As you know, proteins are the basis for the origin of life on our planet. According to the Oparin-Haldane theory, it was the coacervate drop, consisting of peptide molecules, that became the basis for the birth of living things. This is beyond doubt, because the analysis of the internal composition of any representative of the biomass shows that these substances are found in everything: plants, animals, microorganisms, fungi, viruses. Moreover, they are very diverse and macromolecular in nature.
These structures have four names, they are all synonyms:
- proteins;
- proteins;
- polypeptides;
- peptides.
Protein molecules
Their number is truly incalculable. 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.
At the same time, complex molecules also have their own classification.
Gradation of complex peptides
- Glycoproteins are closely related compounds of protein and carbohydrate. into the structure of the moleculeprosthetic groups of mucopolysaccharides are intertwined.
- Lipoproteins are a complex compound of protein and lipid.
- Metalloproteins - metal ions (iron, manganese, copper and others) act as a prosthetic group.
- Nucleoproteins - the connection of protein and nucleic acids (DNA, RNA).
- Phosphoproteins - the conformation of a protein and an orthophosphoric acid residue.
- Chromoproteins - 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).
Each considered group has different structure and properties of proteins. 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 interconnected by specific bonds called peptide bonds. From the side structures of the acids depart branches - radicals. This structure of the molecule was discovered by E. Fischer at the beginning of the 21st 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 performance of their functions, proteins are able to undergo a number of chemical transformations. As a result, they change the structure, and a completely newconnection type.
To break the peptide bond, that is, to break the protein, the structure of the chains, you need to choose very harsh conditions (the action 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.
Detection of the protein structure in the laboratory is carried out using the biuret reaction - exposure of the polypeptide to 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.
Levels of Organization: Primary Structure
As mentioned above, a peptide is a sequence of amino acid residues with or without inclusions, coenzymes. So the primary name is 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 a linear structure. At the same time, the structural features of proteins of such a plan are that such a combination of acids is decisive 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 proteins in this category are somewhat changing. Such a structure can be formed initially from nature or when exposed to primary hard hydrolysis, temperature or other conditions.
This conformation has three varieties:
- Smooth, regular, stereoregular coils built from amino acid residues that twist around the main axis of the connection. They are held together only by hydrogen bonds that occur 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 called alpha structures.
- The composition and structure of proteins of the following type differs from the previous one in that hydrogen bonds are formed not between residues standing side by side on one side of the molecule, but between significantly remote ones, and at a sufficiently large distance. For this reason, the entire structure takes the form of several wavy, serpentine polypeptide chains. There is one feature that a protein must exhibit. The structure of amino acids on the branches should be as short as possible, like glycine or alanine, for example. This type of secondary conformation is called beta sheets for the ability to seem to stick together when forming a common structure.
- Biology refers to the third type of protein structure ascomplex, scattered, disordered fragments that do not have stereoregularity and are capable of changing the structure under the influence of external conditions.
No examples of naturally occurring proteins have been identified.
Tertiary education
This is a fairly complex conformation called "globule". What is such a protein? Its structure is based on the secondary structure, but new types of interactions between the atoms of the groupings are added, and the whole molecule seems to curl up, thus focusing on the fact that the hydrophilic groups are directed inside the globule, and the hydrophobic groups are directed outward.
This explains the charge of the protein molecule in colloidal solutions of water. What types of interactions are there?
- Hydrogen bonds - remain unchanged between the same parts as in the secondary structure.
- Hydrophobic (hydrophilic) interactions - occur when a polypeptide is dissolved in water.
- Ionic attraction - formed between oppositely charged groups of amino acid residues (radicals).
- Covalent interactions - 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, holding and stabilizing their conformation due to various types of chemical interactions. Examples of such peptides:phosphoglycerate kenase, tRNA, alpha-keratin, silk fibroin and others.
Quadternary structure
This is one of the most complex globules that proteins form. The structure and functions of proteins of this kind are very versatile and specific.
What is this conformation? These are several (in some cases dozens) 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 twist and intertwine with each other. In this way, complex conformational globules are obtained, which can contain metal atoms, lipid groups, and carbohydrate groups. Examples of such proteins: DNA polymerase, tobacco virus protein shell, hemoglobin, and others.
All the peptide structures we have considered have their own identification methods in the laboratory, based on modern possibilities of using chromatography, centrifugation, electron and optical microscopy and high computer technologies.
Performed functions
The structure and function 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 at once. However, it is possible to express in a generalized form the main functions of protein molecules in the organisms of living beings:
- Movement support. Single-celled organisms, either organelles, or sometypes of cells are capable of movement, contraction, displacement. This is provided by proteins that are part of the structure of their motor apparatus: cilia, flagella, cytoplasmic membrane. If we talk about cells that are unable to move, then proteins can contribute to their contraction (muscle myosin).
- Nourishing or reserve function. It is the accumulation of protein molecules in the eggs, embryos and seeds of plants 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.
- Energy function. In addition to carbohydrates, proteins can also give strength to the body. With the breakdown of 1 g of the peptide, 17.6 kJ of useful energy is released in the form of adenosine triphosphate (ATP), which is spent on vital processes.
- Signal and regulatory function. It consists in the implementation of careful control over 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.
- 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 are built into the cytoplasmic membranes of cells and exercise strict control over all substances passing through it. Also notify aboutchemical and physical changes in the environment.
- Transport function of peptides. It is carried out by channel proteins and carrier proteins. Their role is obvious - transporting the necessary molecules to places with a low concentration from parts with a high one. A typical example is the transport of oxygen and carbon dioxide through organs and tissues by the protein hemoglobin. They also carry out the delivery of compounds with a low molecular weight through the cell membrane inside.
- Structural function. One of the most important of those that protein performs. The structure of all cells, their organelles is provided precisely by peptides. They, like a frame, set 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.
- Catalytic function. Enzymes do it. Numerous and varied, 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 rotting. Under the action of catalase, peroxidase and other enzymes, this process takes two hours. In general, it is thanks to this role of proteins that anabolism and catabolism are carried out, that is, plastic and energy metabolism.
Protective role
There are several types of threats that proteins are designed to protect the body from.
First, chemicalattack of traumatic reagents, gases, molecules, substances of various spectrum of action. Peptides are able to enter into chemical interaction with them, converting them into a harmless form or simply neutralizing them.
Secondly, the physical threat from wounds - if the protein fibrinogen is not transformed into fibrin in time at the site of injury, then the blood will not clot, which means that blockage will not occur. Then, on the contrary, you will need the plasmin peptide, which is capable of dissolving the clot and restoring the patency of the vessel.
Third, the threat to immunity. The structure and significance of proteins that form immune defenses 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 part of the cell or the whole structure is subjected to immediate investigation by the peptide compound. That is why a person can independently, without the help of medicines, protect himself daily from infections and simple viruses.
Physical properties
The structure of a 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.
- The weight of a molecule is up to 1,000,000 D altons.
- Colloidal systems are formed in an aqueous solution. There, the structure acquires a charge that can vary depending on the acidity of the environment.
- When exposed to harsh conditions (irradiation, acid or alkali, temperature, and so on), they are able to move to other levels of conformations, that isdenature. This process is irreversible in 90% of cases. However, there is also a reverse shift - renaturation.
These are the main properties of the physical characteristics of peptides.
