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. This article will describe simple and complex proteins, identify differences in molecular structure, and also consider their functions in the cell.
What are proteins
From the point of view of biochemistry, these are high-molecular organic polymers, the monomers of which are 20 kinds of different amino acids. They are interconnected 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. A CO-NH chemical bond occurs between them.
If a polypeptide consists of amino acid residues, it forms a simple protein. Polymer molecules additionally containing metal ions, vitamins, nucleotides, carbohydrates are complex proteins. Next weconsider the spatial structure of polypeptides.
Levels of organization of protein molecules
They come in four different configurations. The first structure is linear, it is the simplest and has the form of a polypeptide chain; during its spiralization, additional hydrogen bonds are formed. They stabilize the helix, which is called the secondary structure. The tertiary level of organization has 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, this is the structure of complex proteins that perform various functions in the body.
Diversity 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 first are presented 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 performs the functions of immune protection and is an antibody. These compounds can form complexes that include complex carbohydrates and proteins. Fibrillar simple proteins such as collagen and elastin are part of the connective tissue, cartilage, tendons, and skin. Their main functions are construction and support.
Protein tubulin is part of microtubules, which are components of cilia and flagella of such unicellular organisms as ciliates, euglena, parasitic flagellates. The same protein is found in multicellular organisms (sperm flagella, egg cilia, ciliated epithelium of the small intestine).
Albumin protein performs a storage function (for example, egg white). 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 a high taste and is more he althy. Gluten is contained in the so-called durum wheat varieties. The blood plasma of deep-sea fish contains proteins that prevent them from dying 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 °С.
Proteids
These are complex proteins, which are characterized by great diversity due to the different functions they perform. 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, s alts of heavy metals, concentrated alkalis and acids, complex proteins can change theirspatial form, simplifying it. This phenomenon is called denaturation. The structure of complex proteins is broken, hydrogen bonds are broken, and molecules lose their properties and functions. As a rule, denaturation is irreversible. But for some polypeptides that perform 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 is completely destroyed. This leads to cleavage of the peptide bonds of the primary structure. It is no longer possible to restore the protein and its functions. This phenomenon is called destruction. An example is the boiling of chicken eggs: liquid protein - albumin, which is in the tertiary structure, is completely destroyed.
Protein biosynthesis
Recall once again that the composition of the polypeptides of living organisms includes 20 amino acids, among which there are essential ones. These are lysine, methionine, phenylalanine, etc. They enter the bloodstream from the small intestine after the breakdown of protein products in it. To synthesize non-essential amino acids (alanine, proline, serine), fungi and animals use nitrogen-containing compounds. Plants, being autotrophs, independently form all the necessary compound monomers, which are complex proteins. To do this, they use nitrates, ammonia or free nitrogen in their assimilation reactions. In microorganisms, some species provide themselves with a complete set of amino acids, while in others only some monomers are synthesized. Stagesprotein biosynthesis occurs in the cells of all living organisms. Transcription occurs in the nucleus, and translation occurs in the cytoplasm of the cell.
The first stage - the synthesis of the mRNA precursor occurs with the participation of the enzyme RNA polymerase. It breaks hydrogen bonds between DNA strands, and on one of them, according to the principle of complementarity, it assembles a pre-mRNA molecule. It undergoes slicing, that is, it matures, and then exits the nucleus into the cytoplasm, forming a matrix ribonucleic acid.
For the implementation of the second stage, it is necessary to have special organelles - ribosomes, as well as molecules of informational and transport ribonucleic acids. Another important condition is the presence of ATP molecules, since plastic exchange reactions, which include protein biosynthesis, occur with energy absorption.
Enzymes, their structure and functions
This is a large group of proteins (about 2000) that act as substances that affect the rate of biochemical reactions in cells. They can be simple (trepsin, pepsin) or complex. Complex proteins consist of a coenzyme and an apoenzyme. The specificity of the protein itself with respect to the compounds it acts on determines the coenzyme, and the activity of the proteins is observed only when the protein component is associated with the apoenzyme. The catalytic activity of an 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 according to the principle"key-lock", so the action of enzymes is strictly specific. The functions of complex proteins are both participation in metabolic processes and their use as acceptors.
Classes of complex proteins
They were developed by biochemists based on 3 criteria: physical and chemical properties, functional features and specific structural features of proteins. 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, which were considered by us earlier. The third group includes polypeptides that differ in the chemical composition of prosthetic groups (these are chromoproteins, nucleoproteins, metalloproteins).
Let's consider the properties of complex proteins in more detail. For example, an acidic protein that is part of 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, the S-100 protein, consists of two chains linked by a calcium ion. It is part of the neurons and neuroglia - the supporting tissue of the nervous system. A common property of all acidic proteins is a 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 cell heredity. These proteins are involved in the processes of transcription and translation. Amphiphilic proteins are widely present in cell membranes, forming a lipoprotein bilayer. Thus, having studied the groups of complex proteins considered above, we were convinced that their physicochemical properties are determined by 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 signaling function of proteins, 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 from each other 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. Their apoenzyme 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, membrane enzymes are also glycoproteins. In proteoglycan molecules, the protein part is only 5%, the rest falls on the prosthetic group (heteropolysaccharide). Both parts are connected by a glycosidic bond of the OH-threonine and arginine groups and the NH₂-glutamine and lysine groups. Proteoglycan molecules play a very important role in the water-s alt metabolism of the cell. Belowpresents a table of complex proteins studied by us.
| Glycoproteins | Proteoglycans |
| Structural components of prosthetic groups | |
| 1. Monosaccharides (glucose, galactose, mannose) | 1. Hyaluronic Acid |
| 2. Oligosaccharides (m altose, lactose, sucrose) | 2. Chondroitic acid. |
| 3. Acetylated amino derivatives of monosaccharides | 3. Heparin |
| 4. Deoxysaccharides | |
| 5. Neuramic and sialic acids | |
Metalloproteins
These substances contain ions of one or more metals in their molecules. Consider examples of complex proteins belonging to the above group. These are primarily enzymes such as cytochrome oxidase. It is located on the cristae of mitochondria and activates ATP synthesis. Ferrin and transferrin are proteins containing iron ions. The first deposits them in cells, and the second is a transport protein in the blood. Another metalloprotein is alpha-amelase, it contains calcium ions, is part of saliva and pancreatic juice, participating in the breakdown of starch. Hemoglobin is both a metalloprotein and a chromoprotein. It performs the functions of a transport protein, carrying oxygen. As a result, the compound oxyhemoglobin is formed. When carbon monoxide, otherwise called carbon monoxide, is inhaled, its molecules form a very stable compound with erythrocyte hemoglobin. It quickly spreads through organs and tissues, causing poisoning.cells. As a result, with prolonged inhalation of carbon monoxide, death occurs from suffocation. Hemoglobin also partially transfers 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, hemocyanin is the oxygen-carrying protein. 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 - colored organic substances. Their color depends on chromoform groups that selectively absorb certain spectra of sunlight. In plant cells there are green plastids - chloroplasts containing the pigment chlorophyll. It consists of magnesium atoms and the polyhydric alcohol phytol. They are associated with protein molecules, and the chloroplasts themselves contain thylakoids (plates), or membranes connected in stacks - grana. 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 metabolism, namely in the reactions of assimilation and dissimilation.
Viral proteins
They are kept by representatives of non-cellular life forms that are part of the Vira Realm. Viruses do not have their own protein-synthesizing apparatus. Nucleic acids, DNA or RNA, can cause synthesisown particles by the cell itself infected with the virus. Simple viruses consist only of protein molecules compactly assembled into helical or polyhedral 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 the recognition of specific receptors on the host cell membrane. Additional viral envelopes also include enzyme proteins that ensure DNA replication or RNA transcription. Based on the foregoing, the following conclusion can be drawn: the envelope proteins of viral particles have a specific structure that depends on the membrane proteins of the host cell.
In this article, we have characterized complex proteins, studied their structure and functions in the cells of various living organisms.
