Simple and complex proteins. Structure, functions, properties, characteristics, examples of complex proteins

One of the definitions of life is as follows: "Life is a way of existence of protein bodies." On our planet, without exception, organisms contain such organic substances as proteins. In this paper, simple and complex proteins will be described, differences in the molecular structure will be determined, and their functions in the cell will be examined.

What are proteins?

In terms of biochemistry - it's high molecular organic polymers, monomers of which are 20 kinds of different amino acids. They are joined together by covalent chemical bonds, otherwise called peptide bonds. Since protein monomers are amphoteric compounds, they contain both an amino group and a carboxyl functional group. The chemical bond of CO-NH arises between them.

If the polypeptide consists of the residues of the amino acid units, it forms a simple protein. Polymer molecules that additionally contain metal ions, vitamins, nucleotides, carbohydrates are complex proteins. Next, we consider the spatial structure of polypeptides.

Levels of organization of protein molecules

They are represented by four different configurations. The first structure is linear, it is the simplest and has the appearance of a polypeptide chain, during the spiral it generates additional hydrogen bonds. They stabilize the spiral, which is called the secondary structure. Tertiary level organizations have simple and complex proteins, most plant and animal cells. The last configuration - the Quaternary, arises from the interaction of several molecules of the native structure, united by coenzymes, just such a structure has complex proteins that perform various functions in the body.

A variety of simple proteins

This group of polypeptides is not numerous. Their molecules consist of only amino acid residues. Proteins include, for example, histones and globulins. The first are represented in the structure of the nucleus and are combined with DNA molecules. The second group - globulins - are considered the main components of blood plasma. A protein such as gamma globulin acts as an immune defense and is an antibody. These compounds can form complexes, which include complex carbohydrates and proteins. Such fibrillar simple proteins, like collagen and elastin, are part of connective tissue, cartilage, tendons, and skin. Their main functions are construction and support.

Protein tubulin is a part of microtubules, which are components of cilia and flagella of such unicellular organisms, as infusorians, euglens, parasitic flagellates. The same protein is a part of multicellular organisms (flagella of spermatozoa, cilia of oocytes, ciliate epithelium of small intestine).

Protein albumin performs a storage function (for example, a protein of chicken eggs). In the endosperm of seeds of cereal plants - rye, rice, wheat - protein molecules accumulate. They are called cellular inclusions. These substances use the seed germ at the beginning of their development. In addition, high protein content in wheat grains is a very important indicator of flour quality. Bread, baked from flour rich in gluten, has high taste qualities and is more useful. Gluten contains the so-called solid wheat varieties. The blood plasma of deep-sea fish contains proteins that prevent their death from the cold. They have the properties of antifreeze, preventing the death of the body at low water temperatures. On the other hand, the cell wall of thermophilic bacteria living in geothermal sources contains proteins that can retain their natural configuration (tertiary or quaternary structure) and do not denature in the temperature range from +50 to + 90 ° C.

Proteins

These are complex proteins, which are characterized by great diversity in connection with the various functions performed by them. As noted earlier, this group of polypeptides, in addition to the protein portion, contains a prosthetic group. Under the influence of various factors, such as high temperature, heavy metal salts, concentrated alkalis and acids, complex proteins can change their spatial shape, simplifying it. This phenomenon is called denaturation. The structure of complex proteins is broken, hydrogen bonds break, and molecules lose their properties and functions. As a rule, denaturation is irreversible. But in some polypeptides performing catalytic, motor and signal functions, renaturation is possible - restoration of the natural structure of the protein.

If the action of the destabilizing factor occurs for a long time, the protein molecule collapses completely. This leads to rupture of the peptide bonds of the primary structure. It is already impossible to restore the protein and its functions. This phenomenon is called destruction. An example is the cooking of chicken eggs: the liquid albumen, which is in the tertiary structure, is completely destroyed.

Biosynthesis of proteins

Once again we recall that the composition of polypeptides of living organisms includes 20 amino acids, among which there are irreplaceable. This is lysine, methionine, phenylalanine, etc. They enter the blood from the small intestine after cleavage of protein products in it. To synthesize the interchangeable amino acids (alanine, proline, serine), fungi and animals use nitrogen-containing compounds. Plants, being autotrophs, independently form all the necessary composite monomers, representing complex proteins. For this, nitrates, ammonia or free nitrogen are used in assimilation reactions. In microorganisms, some species provide themselves with a complete amino acid set, while in others only some monomers are synthesized. Stages of protein biosynthesis take place in the cells of all living organisms. In the nucleus there is transcription, and in the cytoplasm of the cell - translation.

The first stage - the synthesis of the precursor of mRNA occurs with the participation of the enzyme RNA polymerase. It breaks hydrogen bonds between DNA strands, and on one of them, on the principle of complementarity, collects a molecule of pre-iRNA. It undergoes a slipping, that is, ripens, and then leaves the nucleus into the cytoplasm, forming a matrix ribonucleic acid.

To implement the second stage, it is necessary to have special organelles - ribosomes, as well as molecules of information and transport ribonucleic acids. Another important condition is the presence of ATP molecules, since the plastic exchange reactions , to which the biosynthesis of proteins belong, occur with the absorption of energy.

Enzymes, their structure and functions

This is a large group of proteins (about 2000), which play the role of substances that affect the rate of biochemical reactions in cells. They can be simple (trypsin, pepsin) or complex. Complex proteins consist of coenzyme and apoenzyme. The specificity of the protein itself with respect to the compounds to which it acts determines coenzyme, and the activity of the proteins is observed only when the protein component is linked to the apoenzyme. The catalytic activity of the enzyme does not depend on the entire molecule, but only on the active site. Its structure corresponds to the chemical structure of the catalyzed substance on the principle of "key-lock", therefore the action of enzymes is strictly specific. Functions of complex proteins consist both in participation in metabolic processes, and in using them as acceptors.

Classes of complex proteins

They were developed by biochemists, based on 3 criteria: physical and chemical properties, functional characteristics and specific structural features of the proteins. The first group includes polypeptides, which differ in their electrochemical properties. They are divided into basic, neutral and acidic. With respect to water, proteins can be hydrophilic, amphiphilic and hydrophobic. The second group includes enzymes, which were considered by us earlier. The third group includes polypeptides differing in the chemical composition of prosthetic groups (these are chromoproteins, nucleoproteins, metalloproteins).

Consider the properties of complex proteins in more detail. So, for example, the acidic protein, which is part of the ribosomes, contains 120 amino acids and is universal. It is found in protein-synthesizing organelles, both prokaryotic and eukaryotic cells. Another representative of this group is the S-100 protein, which consists of two chains bound by a calcium ion. It is a part of neurons and neuroglia - the supporting tissue of the nervous system. A common property of all acidic proteins is the high content of dibasic carboxylic acids: glutamic and aspartic. Alkaline proteins include histones - proteins that are part of the nucleic acids of DNA and RNA. A feature of their chemical composition is a large amount of lysine and arginine. Histones together with the chromatin of the nucleus form chromosomes - the most important structures of heredity of cells. These proteins are involved in the processes of transcription and translation. Amphiphilic proteins are widely represented in cell membranes, forming a lipoprotein bilayer. Thus, having studied the groups of complex proteins discussed above, we were convinced that their physicochemical properties are due to the structure of the protein component and prosthetic groups.

Some complex proteins of cell membranes are able to recognize and react to various chemical compounds, for example antigens,. This is a signal function of the proteins, it is very important for the processes of selective absorption of substances coming from the environment and for its protection.

Glycoproteins and proteoglycans

They are complex proteins that differ in the biochemical composition of the prosthetic groups. If the chemical bonds between the protein component and the carbohydrate part are covalent-glycosidic, such substances are called glycoproteins. The apoenzymes are represented by molecules of mono- and oligosaccharides, examples of such proteins are prothrombin, fibrinogen (proteins involved in blood coagulation). Cortico-and gonadotropic hormones, interferons, membrane enzymes are also glycoproteins. In protein proteoglycan molecules, the protein part is only 5%, the rest is in the prosthetic group (heteropolysaccharide). Both parts are connected by the glycosidic bond of the group OH-threonine and arginine and the group NH₂-glutamine and lysine. Molecules of proteoglycans play a very important role in water-salt metabolism of cells. Below is a table of complex proteins studied by us.

Metalloproteins

These substances contain ions of one or several metals in their molecules. Consider examples of complex proteins belonging to the above-mentioned group. These are primarily enzymes, such as cytochrome oxidase. It is located on the crystals of mitochondria and activates the synthesis of ATP. Ferrin and transferrin - proteins containing iron ions. The first deposits them in cells, and the second is a transport protein of blood. Another metalloprotein is alpha-amelase, it contains calcium ions, is a part of saliva and pancreatic juice, participating in the cleavage of starch. Hemoglobin is both a metalloprotein and a chromoprotein. It functions as a transport protein, carrying oxygen. As a result, an oxyhemoglobin compound is formed. When inhaled carbon monoxide, otherwise called carbon monoxide, its molecules form a very stable compound with hemoglobin of erythrocytes. It quickly spreads to organs and tissues, causing poisoning of cells. As a result, with prolonged inhalation of carbon monoxide comes death from suffocation. Hemoglobin partially tolerates and carbon dioxide, formed in the processes of catabolism. With the blood flow, carbon dioxide enters the lungs and kidneys, and from them into the external environment. In some crustaceans and mollusks, a transport protein that transfers oxygen is called hemocyanin. Instead of iron, it contains copper ions, so the blood of animals is not red, but blue.

Functions of chlorophyll

As we mentioned earlier, complex proteins can form complexes with pigments - colored organic substances. Their color depends on the chromo-form groups, which selectively absorb certain spectra of sunlight. In the cells of plants there are green plastids - chloroplasts containing the pigment chlorophyll. Its composition includes magnesium atoms and polyatomic alcohol phytol. They are associated with protein molecules, and the chloroplasts themselves contain thylakoids (plates), or membranes, bound in stacks-granules. They contain photosynthetic pigments - chlorophylls - and additional carotenoids. Here are all the enzymes used in photosynthetic reactions. Thus, the chromoproteins, to which chlorophyll also belongs, perform the most important functions in the metabolism, namely, in assimilation and dissimilation reactions.

Viral proteins

They contain representatives of non-cellular forms of life that are part of the Kingdom of Vir. Viruses do not have their own protein synthesizing apparatus. Nucleic acids, DNA or RNA, can cause the synthesis of self-particles by the cell itself, infected with the virus. Simple viruses consist only of protein molecules compactly assembled into structures of a spiral or polyhedral shape, such as tobacco mosaic virus. Complex viruses have an additional membrane that forms part of the plasma membrane of the host cell. It can include glycoproteins (hepatitis B virus, smallpox virus). The main function of glycoproteins is the recognition of specific receptors on the membrane of the host cell. Additional virus envelopes also include enzyme proteins that provide DNA reduplication or RNA transcription. Proceeding from the foregoing, we can draw the following conclusion: the proteins of the envelopes of virus particles have a specific structure, depending on the membrane proteins of the host cell.

In this article, we gave a characterization of complex proteins, studied their structure and functions in the cells of various living organisms.