Proteins are organic substances. These macromolecular compounds are characterized by a specific composition and decompose into amino acids upon hydrolysis. Protein molecules can be of various forms, many of them consist of several polypeptide chains. Information about the structure of the protein is encoded in DNA, and the process of synthesis of protein molecules is called translation.
The chemical composition of proteins
The average protein contains:
- 52% carbon;
- 7% hydrogen;
- 12% nitrogen;
- 21% oxygen;
- 3% sulfur.
Protein molecules are polymers. In order to understand their structure, it is necessary to find out what their monomers - amino acids are.
Amino acids
They are usually divided into two categories: constantly meeting and sometimes meeting. The first includes 18 protein monomers and another 2 amides: aspartic and glutamic acid. Sometimes there are only three acids found.
These acids can be classified in different ways: by the nature of the side chains or the charge of their radicals, and they can also be divided by the number of CN and COOH groups.
Primary protein structure
The order of alternation of amino acids in the protein chain determines its subsequent levels of organization, properties and functions. The main type of bond between monomers is peptide. It is formed by the removal of hydrogen from one amino acid and an OH group from another.
The first level of organization of a protein molecule is the sequence of amino acids in it, simply a chain that determines the structure of protein molecules. It consists of a "skeleton" having a regular structure. This is the repeating sequence –NH-CH-CO-. Separate side chains are represented by amino acid (R) radicals; their properties determine the composition of the structure of proteins.
Even if the structure of the protein molecules is the same, they can differ in properties only from the fact that their monomers have a different sequence in the chain. The order of amino acids in a protein is determined by genes and dictates certain biological functions to the protein. The sequence of monomers in the molecules responsible for the same function is often close in different species. Such molecules are the same or similar in organization and perform the same functions in different species of organisms - homologous proteins. The structure, properties and functions of future molecules are laid already at the stage of synthesis of the amino acid chain.
Some common features
The structure of proteins has been studied for a long time, and analysis of their primary structure has allowed us to make some generalizations. A larger number of proteins is characterized by the presence of all twenty amino acids, of which especially a lot of glycine, alanine, aspartic acid, glutamine and little tryptophan, arginine, methionine, histidine. Exceptions are only some groups of proteins, for example, histones. They are needed for packaging DNA and contain a lot of histidine.
The second generalization: in globular proteins there are no general patterns in the alternation of amino acids. But even polypeptides that are far from the biological activity have small identical fragments of molecules.
Secondary structure
The second level of organization of the polypeptide chain is its spatial arrangement, which is supported by hydrogen bonds. The α-helix and β-fold are isolated. Part of the chain does not have an ordered structure; such zones are called amorphous.
The alpha helix of all natural proteins is right-handed. The side radicals of amino acids in a spiral are always turned outward and are located on different sides from its axis. If they are non-polar, they are grouped on one side of the spiral, arcs are obtained that create conditions for the convergence of different spiral sections.
Beta-folds - strongly elongated spirals - tend to be located nearby in the protein molecule and form parallel and non-parallel β-folded layers.
Tertiary protein structure
The third level of organization of a protein molecule is the folding of helices, folds and amorphous regions into a compact structure. This is due to the interaction of the side radicals of the monomers with each other. Such relationships are divided into several types:
- hydrogen bonds are formed between polar radicals;
- hydrophobic - between nonpolar R-groups;
- electrostatic forces of attraction (ionic bonds) - between groups whose charges are opposite;
- disulfide bridges - between cysteine radicals.
The last type of bond (–S = S-) is a covalent interaction. Disulfide bridges strengthen proteins, their structure becomes more durable. But the presence of such relationships is not necessary. For example, cysteine can be very small in the polypeptide chain, or its radicals are located nearby and cannot create a “bridge”.
Fourth level of organization
The quaternary structure is not formed by all proteins. The structure of proteins at the fourth level is determined by the number of polypeptide chains (protomers). They are interconnected by the same bonds as the previous level of organization, except for disulfide bridges. The molecule consists of several protomers, each of them has its own special (or identical) tertiary structure.
All levels of organization determine the functions that the resulting proteins will perform. The structure of proteins at the first level of organization very accurately determines their subsequent role in the cell and the body as a whole.
Protein functions
It is hard to imagine how important the role of proteins in cell activity is. Above, we examined their structure. The functions of proteins directly depend on it.
Performing a construction (structural) function, they form the basis of the cytoplasm of any living cell. These polymers are the main material of all cell membranes when they are complexed with lipids. This also includes cell division into compartments, each of which has its own reactions. The fact is that for each complex of cellular processes its own conditions are required, the pH of the medium plays a particularly important role. Proteins build thin septa that divide the cell into so-called compartments. And the phenomenon itself was called compartmentalization.
The catalytic function is to regulate all cell responses. All enzymes by origin are simple or complex proteins.
Any kind of movement of organisms (muscle work, protoplasm movement in a cell, cilia flicker in protozoa, etc.) is carried out by proteins. The structure of proteins allows them to move, form fibers and rings.
The transport function is that many substances are transported through the cell membrane by special carrier proteins.
The hormonal role of these polymers is immediately understandable: a number of hormones in structure are proteins, for example, insulin, oxytocin.
The reserve function is determined by the fact that proteins are able to form deposits. For example, valgumin eggs, casein of milk, protein seeds of plants - they store a large amount of nutrients.
All tendons, articular joints, skeleton bones, hooves are formed by proteins, which brings us to their next function - supporting.
Protein molecules are receptors, providing selective recognition of certain substances. In this role, glycoproteins and lectins are especially known.
The most important factors of immunity - antibodies and the complement system by origin are proteins. For example, the blood coagulation process is based on changes in the fibrinogen protein. The inner walls of the esophagus and stomach are lined with a protective layer of mucous proteins - licins. Toxins are also proteins of origin. The basis of the skin protecting the body of animals is collagen. All of these protein functions are protective.
Well, the last function is regulatory. There are proteins that control the work of the genome. That is, they regulate transcription and translation.
No matter what important role proteins play, the structure of proteins has been unraveled by scientists for a long time. And now they open up new ways of using this knowledge.