Education, Secondary education and schools
Proteins: Protein structure and function
Proteins are organic substances. These high-molecular compounds are characterized by a certain composition and break down into amino acids during hydrolysis. Protein molecules can be of a variety of forms, many of them are composed 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.
Chemical composition of proteins
The averaged protein contains:
- 52% of 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 are - amino acids.
Amino acids
They are divided into two categories: constantly occurring and sometimes occurring. The first include 18 protein monomers and 2 more amides: aspartic and glutamic acid. Sometimes there are only three acids.
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 groups CN and COOH.
Primary structure of the protein
The sequence of amino acids in the protein chain determines its subsequent levels of organization, properties and functions. The main type of bond between the monomers is peptide. It is formed by the removal of hydrogen from one aminexlota and the OH group from the other.
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 a repeating sequence of -NH-CH-CO-. Individual side chains are represented by amino acid radicals (R), their properties determine the composition of the structure of proteins.
Even if the structure of 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 the protein is determined by genes and dictates certain biological functions to the protein. The sequence of monomers in 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 the future molecules are laid already at the stage of synthesis of the chain of amino acids.
Some common features
The structure of proteins was studied a long time ago, and an analysis of their primary structure allowed some generalizations. For a larger number of proteins, the presence of all twenty amino acids is characteristic , of which there are especially many glycine, alanine, aspartic acid, glutamine and little tryptophan, arginine, methionine, histidine. Exceptions are only a few groups of proteins, for example, histones. They are needed for DNA packaging 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 in biological polypeptides far from biological activity, there are small identical fragments of molecules.
Secondary structure
The second level of organization of the polypeptide chain is its spatial arrangement, which is maintained due to hydrogen bonds. Separate the α-helix and β-fold. Part of the chain does not have an ordered structure, such zones are called amorphous.
The alpha spiral of all natural proteins is right-wound. The side radicals of amino acids in the helix are always facing outwards and are located on opposite sides of its axis. If they are nonpolar, they are grouped on one side of the spiral, resulting in arcs that create conditions for the convergence of different spiral areas.
Beta-folds - strongly elongated spirals - tend to align themselves in the protein molecule side by side and form parallel and nonparallel β-fold layers.
Tertiary structure of the protein
The third level of organization of the protein molecule is the folding of spirals, folds and amorphous areas into a compact structure. This is due to the interaction of the side radicals of the monomers with each other. Such links are divided into several types:
- Hydrogen bonds are formed between polar radicals;
- Hydrophobic - between non-polar R-groups;
- Electrostatic forces of attraction (ionic bonds) - between groups whose charges are opposite;
- Disulfide bridges - between the radicals of cysteine.
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 connections is not necessary. For example, cysteine can be very small in a polypeptide chain, or its radicals are located side by side and can not create a "bridge".
The fourth level of organization
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 linked by the same links as the previous level of the organization, except for the disulfide bridges. The molecule consists of several protomers, each of which has its own (or identical) tertiary structure.
All levels of the organization determine the functions that the resulting proteins will perform. The structure of proteins at the first level of the organization very accurately determines their subsequent role in the cell and the organism as a whole.
Functions of proteins
It is difficult to even imagine how important the role of proteins in the activity of the cell. Above we considered their structure. The functions of proteins depend directly on it.
Performing a structural (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 the division of cells into compartments, in each of which their reactions proceed. The fact is that for each complex of cellular processes their conditions are required, especially the pH of the medium plays a big role. Proteins build thin partitions that divide the cell into so-called compartments. And the phenomenon itself was called compartmentalization.
The catalytic function is to regulate all cell reactions. All enzymes of origin are simple or complex proteins.
Any kind of movement of organisms (the work of muscles, the movement of protoplasm in the cell, the flickering of cilia in the protozoa, etc.) is carried out by proteins. The structure of proteins allows them to move, form fibers and rings.
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, salmon valgum, milk casein, plant seed proteins - they contain a large number of nutrients.
All tendons, articular joints, bones of the skeleton, hoofs are formed by proteins, which leads us to their next function - the supporting one.
Protein molecules are receptors, making selective recognition of certain substances. In this role, glycoproteins and lectins are especially known.
The most important factors of immunity - antibodies and complement system by origin are proteins. For example, the process of blood clotting 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 - lycins. Toxins are also proteins of origin. The basis of the skin, protecting the body of animals, is collagen. All these functions of proteins 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.
Whatever role proteins play, the structure of proteins has been solved by scientists for a long time. And now they are opening up new ways of using this knowledge.
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