Protein: structure and function. Protein Properties

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

There are four names for these structures, all of them are synonyms:

• proteins;
• Proteins
• polypeptides;
• peptides.

Protein molecules

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

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

Moreover, complex molecules also have their own classification.

Complex peptide gradation

1. Glycoproteins are closely related protein and carbohydrate compounds. Prosthetic groups of mucopolysaccharides are woven into the structure of the molecule.
2. Lipoproteins are a complex compound of protein and lipid.
3. Metalloproteins - metal ions (iron, manganese, copper and others) act as a prosthetic group.
4. Nucleoproteins are a bond of protein and nucleic acids (DNA, RNA).
5. Phosphoproteins are the conformation of a protein and phosphoric acid residue.
6. Chromoproteins are very similar to metalloproteins, however, the element that is part of the prosthetic group is a whole colored complex (red - hemoglobin, green - chlorophyll, and so on).

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

The chemical structure of proteins

From this point of view, proteins are a long, massive chain of amino acid residues connected by specific bonds called peptide bonds. Branches — radicals — depart from the side structures of acids. Such a molecular structure was discovered by E. Fisher at the beginning of the XXI century.

Later, proteins, the structure and functions of proteins were studied in more detail. It became clear that there are only 20 amino acids that form the structure of the peptide, but they can be combined in a variety of ways. Hence the diversity of polypeptide structures. In addition, in the process of life and the performance of their functions, proteins are able to undergo a series 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, disrupt the protein, the structure of the chains, it is necessary to select very stringent conditions (the effect of high temperatures, acids or alkalis, a catalyst). This is due to the high strength of covalent bonds in the molecule, namely in the peptide group.

The detection of the protein structure in the laboratory is carried out using a 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 purple color.

There are four main structural organizations, each of which has its own structural features of proteins.

Organization Levels: Primary Structure

As mentioned above, a peptide is a sequence of amino acid residues with or without inclusions, coenzymes. So primary is called such a structure of the molecule, which is natural, natural, is truly amino acids connected by peptide bonds, and nothing more. That is, a polypeptide of linear structure. Moreover, the structural features of proteins of such a plan is that such a combination of acids is crucial for the performance of the functions of a 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 discovered. Examples of peptides with a natural primary structure are insulin, pepsin, chymotrypsin and others.

Secondary conformation

The structure and properties of the proteins of this category are slightly changing. Such a structure can form initially from nature or when exposed to the primary by harsh hydrolysis, temperature or other conditions.

This conformation has three varieties:

1. Smooth, regular, stereoregular turns built from amino acid residues that twist around the main axis of the compound. They are held together only by hydrogen bonds arising between the oxygen of one peptide group and the hydrogen of another. Moreover, the structure is considered correct due to the fact that the turns are evenly repeated every 4 links. Such a structure can be either left-handed or right-handed. But in most known proteins, the dextrorotatory isomer predominates. Such conformations are commonly called alpha structures.
2. The composition and structure of proteins of the following type differs from the previous one in that hydrogen bonds are formed not between the residues that are adjacent to one side of the molecule, but between significantly removed ones, and at a fairly large distance. For this reason, the entire structure takes the form of several wavy, snake-crimped polypeptide chains. There is one feature that 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 their ability to stick together when a common structure is formed.
3. The structure of the protein related to the third type is designated by biology as complex, heterogeneous, disordered fragments that do not have stereoregularity and are able to change the structure under the influence of external conditions.

Examples of proteins having a secondary structure by 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, however, new types of interactions between the atoms of the groups are added, and the whole molecule seems to be folded, focusing, so that the hydrophilic groups are directed inside the globule, and hydrophobic - out.

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

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

Thus, the composition and structure of proteins with a tertiary structure can be described as polypeptide chains folded into globules that hold and stabilize their conformation due to different types of chemical interactions. Examples of such peptides: phosphoglyceratkenase, tRNA, alpha-keratin, silk fibroin 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? These are several (in some cases tens) of large and small polypeptide chains, which are formed independently of each other. But then due to the same interactions that we considered for the tertiary structure, all these peptides twist and intertwine. Thus, complex conformational globules are obtained, which can contain metal atoms, lipid groups, and carbohydrate. Examples of such proteins: DNA polymerase, protein coat of the tobacco virus, hemoglobin and others.

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

Functions Performed

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

1. Providing movement. Unicellular organisms, or organelles, or some types of cells are capable of movement, contraction, movement. This is provided by the proteins that make up the structure of their motor apparatus: cilia, flagella, and cytoplasmic membrane. If we talk about cells that are unable to move, then proteins can contribute to their contraction (muscle myosin).
2. Nutritional or reserve function. It is an accumulation of protein molecules in the eggs, embryos and seeds of plants to further replenish the missing nutrients. When cleaved, peptides provide 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 breaks down, 17.6 kJ of useful energy is released in the form of adenosine triphosphoric acid (ATP), which is spent on vital processes.
4. Signal and regulatory function. It consists in the careful monitoring of the 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 on one side of the membrane and involving the other end in the restructuring. At the same time, the signal and the necessary information are transmitted. Most often, such proteins integrate into the cytoplasmic membranes of cells and exercise strict control over all substances passing through it. Also alert about chemical and physical changes in the environment.
6. The transport function of peptides. It is carried out by channel proteins and carrier proteins. Their role is obvious - transportation of the necessary molecules to places with a low concentration of parts with high. A typical example is the transfer of oxygen and carbon dioxide to organs and tissues by the hemoglobin protein. They also carry out the delivery of compounds with a low molecular weight 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 precisely by peptides. They, like the frame, define the shape and structure. In addition, they 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, an ordinary apple in the stomach could be digested in only two days, with a high probability of decaying at the same time. Under the action of catalase, peroxidase and other enzymes, this process occurs in two hours. In general, it is thanks to this role of proteins that anabolism and catabolism, i.e., plastic and energy metabolism , are carried out .

Protective role

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

Firstly, a chemical attack of traumatic reagents, gases, molecules, substances of various spectrum of action. Peptides are able to enter into chemical interaction with them, transforming into a harmless form or simply neutralizing them.

Secondly, there is a physical threat from wounds - if the protein fibrinogen does not transform in time to fibrin at the site of the injury, then the blood will not clot, which means blockage will not occur. Then, on the contrary, you will need a plasmin peptide that is able to dissolve the clot and restore the patency of the vessel.

Thirdly, a threat to the immune system. The structure and importance of the proteins that form the immune defense are extremely important. Antibodies, immunoglobulins, interferons - all these are important and significant elements of the human lymphatic and immune system. Any foreign particle, harmful molecule, dead part of the cell or the whole structure is immediately examined by the peptide compound. That is why a person can independently, without the help of medicines, protect himself daily against infections and uncomplicated viruses.

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 are reduced to the following characteristics.

1. Molecule weight - up to 1,000,000 Daltons.
2. Colloidal systems are formed in an aqueous solution. There, the structure acquires a charge, which can vary depending on the acidity of the medium.
3. Under the influence of harsh conditions (radiation, acid or alkali, temperature, and so on) they are able to go to other levels of conformations, that is, to denature. This process in 90% of cases is irreversible. However, there is also a reverse shift - renaturation.

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