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 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:
- Primary. Represents an amino acid sequence in a linear chain.
- 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.
- 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.
- 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 general type buildings:
- 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:
Such a classification is not entirely accurate, because according to recent research, many simple proteins are associated with a minimal amount of non-protein compounds. Thus, some proteins contain pigments, carbohydrates, and sometimes lipids, which makes them more like complex protein molecules.
Physicochemical properties of protein
Physicochemical characteristics proteins are determined by the composition and quantity of amino acid residues contained in their molecules. The molecular weights of polypeptides vary greatly: from several thousand to a million or more. The chemical properties of protein molecules are varied, 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 a weakly alkaline protein solution is exposed to copper sulfate, complex compounds are formed 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
Thanks to his physical and chemical properties proteins perform a large number of biological functions, the list of which includes:
- 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 transport many chemical compounds together with blood serum. 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
- Proteins began to be studied in 1728, when the Italian Jacopo Bartolomeo Beccari isolated protein from flour.
- 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.
- Protein molecules have been discovered in Antarctic fish that prevent blood from freezing.
- The resilin protein is ideally elastic and is the basis for the attachment points of insect wings.
- The body has unique chaperone proteins that are capable of restoring the correct native tertiary or quaternary structure of other protein compounds.
- In the cell nucleus there are histones - proteins that take part in chromatin compaction.
- 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.
- Egg white is a striking example of the implementation of a reserve function by protein molecules.
- In a collagen molecule, every third amino acid residue is formed by glycine.
- 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: October 29, 2018 by: Scientific Articles.Ru
Squirrels- 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 great importance has the character of R- radicals (residues) in amino acid molecules. 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.
Ionic (salt) and hydrogen bonds, as well as hydrophobic interaction, play a major role in creating the structure of proteins - special kind contacts between 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 characterized by exceptional complexity, in addition to the primary one, more high levels structural organization: secondary, tertiary, and sometimes quaternary structures.
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:
- By degree of difficulty (simple and complex).
- According to the shape of the molecules (globular and fibrillar proteins).
- According to solubility in individual solvents (water-soluble, soluble in dilute saline solutions - albumins, alcohol-soluble - prolamins, soluble in dilute alkalis and acids - glutelins).
- 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 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, 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 a certain plasticity mechanical strength, are able to maintain 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 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 chemical composition proteins do 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 special practical significance has thermal denaturation of proteins, 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. Hydrolysis reaction to form amino acids in general view can be written like this:
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 occurs with concentrated nitric acid, 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.
Isoelectric point
Amphotericity - acid-base properties of proteins.
Quaternary structure
Many proteins are composed of several subunits (protomers), which may have the same or different amino acid composition. In this case, the proteins have quaternary structure. Proteins usually contain an even number of subunits: two, four, six. The interaction occurs due to ionic, hydrogen bonds, and van der Waals forces. Adult human hemoglobin HbA consists of four pairwise identical subunits ( A 2 β 2).
The quaternary structure provides many biological benefits:
a) there is a saving of genetic material, the length of the structural gene and mRNA, in which information about the primary structure of the protein is recorded, decreases.
b) it is possible to replace subunits, which allows you to change activity
enzyme in connection with changing conditions (to adapt). Hemoglobin
newborn consists of proteins ( A 2 γ 2) . but during the first months the composition becomes like that of an adult (a 2 β 2) .
8.4. Physicochemical properties of protein
Proteins, like amino acids, are amphoteric compounds and have buffering properties.
Proteins can be divided into neutral, acidic and basic.
Neutral proteins contain an equal number of groups prone to ionization: acidic and basic. The isoelectric point of such proteins is in an environment close to neutral if the pH< pI , то белок становится положительно заряженным катионом, pH >pI, then the protein becomes a negatively charged anion.
NH 3 - protein - COOH<-->+ NH 3 - protein - COO –<-->NH 2 - protein - COO –
pH< pI aqueous solution I pH > pI
Acidic proteins contain unequal number of groups prone to ionization: there are more carboxyl groups than amino groups. In an aqueous solution, they acquire a negative charge, and the solution becomes acidic. When acid (H +) is added, the protein first enters the isoelectric point, and then, in excess of acid, it is converted into a cation. In an alkaline environment, such a protein is negatively charged (the charge of the amino group disappears).
Acidic protein
NH 3 - protein - COO – + H + + NH 3 - protein - COO – + H + + NH 3 - protein - COOH
| <--> | <--> |
COO – COON COOH
Aqueous solution pH = p I pH< pI
Protein in excess acid
positively charged
Acidic protein in an alkaline environment is negatively charged
NH 3 - protein - COO – OH – NH 2 - protein - COO –
| <--> |
COO – COO –
pH > pI
Basic proteins contain unequal number of groups prone to ionization: there are more amino groups than carboxyl groups. In an aqueous solution, they acquire a positive charge, and the solution becomes alkaline. When alkali (OH –) is added, the protein first enters the isoelectric point, and then, in excess of alkali, it turns into an anion. In an acidic environment, such a protein is positively charged (the charge of the carboxyl group disappears)
Protein Molecule Shape. Studies of the native conformation of protein molecules have shown that these particles in most cases have a more or less asymmetrical shape. Depending on the degree of asymmetry, i.e. the relationship between the long (b) and short (a) axes of the protein molecule, globular (spherical) and fibrillar (thread-like) proteins are distinguished.
Globular are protein molecules in which the folding of polypeptide chains has led to the formation of a spherical structure. Among them there are strictly spherical, ellipsoidal and rod-shaped. They differ in the degree of asymmetry. For example, egg albumin has b/a = 3, wheat gliadin - 11, and corn zein - 20. Many proteins in nature are globular.
Fibrillar proteins form long, highly asymmetrical filaments. Many of them serve a structural or mechanical function. These are collagen (b/a - 200), keratins, fibroin.
The proteins of each group have their own characteristic properties. Many globular proteins are soluble in water and dilute saline solutions. Soluble fibrillar proteins are characterized by very viscous solutions. Globular proteins, as a rule, have good biological value - they are absorbed during digestion, while many fibrillar proteins are not.
There is no clear boundary between globular and fibrillar proteins. A number of proteins occupy an intermediate position and combine the characteristics of both globular and fibrillar. Such proteins include, for example, muscle myosin (b/a = 75) and blood fibrinogen (b/a = 18). Myosin has a rod-shaped form, similar to the shape of fibrillar proteins, however, like globular proteins, it is soluble in saline solutions. Solutions of myosin and fibrinogen are viscous. These proteins are absorbed during the digestion process. At the same time, actin, a globular muscle protein, is not absorbed.
Protein Denaturation. The native conformation of protein molecules is not rigid, it is quite labile (Latin “labilis” - sliding) and can be seriously disrupted under a number of influences. Violation of the native conformation of a protein, accompanied by a change in its native properties without breaking the peptide bonds, is called denaturation (Latin “denaturare” - deprive of natural properties) of the protein.
Denaturation of proteins can be caused by various reasons, leading to disruption of weak interactions, as well as to the rupture of disulfide bonds that stabilize their native structure.
Heating of most proteins to temperatures above 50°C, as well as ultraviolet and other types of high-energy irradiation, increases vibrations of the atoms of the polypeptide chain, which leads to disruption of various bonds in them. Even mechanical shaking can cause protein denaturation.
Protein denaturation also occurs due to chemical exposure. Strong acids or alkalis affect the ionization of acidic and basic groups, causing disruption of ionic and some hydrogen bonds in protein molecules. Urea (H 2 N-CO-NH 2) and organic solvents - alcohols, phenols, etc. - disrupt the system of hydrogen bonds and weaken hydrophobic interactions in protein molecules (urea - due to disruption of the structure of water, organic solvents - due to establishing contacts with non-polar amino acid radicals). Mercaptoethanol breaks down disulfide bonds in proteins. Heavy metal ions disrupt weak interactions.
During denaturation, the properties of the protein change and, first of all, its solubility decreases. For example, when boiling, proteins coagulate and precipitate from solutions in the form of clots (as when boiling chicken egg). Precipitation of proteins from solutions also occurs under the influence of protein precipitants, which include trichloroacetic acid, Barnstein's reagent (a mixture of sodium hydroxide with copper sulfate), tannin solution, etc.
During denaturation, the water absorption capacity of the protein decreases, i.e. its ability to swell; New chemical groups may appear, for example: when exposed to captoethanol - SH groups. As a result of denaturation, the protein loses its biological activity.
Although primary structure The protein is not affected by denaturation; the changes are irreversible. However, for example, when urea is gradually removed by dialysis from a denatured protein solution, its renaturation occurs: the native structure of the protein is restored, and with it, to one degree or another, its native properties. This denaturation is called reversible.
Irreversible denaturation of proteins occurs during the aging process of organisms. Therefore, for example, plant seeds, even under optimal storage conditions, gradually lose their viability.
Denaturation of proteins occurs when baking bread, drying pasta, vegetables, during cooking, etc. As a result, the biological value of these proteins increases, since denatured (partially destroyed) proteins are more easily absorbed during the digestion process.
Isoelectric point of a protein. Proteins contain various basic and acidic groups that have the ability to ionize. In a strongly acidic environment, the main groups (amino groups, etc.) are actively protonated, and protein molecules acquire a total positive charge, and in a strongly alkaline environment, carboxyl groups easily dissociate, and protein molecules acquire a total negative charge.
The sources of positive charge in proteins are the side radicals of lysine, arginine and histidine residues, and the α-amino group of the N-terminal amino acid residue. Sources of negative charge are side radicals of aspartic and glutamic acid residues, and the a-carboxyl group of the C-terminal amino acid residue.
At a certain pH value of the medium, equality of positive and negative charges on the surface of the protein molecule is observed, i.e., its total electrical charge turns out to be zero. The pH value of the solution at which the protein molecule is electrically neutral is called the isoelectric point of the protein (pi).
Isoelectric points are characteristic constants of proteins. They are determined by their amino acid composition and structure: the number and location of acidic and basic amino acid residues in polypeptide chains. The isoelectric points of proteins, in which acidic amino acid residues predominate, are located in the pH region<7, а белков, в которых преобладают остатки основных аминокислот - в области рН>7. The isoelectric points of most proteins are in a slightly acidic environment.
In the isoelectric state, protein solutions have minimal viscosity. This is due to a change in the shape of the protein molecule. At the isoelectric point, oppositely charged groups attract each other, and the proteins curl into balls. When the pH shifts from the isoelectric point, like-charged groups repel each other and the protein molecules unfold. In the unfolded state, protein molecules give solutions a higher viscosity than when rolled into balls.
At the isoelectric point, proteins have minimal solubility and can easily precipitate.
However, precipitation of proteins at the isoelectric point still does not occur. This is prevented by structured water molecules, which retain a significant portion of hydrophobic amino acid radicals on the surface of protein globules.
Proteins can be precipitated using organic solvents (alcohol, acetone), which disrupt the system of hydrophobic contacts in protein molecules, as well as high concentrations of salts (salting out method), which reduce the hydration of protein globules. In the latter case, part water is coming on the dissolution of salt and ceases to participate in the dissolution of protein. Due to the lack of solvent, such a solution becomes supersaturated, which entails the precipitation of part of it. Protein molecules begin to stick together and, forming increasingly larger particles, gradually precipitate from the solution.
Optical properties of protein. Protein solutions have optical activity, i.e. the ability to rotate the plane of polarization of light. This property of proteins is due to the presence of asymmetry elements in their molecules - asymmetric carbon atoms and a right-handed α-helix.
When a protein denatures, its optical properties change, which is associated with the destruction of the α-helix. The optical properties of completely denatured proteins depend only on the presence of asymmetric carbon atoms in them.
By the difference in the optical properties of a protein before and after denaturation, the degree of its helicalization can be determined.
Qualitative reactions to proteins. Proteins are characterized by color reactions due to the presence of certain chemical groups in them. These reactions are often used to detect proteins.
When copper sulfate and alkali are added to a protein solution, a lilac color appears due to the formation of complexes of copper ions with the peptide groups of the protein. Since this reaction is produced by biuret (H 2 N-CO-NH-CO-NH 2), it is called biuret. It is often used for the quantitative determination of protein, along with the I. Kjeldahl method, since the intensity of the resulting color is proportional to the protein concentration in the solution.
When protein solutions are heated with concentrated nitric acid, a yellow color appears due to the formation of nitro derivatives of aromatic amino acids. This reaction is called xanthoprotein(Greek “xanthos” - yellow).
When heated, many protein solutions react with mercury nitrate solution, which forms crimson-colored complex compounds with phenols and their derivatives. This is a qualitative Millon reaction to tyrosine.
As a result of heating most protein solutions with lead acetate in an alkaline environment, a black precipitate of lead sulfide precipitates. This reaction is used to detect sulfur-containing amino acids and is called the Foll reaction.
Before talking about the properties of proteins, it is worth giving short definition this concept. These are high-molecular organic substances that consist of alpha-amino acids connected by a peptide bond. Proteins are important part nutrition of humans and animals, since not all amino acids are produced by the body - some come from food. What are their properties and functions?
Amphotericity
This is the first feature of proteins. Amphotericity refers to their ability to exhibit both acidic and basic properties.
Proteins in their structure have several types of chemical groups that are capable of ionizing H 2 O in solution. These include:
- Carboxyl residues. Glutamic and aspartic acids, to be more precise.
- Nitrogen containing groups. The ε-amino group of lysine, the arginine residue CNH(NH 2) and the imidazole residue of a heterocyclic alpha amino acid called histidine.
Each protein has such a feature as an isoelectric point. This concept refers to the acidity of the environment, in which the surface or molecule does not have electric charge. Under these conditions, protein hydration and solubility are minimized.
The indicator is determined by the ratio of basic and acidic amino acid residues. In the first case, the point falls on the alkaline region. In the second - sour.
Solubility
Based on this property, proteins are divided into a small classification. Here's what they are like:
- Soluble. They are called albumins. They are moderately soluble in concentrated salt solutions and coagulate when heated. This reaction is called denaturation. The molecular weight of albumins is about 65,000. They do not contain carbohydrates. And substances that consist of albumin are called albuminoids. These include egg whites, plant seeds and blood serum.
- Insoluble. They are called scleroproteins. A striking example is keratin, a fibrillar protein with mechanical strength second only to chitin. It is this substance that makes up nails, hair, the rhamphotheca of bird beaks and feathers, as well as the horns of a rhinoceros. This group of proteins also includes cytokeratins. This is the structural material of the intracellular filaments of the cytoskeleton of epithelial cells. Another insoluble protein includes a fibrillar protein called fibroin.
- Hydrophilic. They actively interact with water and absorb it. These include proteins of the intercellular substance, nucleus and cytoplasm. Including the notorious fibroin and keratin.
- Hydrophobic. They repel water. These include proteins that are components of biological membranes.
Denaturation
This is the name given to the process of modification of a protein molecule under the influence of certain destabilizing factors. However, the amino acid sequence remains the same. But proteins lose their natural properties (hydrophilicity, solubility, etc.).
It is worth noting that any significant change in external conditions can lead to disturbances in protein structures. Most often, denaturation is provoked by an increase in temperature, as well as the effect of alkali, strong acid, radiation, salts of heavy metals and even certain solvents on the protein.
Interestingly, denaturation often leads to protein particles aggregating into larger ones. A striking example is, for example, scrambled eggs. Everyone knows how, during the frying process, protein is formed from a clear liquid.
We should also talk about such a phenomenon as renaturation. This process is the reverse of denaturation. During it, proteins return to their natural structure. And it really is possible. A group of chemists from the USA and Australia have found a way to renature a hard-boiled egg. This will only take a few minutes. And for this you will need urea (carbonic acid diamide) and centrifugation.
Structure
It is necessary to talk about it separately, since we're talking about about the importance of proteins. There are four levels of structural organization in total:
- Primary. Refers to the sequence of amino acid residues in a polypeptide chain. The main feature is conservative motives. This is the name given to stable combinations of amino acid residues. They are found in many complex and simple proteins.
- Secondary. This refers to the ordering of any local fragment of a polypeptide chain, which is stabilized by hydrogen bonds.
- Tertiary. This designates the spatial structure of the polypeptide chain. This level consists of some secondary elements (they are stabilized different types interactions, where hydrophobic ones are the most important). Here, ionic, hydrogen, and covalent bonds participate in stabilization.
- Quaternary. It is also called domain or subunit. This level consists of the relative arrangement of polypeptide chains as part of a whole protein complex. It is interesting that proteins with a quaternary structure contain not only identical, but also different chains of polypeptides.
This division was proposed by a Danish biochemist named K. Lindström-Lang. And even if it is considered outdated, they still continue to use it.
Types of structure
When talking about the properties of proteins, it should also be noted that these substances are divided into three groups according to the type of structure. Namely:
- Fibrillar proteins. They have a thread-like elongated structure and a large molecular weight. Most of them are not soluble in water. The structure of these proteins is stabilized by interactions between polypeptide chains (they consist of at least two amino acid residues). It is fibrillar substances that form the polymer, fibrils, microtubules and microfilaments.
- Globular proteins. The type of structure determines their solubility in water. A general shape molecules are spherical.
- Membrane proteins. The structure of these substances has an interesting feature. They have domains that cross the cell membrane, but parts of them protrude into the cytoplasm and intercellular environment. These proteins play the role of receptors - they transmit signals and are responsible for the transmembrane transport of nutrients. It is important to note that they are very specific. Each protein allows only a specific molecule or signal to pass through.
Simple
You can also tell us a little more about them. Simple proteins consist only of polypeptide chains. These include:
- Protamine. Nuclear low molecular weight protein. Its presence protects DNA from the action of nucleases - enzymes that attack nucleic acids.
- Histones. Strongly basic simple proteins. They are concentrated in the nuclei of plant and animal cells. They take part in the “packaging” of DNA strands in the nucleus, as well as in processes such as repair, replication and transcription.
- Albumin. They have already been discussed above. The most famous albumins are whey and egg.
- Globulin. Participates in blood clotting, as well as other immune reactions.
- Prolamins. These are the reserve proteins of cereals. Their names are always different. In wheat they are called ptyalins. In barley - hordeins. Oats have avsnins. Interestingly, prolamins are divided into their own protein classes. There are only two of them: S-rich (with sulfur content) and S-poor (without it).
Complex
What about complex proteins? They contain prosthetic groups or those without amino acids. These include:
- Glycoproteins. They contain carbohydrate residues with covalent bonds. These complex proteins are the most important structural components cell membranes. These also include many hormones. And glycoproteins of erythrocyte membranes determine blood type.
- Lipoproteins. They consist of lipids (fat-like substances) and play the role of “transport” of these substances in the blood.
- Metalloproteins. These proteins are of great importance in the body, since without them iron metabolism does not occur. Their molecules contain metal ions. And typical representatives of this class are transferrin, hemosiderin and ferritin.
- Nucleoproteins. They consist of RKN and DNA that do not have a covalent bond. A striking representative is chromatin. It is in its composition that genetic information is realized, DNA is repaired and replicated.
- Phosphoproteins. They consist of phosphoric acid residues linked covalently. An example is casein, which is initially contained in milk as a calcium salt (in bound form).
- Chromoproteins. They have a simple structure: a protein and a colored component belonging to the prosthetic group. They take part in cellular respiration, photosynthesis, redox reactions, etc. Also, without chromoproteins, energy accumulation does not occur.
Metabolism
Much has already been said above about the physicochemical properties of proteins. Their role in metabolism also needs to be mentioned.
There are amino acids that are essential because they are not synthesized by living organisms. Mammals themselves obtain them from food. During its digestion, the protein is destroyed. This process begins with denaturation when it is placed in an acidic environment. Then - hydrolysis, in which enzymes participate.
Certain amino acids that the body ultimately receives are involved in the process of protein synthesis, the properties of which are necessary for its full existence. And the remainder is processed into glucose - a monosaccharide, which is one of the main sources of energy. Protein is very important when dieting or fasting. If it is not supplied with food, the body will begin to “eat itself” - process its own proteins, especially muscle proteins.
Biosynthesis
When considering the physicochemical properties of proteins, it is necessary to focus on such a topic as biosynthesis. These substances are formed on the basis of the information encoded in genes. Any protein is a unique sequence of amino acid residues determined by the gene encoding it.
How does this happen? A gene that codes for a protein transfers information from DNA to RNA. This is called transcription. In most cases, synthesis then occurs on ribosomes - this is the most important organelle of a living cell. This process is called translation.
There is also the so-called non-ribosomal synthesis. It is also worth mentioning, since we are talking about the importance of proteins. This type of synthesis is observed in some bacteria and lower fungi. The process is carried out through a high molecular weight protein complex (known as NRS synthase), and ribosomes do not take part in this.
And, of course, there is also chemical synthesis. It can be used to synthesize short proteins. Methods such as chemical ligation are used for this. This is the opposite of the notorious biosynthesis on ribosomes. The same method can be used to obtain inhibitors of certain enzymes.
In addition, thanks to chemical synthesis, it is possible to introduce into proteins those amino acid residues that are not found in ordinary substances. Let us accept those whose side chains have fluorescent labels.
It is worth mentioning that the methods of chemical synthesis are not flawless. There are certain restrictions. If a protein contains more than 300 residues, then the artificially synthesized substance will most likely have the wrong structure. And this will affect the properties.
Substances of animal origin
They need to be considered Special attention. Animal protein is a substance found in eggs, meat, dairy products, poultry, seafood and fish. They contain all the amino acids needed by the body, including the 9 essential ones. Here are a number of the most important functions that animal protein performs:
- Catalysis of multitudes chemical reactions. This substance launches them and accelerates them. Enzymatic proteins are “responsible” for this. If the body does not receive enough of them, then oxidation and reduction, the joining and breaking of molecular bonds, as well as the transportation of substances will not proceed fully. It is interesting that only a small part of amino acids enter into various types of interactions. And an even smaller amount (3-4 residues) is directly involved in catalysis. All enzymes are divided into six classes - oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases. Each of them is responsible for one or another reaction.
- Formation of the cytoskeleton, which forms the structure of cells.
- Immune, chemical and physical protection.
- Transporting important components necessary for cell growth and development.
- Transmission of electrical impulses that are important for the functioning of the whole organism, since without them cell interaction is impossible.
And these are not all possible functions. But even so, the significance of these substances is clear. Protein synthesis in cells and in the body is impossible if a person does not eat its sources. And they are turkey meat, beef, lamb, rabbit. A lot of protein is also found in eggs, sour cream, yogurt, cottage cheese, and milk. You can also activate protein synthesis in the body's cells by adding ham, offal, sausage, stew and veal to your diet.
