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, all organisms contain such organic substances as proteins. In this article, simple and complex proteins will be described, differences in the molecular structure will be determined, and their functions in the cell will be considered.

What are proteins

From the point of view of biochemistry, these are high molecular weight organic polymers, the monomers of which are 20 types of various amino acids. They are interconnected by covalent chemical bonds, otherwise called peptide. Since protein monomers are amphoteric compounds, they contain both an amino group and a carboxyl functional group. The chemical bond CO-NH occurs between them.

If the polypeptide consists of residues of amino acid units, it forms a simple protein. Polymer molecules, additionally containing metal ions, vitamins, nucleotides, carbohydrates, are complex proteins. Next, we consider the spatial structure of the 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 its helix formation of additional hydrogen bonds occurs. They stabilize the spiral, which is called the secondary structure. Simple and complex proteins, most plant and animal cells, have a tertiary level of organization. The last configuration is a Quaternary, which occurs when several molecules of the native structure, combined by coenzymes, interact; this is the structure of complex proteins that perform various functions in the body.

Variety of Simple Proteins

This group of polypeptides is not numerous. Their molecules consist only of amino acid residues. Proteins include, for example, histones and globulins. The former 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 serves 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, skin. Their main functions are construction and support.

Protein tubulin is part of microtubules, which are components of cilia and flagella of unicellular organisms such as ciliates, euglena, parasitic flagellates. The same protein is part of multicellular organisms (sperm flagella, cilia of the ovum, ciliary epithelium of the small intestine).

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

Proteids

These are complex proteins, which are characterized by a wide variety in connection with the various functions performed by them. As noted earlier, this group of polypeptides, in addition to the protein part, contains a prosthetic group. Under the influence of various factors, such as high temperature, salts of heavy metals, 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 that perform catalytic, motor and signal functions, renaturation is possible - restoration of the natural structure of the proteid.

If the effect of the destabilizing factor occurs for a long time, the protein molecule is completely destroyed. This leads to the breaking 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 eggs: liquid protein - albumin, located in the tertiary structure, is completely destroyed.

Protein biosynthesis

Once again, we recall that the composition of polypeptides of living organisms includes 20 amino acids, among which are essential. These are lysine, methionine, phenylalanine, etc. They enter the blood from the small intestine after the breakdown of protein products in it. To synthesize essential amino acids (alanine, proline, serine), fungi and animals use nitrogen-containing compounds. Plants, being autotrophs, independently form all the necessary composite monomers, which are complex proteins. To do this, they use nitrates, ammonia or free nitrogen in assimilation reactions. In microorganisms, some species provide themselves with a complete amino acid set, while in others, only some monomers are synthesized. The stages of protein biosynthesis proceed in the cells of all living organisms. Transcription occurs in the nucleus, and translation occurs in the cell cytoplasm.

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

For the second stage, the presence of 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 protein biosynthesis belongs, occur with energy absorption.

Enzymes, their structure and functions

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

Complex protein classes

They were developed by biochemists on the basis of 3 criteria: physicochemical properties, functional features and the specificity of the structural features of proteids. The first group includes polypeptides that differ in electrochemical properties. They are divided into basic, neutral and acidic. In relation to water, proteins can be hydrophilic, amphiphilic and hydrophobic. The second group includes enzymes that we examined earlier. The third group includes polypeptides that differ 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 of both prokaryotic and eukaryotic cells. Another representative of this group - protein S-100, consists of two chains connected by a calcium ion. It is 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 transcription and translation. Amphiphilic proteins are widely represented in cell membranes, forming a lipoprotein bilayer. Thus, having studied the above-considered groups of complex proteins, 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 respond to various chemical compounds, such as antigens. This is a signal function of proteids, it is very important for the processes of selective absorption of substances coming from the external environment, and for its protection.

Glycoproteins and proteoglycans

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

Metalloproteins

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

Chlorophyll Functions

As we mentioned earlier, complex proteins can form complexes with pigments - dyed organic substances. Their color depends on the chromoform groups, which selectively absorb certain spectra of sunlight. In plant cells, there are green plastids - chloroplasts containing the chlorophyll pigment. It consists of magnesium atoms and polyhydric alcohol phytol. They are associated with protein molecules, and the chloroplasts themselves contain thylakoids (plates), or membranes bound in piles - grains. They contain photosynthetic pigments - chlorophylls - and additional carotenoids. Here are all the enzymes used in photosynthetic reactions. Thus, chromoproteins, which include chlorophyll, perform the most important functions in the metabolism, namely in the assimilation and dissimilation reactions.

Viral proteins

They contain representatives of non-cellular life forms 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 their own particles by the virus-infected cell itself. Simple viruses consist only of protein molecules compactly assembled into spiral or multifaceted structures, such as the tobacco mosaic virus. Complex viruses have an additional membrane that forms part of the plasma membrane of the host cell. It may include glycoproteins (hepatitis B virus, smallpox virus). The main function of glycoproteins is to recognize specific receptors on the membrane of the host cell. Additional viral envelopes also include enzyme proteins that provide DNA reduplication or RNA transcription. Based on the foregoing, we can draw the following conclusion: the proteins of the envelopes of viral particles have a specific structure, depending on the membrane proteins of the host cell.

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