Proteins are organic substances. These macromolecular compounds are characterized by a certain composition and decompose into amino acids upon hydrolysis. Protein molecules come in a wide variety of shapes, many of which are made up of multiple polypeptide chains. Information about the structure of a protein is encoded in DNA, and the process of synthesis of protein molecules is called translation.
Chemical composition of proteins
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 know what their monomers, amino acids, are.
Amino acids
They are usually divided into two categories: constantly occurring and occasionally occurring. The former include 18 protein monomers and 2 more amides: aspartic and glutamic acids. Sometimes there are only three acids.
These acids can be classified in many ways: by the nature of the side chains or the charge of their radicals, they can also be divided by the number of CN and COOH groups.
Protein primary structure
The sequence of amino acids in a protein chain determinesits subsequent levels of organization, properties and functions. The main type of bond between monomers is peptide. It is formed by splitting off 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" that has a regular structure. This is a repeating sequence -NH-CH-CO-. Separate 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 a 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 - the same or similar in organization and performing the same functions in different types of organisms - are homologous proteins. The structure, properties and functions of future molecules are laid down 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 the analysis of their primary structure allowed us to make some generalizations. Most proteins are characterized by the presence of all twenty amino acids, of which there are especially many glycine, alanine, aspartic acid, glutamine and little tryptophan, arginine, methionine,histidine. The only exceptions are certain groups of proteins, for example, histones. They are needed for DNA packaging and contain a lot of histidine.
Second generalization: in globular proteins there are no general patterns in the alternation of amino acids. But even polypeptides that are distant in 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. Allocate α-helix and β-fold. 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. Side radicals of amino acids in the helix always face outward and are located on opposite sides of its axis. If they are non-polar, they are grouped on one side of the spiral, resulting in arcs that create conditions for the convergence of different spiral sections.
Beta-folds - strongly elongated spirals - tend to be located side by side in the protein molecule and form parallel and non-parallel β-pleated layers.
Tertiary protein structure
The third level of organization of a protein molecule is the folding of spirals, folds and amorphous sections into a compact structure. This is due to the interaction of the side radicals of the monomers with each other. Such connections are divided into several types:
- hydrogen bonds form between polar radicals;
- hydrophobic– between non-polar 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 such connections are not required. For example, there may be very little cysteine in the polypeptide chain, or its radicals are located nearby and cannot create a "bridge".
The fourth level of organization
Not all proteins form a quaternary structure. 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. A molecule consists of several protomers, each of which 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's hard to even 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 building (structural) function, they form the basis of the cytoplasm of any living cell. These polymers are the main material of all cell membranes whenare complexed with lipids. This also includes the division of the cell into compartments, each of which has its own reactions. The fact is that each complex of cellular processes requires its own conditions, especially the pH of the medium plays an important role. Proteins build thin partitions that divide the cell into so-called compartments. And the phenomenon itself is called compartmentalization.
The catalytic function is to regulate all reactions of the cell. All enzymes are either simple or complex proteins in origin.
Any kind of movement of organisms (work of muscles, movement of protoplasm in a cell, flickering of cilia 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 clear: a number of hormones are proteins in structure, for example, insulin, oxytocin.
Spare function is determined by the fact that proteins are able to form deposits. For example, egg valgumin, milk casein, plant seed proteins - they store a large amount of nutrients.
All tendons, articular joints, bones of the skeleton, hooves are formed by proteins, which brings us to their next function - supporting.
Protein molecules are receptors, carrying out selective recognition of certain substances. In this role, glycoproteins and lectins are especially known.
The most importantimmunity factors - antibodies and the 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 - licins. Toxins are also proteins in origin. The basis of the skin that protects 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 how important the proteins are, the structure of proteins has been unraveled by scientists for a long time. And now they are discovering new ways to use this knowledge.
