Protein: structure and function. Properties of proteins

As you know, proteins are the basis for the birth of life on our planet. According to the theory of Oparin-Haldane, it is the coacervate drop consisting of peptide molecules that became the basis for the birth of the living. This is unquestionable, because the analysis of the internal composition of any representative of biomass shows that these substances exist in everything: plants, animals, microorganisms, fungi, viruses. And they are very diverse and macromolecular in nature.

The names of these structures are four, they are all synonymous:

• Proteins;
• Proteins;
• Polypeptides;
• Peptides.

Protein Molecules

Their number is truly innumerable. Moreover, all protein molecules can be divided into two large groups:

• Simple - consist only of amino acid sequences linked by peptide bonds;
• Complex - the structure and structure of the protein are characterized by additional protolytic (prosthetic) groups, called cofactors.

In this case, complex molecules also have their own classification.

Gradation of complex peptides

1. Glycoproteins are closely related protein and carbohydrate compounds. The structure of the molecule is interwoven with prosthetic groups of mucopolysaccharides.
2. Lipoproteins - a complex compound of protein and lipid.
3. Metalloproteids - as a prosthetic group are metal ions (iron, manganese, copper and others).
4. Nucleoproteins - the relationship of protein and nucleic acids (DNA, RNA).
5. Phosphoproteins - the conformation of protein and the residue of orthophosphoric acid.
6. Chromoproteins are very similar to metalloproteids, but the element that is part of the prosthetic group is an entire colored complex (red - hemoglobin, green - chlorophyll, etc.).

In each group examined, the structure and properties of proteins are different. The functions they perform also vary depending on the type of molecule.

Chemical structure of proteins

From this point of view, proteins are a long, massive chain of amino acid residues linked together by specific bonds called peptide bonds. From the side structures of acids branch off - radicals. This structure of the molecule was discovered by E. Fisher at the beginning of the 21st century.

Later proteins, structure and functions of proteins were studied in more detail. It became clear that the amino acids that make up the peptide structure are only 20, but they can be combined in a variety of ways. Hence the diversity of polypeptide structures. In addition, in the process of living and performing their functions, proteins are able to undergo a number of chemical transformations. As a result, they change the structure, and a completely new type of connection appears.

To break the peptide bond, that is, to break the protein, the structure of the chains, it is necessary to select very stringent conditions (the action of high temperatures, acids or alkalis, a catalyst). This is due to the high strength of the covalent bonds in the molecule, namely in the peptide group.

Detection of the protein structure under laboratory conditions is carried out with the help of biuret reaction - exposure to the polypeptide with freshly precipitated copper (II) hydroxide . The complex of the peptide group and the copper ion gives a bright violet color.

There are four main structural organizations, each of which has its own peculiarities in the structure of proteins.

Levels of organization: primary structure

As mentioned above, the peptide is a sequence of amino acid residues with inclusions, coenzymes or without them. So the primary structure is called a molecule structure, which is natural, natural, is truly amino acids linked by peptide bonds, and nothing more. That is, the polypeptide is linear. In this case, the peculiarities of the structure of proteins of such a plan are that such a combination of acids is the determining factor for the function of the protein molecule. Due to the presence of these features it is possible not only to identify the peptide, but also to predict the properties and role of a completely new, not yet open. Examples of peptides possessing a natural primary structure are insulin, pepsin, chymotrypsin and others.

Secondary conformation

The structure and properties of proteins in this category vary slightly. Such a structure can be formed initially from nature, either when exposed to primary hard hydrolysis, temperature, or other conditions.

This conformation has three varieties:

1. Smooth, regular, stereoregular turns, built from amino acid residues, which twist around the main axis of the joint. They are retained together only by hydrogen bonds arising between the oxygen of one peptide group and the other hydrogen. And the structure is considered correct because the turns are evenly repeated every 4 links. Such a structure can be either left-handed or right-wound. But in most known proteins, the dextrorotatory isomer predominates. Such conformation is usually called alpha structures.
2. The composition and structure of the proteins of the next type differs from the previous one in that the hydrogen bonds are formed not between a number of residues on one side of the molecule but between considerably distant ones, and a sufficiently large distance. For this reason, the whole structure takes the form of several undulating, serpentine-polypeptide chains. There is one feature that a protein should exhibit. The structure of amino acids on the branches should be as short as possible, as in glycine or alanine, for example. This type of secondary conformation is called beta sheets for the ability to stick together when forming a common structure.
3. The structure of the protein belonging to the third type denotes as complex, scattered, disordered fragments that do not possess stereoregularity and are capable of changing the structure under the influence of external conditions.

Examples of proteins that have a secondary structure from nature have not been identified.

Tertiary education

This is a rather complex conformation, called the "globule." What is such a protein? Its structure is based on the secondary structure, but new types of interactions between the grouping atoms are added, and the entire molecule seems to be folded, thus orienting itself to ensure that hydrophilic groups are directed into the globule and hydrophobic groupings are directed outwards.

This explains the charge of the protein molecule in colloidal water solutions. What types of interactions are there?

1. Hydrogen bonds remain unchanged between the same parts as in the secondary structure.
2. Hydrophobic (hydrophilic) interactions - arise when the polypeptide is dissolved in water.
3. Ionic attraction - formed between the differently charged groups of amino acid residues (radicals).
4. Covalent interactions - can form between specific acid sites - molecules of cysteine, or rather, their tails.

Thus, the composition and structure of proteins possessing a tertiary structure can be described as polypeptide chains folded into globules, which retain and stabilize their conformation through different types of chemical interactions. Examples of such peptides are: phosphoglycerate kenase, tRNA, alpha-keratin, fibroin silk, and others.

Quaternary structure

This is one of the most complex globules that proteins form. The structure and functions of proteins of this kind are very multifaceted and specific.

What is such a conformation? This is a few (in some cases dozens) of large and small polypeptide chains that are formed independently of each other. But then, due to the same interactions that we considered for the tertiary structure, all these peptides are twisted and intertwined. In this way, complex conformational globules are obtained that can contain both metal atoms, lipid groups, and carbohydrate groups. Examples of such proteins are: DNA polymerase, tobacco envelope protein, hemoglobin, and others.

All the peptide structures examined by us have their own identification methods in the laboratory, based on modern possibilities of using chromatography, centrifugation, electronic and optical microscopy and high computer technologies.

Performed functions

The structure and functions of proteins are closely correlated with each other. That is, each peptide plays a certain role, unique and specific. There are also those who are able to perform several significant operations in one living cell. However, it is possible to express in a generalized form the basic functions of protein molecules in the organisms of living beings:

1. Providing traffic. Unicellular organisms, or organelles, or some types of cells are capable of movement, contraction, movement. This is provided by proteins that are part of the structure of their motor apparatus: cilia, flagella, cytoplasmic membrane. If we talk about inability to move cells, the proteins can contribute to their reduction (muscle myosin).
2. Nutritional or reserve function. It is an accumulation of protein molecules in eggs, embryos and plant seeds to further replenish the missing nutrients. When cleaved, peptides give amino acids and biologically active substances that are necessary for the normal development of living organisms.
3. Energy function. In addition to carbohydrates, proteins can also give strength to the body. When 1 g of the peptide is disintegrated, 17.6 kJ of useful energy is released in the form of adenosine triphosphate (ATP), which is expended on vital processes.
4. Signal and regulatory function. It involves the implementation of careful monitoring of the ongoing processes and the transmission of signals from cells to tissues, from them to organs, from the latter to systems and so on. A typical example is insulin, which strictly fixes the amount of glucose in the blood.
5. Receptor function. It is carried out by changing the conformation of the peptide from one side of the membrane and involving in the restructuring of the other end. At the same time, the signal and the necessary information are transmitted. Most often such proteins are built into the cytoplasmic membranes of cells and exercise strict control over all substances passing through it. Also notify of chemical and physical changes in the environment.
6. Transport function of peptides. It is carried out by protein channels and carrier proteins. Their role is obvious - transporting the necessary molecules to places with low concentration from parts with high. A typical example is the transfer of oxygen and carbon dioxide to organs and tissues by the protein hemoglobin. They also deliver compounds with a low molecular mass through the cell membrane inside.
7. Structural function. One of the most important of those that performs protein. The structure of all cells, their organelles is provided by peptides. They, like a skeleton, define shape and structure. In addition, they also support it and modify it if necessary. Therefore, for growth and development, all living organisms need proteins in the diet. These peptides include elastin, tubulin, collagen, actin, keratin and others.
8. Catalytic function. It is performed by enzymes. Numerous and diverse, they accelerate all chemical and biochemical reactions in the body. Without their participation, the usual apple in the stomach could only be digested for two days, with a high probability of decaying at the same time. Under the action of catalase, peroxidase and other enzymes, this process takes place in two hours. In general, it is thanks to this role of proteins that anabolism and catabolism, that is, plastic and energy metabolism , are realized .

Protective role

There are several types of threats, from which proteins are designed to protect the body.

First, chemical attack of traumatic reagents, gases, molecules, substances of different spectrum of action. Peptides are able to interact with them in chemical interaction, translating into an inoffensive form or simply neutralizing.

Secondly, the physical threat from the wounds - if the protein fibrinogen does not in time transform into fibrin at the site of the injury, then the blood does not curdle, which means that blockage does not occur. Then, on the contrary, a plasmin peptide is needed, which can dissolve the clot and restore the permeability of the vessel.

Third, the threat to immunity. The structure and importance of proteins forming immune defense are extremely important. Antibodies, immunoglobulins, interferons are all important and significant elements of the human lymphatic and immune system. Any foreign particle, harmful molecule, dead cell part or whole structure undergoes immediate investigation from the peptide compound. That is why a person can independently protect himself from infections and simple viruses without medication.

Physical properties

The structure of the cell protein is very specific and depends on the function performed. But the physical properties of all peptides are similar and boil down to the following characteristics.

1. The molecular weight is up to 1,000,000 Daltons.
2. In aqueous solution, colloidal systems are formed . There, the structure acquires a charge capable of varying depending on the acidity of the medium.
3. Under the influence of harsh conditions (irradiation, acid or alkali, temperature, etc.), they are able to pass to other levels of conformations, that is, to denature. This process is 90% irreversible. However, there is also a reverse shift - renaturation.

These are the main properties of the physical characteristics of peptides.