temperature, duration of heating and humidity. This must be remembered when developing heat treatment regimes for food raw materials, semi-finished products, and sometimes finished products. Thermal denaturation processes play a special role in blanching plant materials, drying grain, baking bread, and producing pasta. Protein denaturation can also be caused by mechanical action (pressure, rubbing, shaking, ultrasound). Protein denaturation is caused by the action of chemical reagents (acids, alkalis, alcohol, acetone). All these techniques are widely used in food and biotechnology.

Qualitative reactions for squirrels:

a) When the protein burns, it smells like burnt feathers.

b) Protein +HNO 3 → yellow color

c) Protein solution + NaOH + CuSO 4 → purple color

Hydrolysis

Protein + H 2 O → mixture of amino acids

Functions of proteins in nature:

· catalytic (enzymes);

· regulatory (hormones);

· structural (wool keratin, silk fibroin, collagen);

motor (actin, myosin);

transport (hemoglobin);

· spare (casein, egg albumin);

· protective (immunoglobulins), etc.

Hydration

The process of hydration means the binding of water by proteins, and they exhibit hydrophilic properties: they swell, their mass and volume increase. The swelling of the protein is accompanied by its partial dissolution. The hydrophilicity of individual proteins depends on their structure. The hydrophilic amide (–CO–NH–, peptide bond), amine (NH 2) and carboxyl (COOH) groups present in the composition and located on the surface of the protein macromolecule attract water molecules, strictly orienting them to the surface of the molecule. By surrounding protein globules, a hydration (aqueous) shell prevents the stability of protein solutions. At the isoelectric point, proteins have least ability bind water, the hydration shell around the protein molecules is destroyed, so they combine to form large aggregates. Aggregation of protein molecules also occurs when they are dehydrated using certain organic solvents, such as ethyl alcohol. This leads to the precipitation of proteins. When the pH of the environment changes, the protein macromolecule becomes charged and its hydration capacity changes.

With limited swelling, concentrated protein solutions form complex systems called jellies. Jellies are not fluid, elastic, have plasticity, a certain mechanical strength, and are able to retain their shape. Globular proteins can be completely hydrated, dissolving in water (for example, milk proteins), forming solutions with low concentrations. The hydrophilic properties of proteins have great importance in biology and Food Industry. A very mobile jelly, built mainly from protein molecules, is the cytoplasm - the semi-liquid contents of the cell. Highly hydrated jelly is raw gluten isolated from wheat dough, it contains up to 65% water. Hydrophilicity, the main quality of wheat grain, grain proteins and flour, plays a big role in the storage and processing of grain, and in baking. The dough, which is obtained in bakery production, is a protein swollen in water, a concentrated jelly containing starch grains.

Foaming

The foaming process is the ability of proteins to form highly concentrated liquid-gas systems called foams. The stability of foam, in which protein is a foaming agent, depends not only on its nature and concentration, but also on temperature. Proteins are widely used as foaming agents in the confectionery industry (marshmallows, marshmallows, souffles). Bread has a foam structure, and this affects its taste properties.

Combustion

Proteins burn to form nitrogen, carbon dioxide and water, as well as some other substances. Combustion is accompanied by the characteristic smell of burnt feathers.

Color reactions.

• Xanthoprotein – interaction of aromatic and heteroatomic cycles in a protein molecule with concentrated nitric acid occurs, accompanied by the appearance of a yellow color;
• Biuret - weakly alkaline solutions of proteins interact with a solution of copper(II) sulfate to form complex compounds between Cu 2+ ions and polypeptides. The reaction is accompanied by the appearance of a violet-blue color;
• When proteins are heated with alkali in the presence of lead salts, a black precipitate that contains sulfur precipitates.



§ 9. PHYSICAL AND CHEMICAL PROPERTIES OF PROTEINS

Proteins are very large molecules; in size they can be second only to individual representatives of nucleic acids and polysaccharides. Table 4 shows the molecular characteristics of some proteins.

Table 4

Molecular characteristics of some proteins

	Relative molecular weight	Number of circuits	Number of amino acid residues
Ribonuclease
Myoglobin
Chymotrypsin
Hemoglobin
Glutamate dehydrogenase

Protein molecules can contain the most different quantities amino acid residues - from 50 to several thousand; the relative molecular weights of proteins also vary greatly - from several thousand (insulin, ribonuclease) to a million (glutamate dehydrogenase) or more. The number of polypeptide chains in proteins can range from one to several tens and even thousands. Thus, the tobacco mosaic virus protein includes 2120 protomers.

Knowing the relative molecular weight of a protein, one can approximately estimate how many amino acid residues are included in its composition. The average relative molecular weight of the amino acids forming a polypeptide chain is 128. When a peptide bond is formed, a water molecule is eliminated, therefore, the average relative weight of an amino acid residue will be 128 - 18 = 110. Using these data, it can be calculated that a protein with a relative molecular weight of 100,000 will consist of approximately 909 amino acid residues.

Electrical properties of protein molecules

The electrical properties of proteins are determined by the presence of positively and negatively charged amino acid residues on their surface. The presence of charged protein groups determines the total charge of the protein molecule. If negatively charged amino acids predominate in proteins, then its molecule in a neutral solution will have a negative charge; if positively charged ones predominate, the molecule will have a positive charge. The total charge of a protein molecule also depends on the acidity (pH) of the medium. With an increase in the concentration of hydrogen ions (increase in acidity), the dissociation of carboxyl groups is suppressed:

and at the same time the number of protonated amino groups increases;

Thus, as the acidity of the medium increases, the number of negatively charged groups on the surface of the protein molecule decreases and the number of positively charged groups increases. A completely different picture is observed with a decrease in the concentration of hydrogen ions and an increase in the concentration of hydroxide ions. The number of dissociated carboxyl groups increases

and the number of protonated amino groups decreases

So, by changing the acidity of the medium, you can change the charge of the protein molecule. With an increase in the acidity of the environment in a protein molecule, the number of negatively charged groups decreases and the number of positively charged ones increases, the molecule gradually loses its negative charge and acquires a positive charge. When the acidity of the solution decreases, the opposite picture is observed. It is obvious that at certain pH values ​​the molecule will be electrically neutral, i.e. the number of positively charged groups will be equal to the number of negatively charged groups, and the total charge of the molecule will be equal to zero(Fig. 14).

The pH value at which the total charge of the protein is zero is called the isoelectric point and is designatedpI.

Rice. 14. In the state of the isoelectric point, the total charge of the protein molecule is zero

The isoelectric point for most proteins is in the pH range from 4.5 to 6.5. However, there are exceptions. Below are the isoelectric points of some proteins:

At pH values ​​below the isoelectric point, the protein carries a total positive charge; above it, it carries a total negative charge.

At the isoelectric point, the solubility of a protein is minimal, since its molecules in this state are electrically neutral and there are no mutual repulsion forces between them, so they can “stick together” due to hydrogen and ionic bonds, hydrophobic interactions, van der Waals forces. At pH values ​​different from pI, the protein molecules will carry the same charge - either positive or negative. As a result of this, electrostatic repulsion forces will exist between the molecules, preventing them from sticking together, and solubility will be higher.

Protein solubility

Proteins can be soluble or insoluble in water. The solubility of proteins depends on their structure, pH value, salt composition of the solution, temperature and other factors and is determined by the nature of those groups that are located on the surface of the protein molecule. Insoluble proteins include keratin (hair, nails, feathers), collagen (tendon), fibroin (click, spider web). Many other proteins are water soluble. Solubility is determined by the presence of charged and polar groups on their surface (-COO -, -NH 3 +, -OH, etc.). Charged and polar groups of proteins attract water molecules, and a hydration shell is formed around them (Fig. 15), the existence of which determines their solubility in water.

Rice. 15. Formation of a hydration shell around a protein molecule.

Protein solubility is affected by the presence of neutral salts (Na 2 SO 4, (NH 4) 2 SO 4, etc.) in solution. At low salt concentrations, protein solubility increases (Fig. 16), since under such conditions the degree of dissociation of polar groups increases and charged groups of protein molecules are shielded, thereby reducing protein-protein interaction, which promotes the formation of aggregates and protein precipitation. At high salt concentrations, protein solubility decreases (Fig. 16) due to the destruction of the hydration shell, leading to aggregation of protein molecules.

Rice. 16. Dependence of protein solubility on salt concentration

There are proteins that dissolve only in salt solutions and do not dissolve in pure water, such proteins are called globulins. There are other proteins - albumins, unlike globulins, they are highly soluble in clean water.
The solubility of proteins also depends on the pH of solutions. As we have already noted, proteins have minimal solubility at the isoelectric point, which is explained by the absence of electrostatic repulsion between protein molecules.
Under certain conditions, proteins can form gels. When a gel is formed, the protein molecules form a dense network, the internal space of which is filled with a solvent. Gels are formed, for example, by gelatin (this protein is used to make jelly) and milk proteins when making curdled milk.
Temperature also affects protein solubility. When exposed to high temperatures, many proteins precipitate due to disruption of their structure, but we will talk about this in more detail in the next section.

Protein denaturation

Let's consider a phenomenon that is well known to us. When the egg white is heated, it gradually becomes cloudy and then forms a solid curd. The coagulated egg white - egg albumin - after cooling turns out to be insoluble, while before heating the egg white was well soluble in water. The same phenomena occur when almost all globular proteins are heated. The changes that occur during heating are called denaturation. Proteins in their natural state are called native proteins, and after denaturation - denatured.
During denaturation, the native conformation of proteins is disrupted as a result of the rupture of weak bonds (ionic, hydrogen, hydrophobic interactions). As a result of this process, the quaternary, tertiary and secondary structures of the protein can be destroyed. The primary structure is preserved (Fig. 17).

Rice. 17. Protein denaturation

During denaturation, hydrophobic amino acid radicals located deep in the molecule in native proteins appear on the surface, resulting in conditions for aggregation. Aggregates of protein molecules precipitate. Denaturation is accompanied by loss of biological function of the protein.

Protein denaturation can be caused not only by elevated temperature, but also by other factors. Acids and alkalis can cause protein denaturation: as a result of their action, ionogenic groups are recharged, which leads to the breaking of ionic and hydrogen bonds. Urea destroys hydrogen bonds, which results in proteins losing their native structure. Denaturing agents are organic solvents and heavy metal ions: organic solvents destroy hydrophobic bonds, and heavy metal ions form insoluble complexes with proteins.

Along with denaturation, there is also a reverse process - renaturation. When the denaturing factor is removed, the original native structure can be restored. For example, when the solution is slowly cooled to room temperature, the native structure and biological function of trypsin is restored.

Proteins can also denature in a cell during normal life processes. It is clear that the loss of the native structure and function of proteins is an extremely undesirable event. In this regard, it is worth mentioning special proteins - chaperones. These proteins are able to recognize partially denatured proteins and, by binding to them, restore their native conformation. Chaperones also recognize proteins that have advanced in denaturation and transport them to lysosomes, where they are broken down (degraded). Chaperones play important role and in the process of formation of tertiary and quaternary structures during protein synthesis.

Interesting to know! Currently, a disease such as mad cow disease is often mentioned. This disease is caused by prions.They can cause other diseases of a neurodegenerative nature in animals and humans. Prions are infectious agents of protein nature. A prion entering a cell causes a change in the conformation of its cellular counterpart, which itself becomes a prion. This is how the disease arises. The prion protein differs from the cellular protein in its secondary structure. The prion form of the protein has mainlyb-folded structure, and cellular -a

-spiral. These are high-molecular (molecular weight varies from 5-10 thousand to 1 million or more) natural polymers, the molecules of which are built from amino acid residues connected by an amide (peptide) bond.

Proteins are also called proteins (Greek “protos” - first, important). The number of amino acid residues in a protein molecule varies greatly and sometimes reaches several thousand. Each protein has its own inherent sequence of amino acid residues.

Proteins perform a variety of biological functions: catalytic (enzymes), regulatory (hormones), structural (collagen, fibroin), motor (myosin), transport (hemoglobin, myoglobin), protective (immunoglobulins, interferon), storage (casein, albumin, gliadin) and others.

Proteins are the basis of biomembranes, the most important component of the cell and cellular components. They play a key role in the life of the cell, constituting, as it were, the material basis of its chemical activity.

The exceptional property of protein is self-organization of structure, i.e. its ability to spontaneously create a certain spatial structure characteristic only of a given protein. Essentially, all the activities of the body (development, movement, performance of various functions, and much more) are associated with protein substances. It is impossible to imagine life without proteins.

Proteins are the most important component food for humans and animals, supplier of essential amino acids.

Protein structure

In the spatial structure of proteins, the nature of the R- radicals (residues) in amino acid molecules is of great importance. Nonpolar amino acid radicals are usually located inside the protein macromolecule and cause hydrophobic interactions; polar radicals containing ionic (ion-forming) groups are usually found on the surface of a protein macromolecule and characterize electrostatic (ionic) interactions. Polar nonionic radicals (for example, containing alcohol OH groups, amide groups) can be located both on the surface and inside the protein molecule. They participate in the formation of hydrogen bonds.

In protein molecules, α-amino acids are linked to each other by peptide (-CO-NH-) bonds:

Polypeptide chains constructed in this way or individual sections within a polypeptide chain can, in some cases, be additionally linked to each other by disulfide (-S-S-) bonds or, as they are often called, disulfide bridges.

A major role in creating the structure of proteins is played by ionic (salt) and hydrogen bonds, as well as hydrophobic interaction - a special type of contact between the hydrophobic components of protein molecules in an aqueous environment. All these bonds have varying strengths and ensure the formation of a complex, large protein molecule.

Despite the difference in the structure and functions of protein substances, their elemental composition varies slightly (in% by dry weight): carbon - 51-53; oxygen - 21.5-23.5; nitrogen - 16.8-18.4; hydrogen - 6.5-7.3; sulfur - 0.3-2.5.

Some proteins contain small amounts of phosphorus, selenium and other elements.

The sequence of amino acid residues in a polypeptide chain is called primary protein structure.

A protein molecule can consist of one or more polypeptide chains, each of which contains a different number of amino acid residues. Given the number of possible combinations, the variety of proteins is almost limitless, but not all of them exist in nature.

The total number of different types of proteins in all types of living organisms is 10 11 -10 12. For proteins whose structure is extremely complex, in addition to the primary one, higher levels of structural organization are also distinguished: secondary, tertiary, and sometimes quaternary structure.

Secondary structure most proteins possess, although not always along the entire length of the polypeptide chain. Polypeptide chains with a certain secondary structure can be differently located in space.

In formation tertiary structure In addition to hydrogen bonds, ionic and hydrophobic interactions play an important role. Based on the nature of the “packaging” of the protein molecule, they are distinguished globular, or spherical, and fibrillar, or filamentous proteins (Table 12).

For globular proteins, an a-helical structure is more typical; the helices are curved, “folded.” The macromolecule has a spherical shape. They dissolve in water and saline solutions to form colloidal systems. Most proteins in animals, plants and microorganisms are globular proteins.

For fibrillar proteins, a filamentous structure is more typical. They are generally insoluble in water. Fibrillar proteins usually perform structure-forming functions. Their properties (strength, stretchability) depend on the method of packing the polypeptide chains. Examples of fibrillar proteins are myosin and keratin. In some cases, individual protein subunits form complex ensembles with the help of hydrogen bonds, electrostatic and other interactions. In this case, it is formed quaternary structure proteins.

An example of a protein with a quaternary structure is blood hemoglobin. Only with such a structure does it perform its functions - binding oxygen and transporting it to tissues and organs.

However, it should be noted that in the organization of higher protein structures, an exclusive role belongs to the primary structure.

Protein classification

There are several classifications of proteins:

1. By degree of difficulty (simple and complex).
2. According to the shape of the molecules (globular and fibrillar proteins).
3. According to solubility in individual solvents (water-soluble, soluble in dilute saline solutions - albumins, alcohol-soluble - prolamins, soluble in dilute alkalis and acids - glutelins).
4. According to the functions performed (for example, storage proteins, skeletal proteins, etc.).

Properties of proteins

Proteins are amphoteric electrolytes. At a certain pH value (called the isoelectric point), the number of positive and negative charges in the protein molecule is equal. This is one of the main properties of protein. Proteins at this point are electrically neutral, and their solubility in water is the lowest. The ability of proteins to reduce solubility when their molecules reach electrical neutrality is used for isolation from solutions, for example, in the technology for obtaining protein products.

Hydration. The process of hydration means the binding of water by proteins, and they exhibit hydrophilic properties: they swell, their mass and volume increase. The swelling of individual proteins depends solely on their structure. The hydrophilic amide (-CO-NH-, peptide bond), amine (-NH 2) and carboxyl (-COOH) groups present in the composition and located on the surface of the protein macromolecule attract water molecules, strictly orienting them on the surface of the molecule. The hydration (aqueous) shell surrounding protein globules prevents aggregation and sedimentation, and therefore contributes to the stability of protein solutions. At the isoelectric point, proteins have the least ability to bind water; the hydration shell around protein molecules is destroyed, so they combine to form large aggregates. Aggregation of protein molecules also occurs when they are dehydrated with the help of certain organic solvents, for example, ethyl alcohol. This leads to the precipitation of proteins. When the pH of the environment changes, the protein macromolecule becomes charged and its hydration capacity changes.

With limited swelling, concentrated protein solutions form complex systems called jellies.

Jellies are not fluid, elastic, have plasticity, a certain mechanical strength, and are able to retain their shape. Globular proteins can be completely hydrated and dissolved in water (for example, milk proteins), forming solutions with low concentrations. The hydrophilic properties of proteins, i.e. their ability to swell, form jellies, stabilize suspensions, emulsions and foams, are of great importance in biology and the food industry. A very mobile jelly, built mainly from protein molecules, is cytoplasm - raw gluten isolated from wheat dough; it contains up to 65% water. The different hydrophilicity of gluten proteins is one of the signs characterizing the quality of wheat grain and flour obtained from it (the so-called strong and weak wheat). The hydrophilicity of grain and flour proteins plays an important role in the storage and processing of grain and in baking. The dough, which is obtained in bakery production, is a protein swollen in water, a concentrated jelly containing starch grains.

Denaturation of proteins. During denaturation under the influence of external factors (temperature, mechanical stress, the action of chemical agents and a number of other factors), a change occurs in the secondary, tertiary and quaternary structures of the protein macromolecule, i.e. its native spatial structure. The primary structure, and therefore the chemical composition of the protein, does not change. Physical properties change: solubility and hydration ability decrease, biological activity is lost. The shape of the protein macromolecule changes and aggregation occurs. At the same time, the activity of certain chemical groups increases, the effect of proteolytic enzymes on proteins is facilitated, and therefore it is easier to hydrolyze.

In food technology, thermal denaturation of proteins is of particular practical importance, the degree of which depends on temperature, duration of heating and humidity. This must be remembered when developing heat treatment regimes for food raw materials, semi-finished products, and sometimes finished products. Thermal denaturation processes play a special role in blanching plant materials, drying grain, baking bread, and producing pasta. Protein denaturation can also be caused by mechanical action (pressure, rubbing, shaking, ultrasound). Finally, the denaturation of proteins is caused by the action of chemical reagents (acids, alkalis, alcohol, acetone). All these techniques are widely used in food and biotechnology.

Foaming. The foaming process refers to the ability of proteins to form highly concentrated liquid-gas systems called foams. The stability of foam, in which protein is a foaming agent, depends not only on its nature and concentration, but also on temperature. Proteins are widely used as foaming agents in the confectionery industry (marshmallows, marshmallows, soufflés). Bread has a foam structure, and this affects its taste.

Protein molecules, under the influence of a number of factors, can be destroyed or interact with other substances to form new products. For the food industry, two important processes can be distinguished:

1) hydrolysis of proteins under the action of enzymes;

2) interaction of amino groups of proteins or amino acids with carbonyl groups of reducing sugars.

Under the influence of protease enzymes that catalyze the hydrolytic breakdown of proteins, the latter break down into simpler products (poly- and dipeptides) and ultimately into amino acids. The rate of protein hydrolysis depends on its composition, molecular structure, enzyme activity and conditions.

Protein hydrolysis. The hydrolysis reaction with the formation of amino acids in general can be written as follows:

Combustion. Proteins burn to produce nitrogen, carbon dioxide and water, as well as some other substances. Combustion is accompanied by the characteristic smell of burnt feathers.

Color reactions to proteins. For the qualitative determination of protein, the following reactions are used:

1) xantoprotein, in which the interaction of aromatic and heteroatomic cycles in a protein molecule with concentrated nitric acid occurs, accompanied by the appearance of a yellow color.

2) biuret, in which weakly alkaline solutions of proteins interact with a solution of copper (II) sulfate to form complex compounds between Cu 2+ ions and polypeptides. The reaction is accompanied by the appearance of a violet-blue color.

Proteins are biopolymers, the monomers of which are alpha amino acid residues connected to each other through peptide bonds. The amino acid sequence of each protein is strictly defined; in living organisms it is encrypted using a genetic code, based on the reading of which the biosynthesis of protein molecules occurs. 20 amino acids are involved in the construction of proteins.

The following types of structure of protein molecules are distinguished:

1. Primary. Represents an amino acid sequence in a linear chain.
2. Secondary. This is a more compact arrangement of polypeptide chains using the formation of hydrogen bonds between peptide groups. There are two variants of the secondary structure - alpha helix and beta fold.
3. Tertiary. It is the arrangement of a polypeptide chain into a globule. In this case, hydrogen and disulfide bonds are formed, and the stabilization of the molecule is realized due to hydrophobic and ionic interactions of amino acid residues.
4. Quaternary. A protein consists of several polypeptide chains that interact with each other through non-covalent bonds.

Thus, amino acids connected in a certain sequence form a polypeptide chain, individual parts of which curl into a spiral or form folds. Such elements of secondary structures form globules, forming the tertiary structure of the protein. Individual globules interact with each other, forming complex protein complexes with a quaternary structure.

Protein classification

There are several criteria by which protein compounds can be classified. Based on their composition, simple and complex proteins are distinguished. Complex protein substances contain non-amino acid groups, the chemical nature of which can be different. Depending on this, they distinguish:

• glycoproteins;
• lipoproteins;
• nucleoproteins;
• metalloproteins;
• phosphoproteins;
• chromoproteins.

There is also a classification according to the general type of structure:

• fibrillar;
• globular;
• membrane.

Proteins are simple (single-component) proteins consisting only of amino acid residues. Depending on their solubility, they are divided into the following groups:

Protein name	Solubility	Examples
Prolamins	Slightly soluble in water, well soluble in 70-80% ethanol. Contains a large amount of arginine.	Proteins are characteristic of cereal seeds (hordein is found in barley, zein in corn).
Histones	Dissolves in weak hydrochloric acid.	They are present in the chromatin that makes up chromosomes.
Scleroproteins (protenoids)	Insoluble in water, saline liquids, diluted acids and alkalis.	Contained in supporting tissues (bones, hair, cartilage, tendons). Includes a lot of proline, alanine, glycine.
Albumin	Dissolves in water and salt solutions.	Lactoalbumin is milk protein, seroalbumin is blood serum.
Globulins	They dissolve well in low concentration salt solutions.	They are part of legumes and oilseed plants (phaseolin - bean seeds, legumin - pea seeds, etc.).
Protamines	Dissolves in acids.	They are found in considerable quantities in the germ cells of humans and animals. For example, in the sperm of fish (salmin - the salmon family).
Glutelins	Soluble in alkalis.	Contained in plants: fruits and green parts. Wheat grain gliadin, together with glutenin, forms gluten, due to which the dough becomes elastic.[ЗМ1]

Such a classification is not entirely accurate, because according to recent research, many simple proteins are associated with minimum quantity non-protein compounds. Thus, some proteins contain pigments, carbohydrates, and sometimes lipids, which makes them more like complex protein molecules.

Physicochemical properties of protein

The physicochemical properties of proteins are determined by the composition and quantity of amino acid residues contained in their molecules. Molecular masses polypeptides vary greatly: from several thousand to a million or more. Chemical properties Protein molecules are diverse, including amphotericity, solubility, and the ability to denature.

Amphotericity

Since proteins contain both acidic and basic amino acids, the molecule will always contain free acidic and free basic groups (COO- and NH3+, respectively). The charge is determined by the ratio of basic and acidic amino acid groups. For this reason, proteins are charged “+” if the pH decreases, and vice versa, “-” if the pH increases. In the case where the pH corresponds to the isoelectric point, the protein molecule will have zero charge. Amphotericity is important for biological functions, one of which is maintaining blood pH levels.

Solubility

The classification of proteins according to their solubility properties has already been given above. The solubility of protein substances in water is explained by two factors:

• charge and mutual repulsion of protein molecules;
• the formation of a hydration shell around the protein - water dipoles interact with charged groups on the outer part of the globule.

Denaturation

The physicochemical property of denaturation is the process of destruction of the secondary, tertiary structure of a protein molecule under the influence of a number of factors: temperature, the action of alcohols, salts of heavy metals, acids and other chemical agents.

Important! The primary structure is not destroyed during denaturation.

Chemical properties of proteins, qualitative reactions, reaction equations

The chemical properties of proteins can be considered using the example of reactions for their qualitative detection. Qualitative reactions make it possible to determine the presence of a peptide group in a compound:

1. Xanthoprotein. When a protein is exposed to high concentrations of nitric acid, a precipitate is formed, which turns yellow when heated.

2. Biuret. When copper sulfate acts on a weakly alkaline protein solution, complex compounds between copper ions and polypeptides, which is accompanied by the solution turning violet-blue. The reaction is used in clinical practice to determine the concentration of protein in blood serum and other biological fluids.

Another important chemical property is the detection of sulfur in protein compounds. For this purpose, an alkaline protein solution is heated with lead salts. This produces a black precipitate containing lead sulfide.

Biological significance of protein

Due to their physical and chemical properties, proteins perform a large number of biological functions, which include:

• catalytic (protein enzymes);
• transport (hemoglobin);
• structural (keratin, elastin);
• contractile (actin, myosin);
• protective (immunoglobulins);
• signaling (receptor molecules);
• hormonal (insulin);
• energy.

Proteins are important for the human body because they participate in the formation of cells, provide muscle contraction in animals, and carry many proteins together with blood serum. chemical compounds. In addition, protein molecules are a source of essential amino acids and perform a protective function, participating in the production of antibodies and the formation of immunity.

TOP 10 little-known facts about protein

1. Proteins began to be studied in 1728, when the Italian Jacopo Bartolomeo Beccari isolated protein from flour.
2. Recombinant proteins are now widely used. They are synthesized by modifying the genome of bacteria. In particular, insulin, growth factors and other protein compounds that are used in medicine are obtained in this way.
3. Protein molecules have been discovered in Antarctic fish that prevent blood from freezing.
4. The resilin protein is ideally elastic and is the basis for the attachment points of insect wings.
5. The body has unique chaperone proteins that are capable of restoring the correct native tertiary or quaternary structure of other protein compounds.
6. In the cell nucleus there are histones - proteins that take part in chromatin compaction.
7. The molecular nature of antibodies - special protective proteins (immunoglobulins) - began to be actively studied in 1937. Tiselius and Kabat used electrophoresis and proved that in immunized animals the gamma fraction was increased, and after absorption of the serum by the provoking antigen, the distribution of proteins among the fractions returned to the picture of the intact animal.
8. Egg white - shining example implementation of the reserve function by protein molecules.
9. In a collagen molecule, every third amino acid residue is formed by glycine.
10. In the composition of glycoproteins, 15-20% are carbohydrates, and in the composition of proteoglycans their share is 80-85%.

Conclusion

Proteins are the most complex compounds, without which it is difficult to imagine the life of any organism. More than 5,000 protein molecules have been identified, but each individual has its own set of proteins and this distinguishes it from other individuals of its species.

The most important chemical and physical properties of proteins updated: March 21, 2019 by: Scientific Articles.Ru

On the physical properties of proteins such as ionization,hydration, solubility Various methods for the isolation and purification of proteins are based.

Since proteins contain ionic, i.e. amino acid residues capable of ionization (arginine, lysine, glutamic acid, etc.), therefore, they are polyelectrolytes. With acidification, the degree of ionization of anionic groups decreases, and that of cationic groups increases; with alkalization, the opposite pattern is observed. At a certain pH, the number of negatively and positively charged particles becomes equal, this state is called isoelectric(the total charge of the molecule is zero). The pH value at which the protein is in an isoelectric state is called isoelectric point and denote pI. One of the methods for their separation is based on the different ionization of proteins at a certain pH value - the method electrophoresis.

Polar groups of proteins (ionic and nonionic) are able to interact with water and become hydrated. The amount of water associated with protein reaches 30-50 g per 100 g of protein. There are more hydrophilic groups on the surface of the protein. Solubility depends on the number of hydrophilic groups in the protein, on the size and shape of the molecules, and on the magnitude of the total charge. The combination of all these physical properties of the protein makes it possible to use the method molecular sieves or gel filtration for protein separation. Method dialysis used to purify proteins from low molecular weight impurities and is based on the large size of protein molecules.

The solubility of proteins also depends on the presence of other solutes, for example, neutral salts. At high concentrations of neutral salts, proteins precipitate, and for precipitation ( salting out) different proteins require different concentrations of salt. This is due to the fact that charged protein molecules adsorb ions of opposite charge. As a result, the particles lose their charges and electrostatic repulsion, resulting in protein precipitation. The salting out method can be used to fractionate proteins.

6. Structural organization protein molecules

Proteins are very large molecules, the molar mass of proteins ranges from 6000 to 1 million grams/mol (see table).

Some proteins may contain chemical groups of a non-protein nature. Such proteins are called complex or holoproteins. The non-amino acid part of proteins is called prosthetic group, the protein part - apoenzyme. Complex proteins are classified by prosthetic group. For example, lipoproteins are proteins containing a group - lipid; metalloproteins contain metal ions; Chromoproteins contain a chromophore, a colored group of non-protein nature. A special case when the chromophore is heme. These proteins include hemoglobin and cytochromes. Prosthetic groups play an important role in the functioning of complex proteins.

Simple proteins can be classified according to the shape of their molecules and their ability to dissolve in water. globular And fibrillar. Globular proteins are globule-shaped and generally soluble in water . Fibrillar proteins have the shape of an elongated fiber - fibrils and are insoluble in water. Fibrillar proteins perform mainly supporting functions, providing tissue strength; globular proteins are more diverse in function.

6.1. Primary structure of proteins

Primary protein structure call the composition and sequence of amino acid residues in a protein molecule. Amino acids in protein are linked by peptide bonds.

All molecules of a given individual protein are identical in amino acid composition, sequence of amino acid residues and length of the polypeptide chain. Establishing the amino acid sequence of proteins is a labor-intensive task. We will talk about this topic in more detail at the seminar. Insulin was the first protein for which the amino acid sequence was determined. Bovine insulin has a molar mass of about 5700. Its molecule consists of two polypeptide chains: an A chain containing 21 aa, and a B chain containing 30 aa, these two chains are connected by two disulfide (-S-S-) connections. Even small changes in the primary structure can significantly change the properties of a protein. Sickle cell disease is the result of a change in just 1 amino acid in the hemoglobin b-chain (Glu ® Val).

Species specificity of the primary structure

When studying amino acid sequences homologous proteins isolated from different types, several important conclusions were drawn. Homologous proteins are those proteins that perform the same functions in different species. An example is hemoglobin: in all vertebrates it performs the same function related to oxygen transport. Homologous proteins from different species usually have polypeptide chains of the same or nearly the same length. In the amino acid sequences of homologous proteins, the same amino acids are always found in many positions - they are called invariant remainders. However, significant differences are observed in other positions of proteins: in these positions, amino acids vary from species to species; These amino acid residues are called variable. The entire set of similarities in the amino acid sequences of homologous proteins is combined into the concept sequence homology. The presence of such homology suggests that the animals from which homologous proteins were isolated have a common evolutionary origin. An interesting example is a complex protein - cytochrome c- a mitochondrial protein that participates as an electron carrier in biological oxidation processes. M » 12500, contains » 100 a.k. A.K. were installed. sequences for 60 species. 27 a.k. - are the same, this indicates that all these residues play an important role in determining the biological activity of cytochrome c. The second important conclusion drawn from the analysis of amino acid sequences is that the number of residues at which cytochrome c differs from any two species is proportional to the phylogenetic difference between these species. For example, the cytochrome c molecules of horse and yeast differ in 48 aa, in duck and chicken - in 2 aa, and in chicken and turkey they do not differ. Information on the number of differences in the amino acid sequences of homologous proteins from different species is used to construct evolutionary maps reflecting the successive stages of origin and development various types animals and plants in the process of evolution.

Spatial arrangement of polypeptide chains (Conformation of peptide chains in proteins)

The term conformation is used to describe the spatial arrangement of substituent groups in an organic molecule that can freely change their position in space without breaking any bonds.

The peptide chain has significant flexibility. As a result, within chain interactions it acquires a certain spatial structure (conformation). In proteins, there are two levels of spatial organization for one polypeptide chain: secondary and tertiary protein structures. For proteins containing several polypeptide chains, it is possible to consider the spatial arrangement of these chains relative to each other - the quaternary structure of the protein.