Proteins are one of the important organic elements of any living cell of the body. They perform many functions: supporting, signaling, enzymatic, transport, structural, receptor, etc. The primary, secondary, tertiary and quaternary structures of proteins have become important evolutionary adaptations. What are these molecules made of? Why is the correct conformation of proteins in the cells of the body so important?
Structural components of proteins
The monomers of any polypeptide chain are amino acids (AA). These low molecular weight organic compounds are quite common in nature and can exist as independent molecules that perform their own functions. Among them are the transport of substances, reception, inhibition or activation of enzymes.
There are about 200 biogenic amino acids in total, but only 20 of them can be protein monomers. They dissolve easily in water, have a crystalline structure, and many taste sweet.
C chemicalFrom the point of view of AA, these are molecules that necessarily contain two functional groups: -COOH and -NH2. With the help of these groups, amino acids form chains, connecting to each other with a peptide bond.
Each of the 20 proteinogenic amino acids has its own radical, depending on which the chemical properties vary. According to the composition of such radicals, all AAs are classified into several groups.
- Nonpolar: isoleucine, glycine, leucine, valine, proline, alanine.
- Polar and uncharged: threonine, methionine, cysteine, serine, glutamine, asparagine.
- Aromatic: tyrosine, phenylalanine, tryptophan.
- Polar and negatively charged: glutamate, aspartate.
- Polar and positively charged: arginine, histidine, lysine.
Any level of organization of the protein structure (primary, secondary, tertiary, quaternary) is based on a polypeptide chain consisting of AA. The only difference is how this sequence is folded in space and with the help of what chemical bonds this conformation is maintained.
Protein primary structure
Any protein is formed on ribosomes - non-membrane cell organelles that are involved in the synthesis of the polypeptide chain. Here, amino acids are connected to each other using a strong peptide bond, forming a primary structure. However, this primary protein structure is very different from the quaternary one, so further maturation of the molecule is necessary.
Proteins likeelastin, histones, glutathione, already with such a simple structure, are able to perform their functions in the body. For the vast majority of proteins, the next step is the formation of a more complex secondary conformation.
Secondary protein structure
The formation of peptide bonds is the first step in the maturation of most proteins. In order for them to perform their functions, their local conformation must undergo some changes. This is achieved with the help of hydrogen bonds - fragile, but at the same time numerous connections between the basic and acid centers of amino acid molecules.
This is how the secondary structure of the protein is formed, which differs from the quaternary in its simplicity of assembly and local conformation. The latter means that not the entire chain is subjected to transformation. Hydrogen bonds can form at several sites of different distances from each other, and their shape also depends on the type of amino acids and the method of assembly.
Lysozyme and pepsin are representatives of proteins that have a secondary structure. Pepsin is involved in digestion, and lysozyme performs a protective function in the body, destroying the cell walls of bacteria.
Features of the secondary structure
Local conformations of the peptide chain can differ from each other. Several dozen have already been studied, and three of them are the most common. Among them are alpha helix, beta layers and beta twist.
Alpha spiral –one of the most common conformations of the secondary structure of most proteins. It is a rigid rod frame with a stroke of 0.54 nm. Amino acid radicals point outward
Right-handed spirals are most common, and left-handed counterparts can sometimes be found. The shaping function is performed by hydrogen bonds, which stabilize the curls. The chain that forms the alpha helix contains very little proline and polar charged amino acids.
- The beta turn is isolated into a separate conformation, although it can be called part of the beta layer. The bottom line is the bending of the peptide chain, which is supported by hydrogen bonds. Usually the place of the bend itself consists of 4-5 amino acids, among which the presence of proline is mandatory. This AK is the only one with a rigid and short skeleton, which allows it to form a self-turn.
- The beta layer is a chain of amino acids that forms several bends and stabilizes them with hydrogen bonds. This conformation is very similar to a sheet of paper folded into an accordion. Most often, aggressive proteins have this shape, but there are many exceptions.
Distinguish between parallel and antiparallel beta-layer. In the first case, the C- and N- ends at the bends and at the ends of the chain coincide, and in the second case they do not.
Tertiary structure
Further protein packaging leads to the formation of a tertiary structure. This conformation is stabilized with the help of hydrogen, disulfide, hydrophobic and ionic bonds. Their large number allows twisting the secondary structure into a more complex one.form and stabilize it.
Separate globular and fibrillar proteins. The molecule of globular peptides is a spherical structure. Examples: albumin, globulin, histones in tertiary structure.
Fibrillar proteins form strong strands, the length of which exceeds their width. Such proteins most often perform structural and shaping functions. Examples are fibroin, keratin, collagen, elastin.
The structure of proteins in the quaternary structure of the molecule
If several globules combine into one complex, the so-called quaternary structure is formed. This conformation is not typical for all peptides, and it is formed when it is necessary to perform important and specific functions.
Each globule in a complex protein is a separate domain or protomer. Collectively, the structure of proteins of the quaternary structure of a molecule is called an oligomer.
Usually, such a protein has several stable conformations that constantly change each other, either depending on the impact of any external factors, or when it is necessary to perform different functions.
An important difference between the tertiary and quaternary structure of a protein is intermolecular bonds, which are responsible for connecting several globules. In the center of the entire molecule, there is often a metal ion, which directly affects the formation of intermolecular bonds.
Additional protein structures
Not always a chain of amino acids is enough to perform the functions of a protein. ATIn most cases, other substances of organic and inorganic nature are attached to such molecules. Since this feature is characteristic of the overwhelming number of enzymes, the composition of complex proteins is usually divided into three parts:
- Apoenzyme is the protein part of the molecule, which is an amino acid sequence.
- Coenzyme is not a protein, but an organic part. It can include various types of lipids, carbohydrates, or even nucleic acids. This includes representatives of biologically active compounds, among which there are vitamins.
- Cofactor - an inorganic part, represented in the vast majority of cases by metal ions.
The structure of proteins in the quaternary structure of a molecule requires the participation of several molecules of different origin, so many enzymes have three components at once. An example is phosphokinase, an enzyme that ensures the transfer of a phosphate group from an ATP molecule.
Where is the quaternary structure of a protein molecule formed?
The polypeptide chain begins to be synthesized on the ribosomes of the cell, but further maturation of the protein occurs in other organelles. The newly formed molecule must enter the transport system, which consists of the nuclear membrane, ER, Golgi apparatus and lysosomes.
The complication of the spatial structure of the protein occurs in the endoplasmic reticulum, where not only various types of bonds are formed (hydrogen, disulfide, hydrophobic, intermolecular, ionic), but also coenzyme and cofactor are added. This forms a quaternaryprotein structure.
When the molecule is completely ready to work, it enters either the cytoplasm of the cell or the Golgi apparatus. In the latter case, these peptides are packaged into lysosomes and transported to other compartments of the cell.
Examples of oligomeric proteins
Quaternary structure is the structure of proteins, which is designed to contribute to the performance of vital functions in a living organism. The complex conformation of organic molecules allows, first of all, to influence the work of many metabolic processes (enzymes).
Biologically important proteins are hemoglobin, chlorophyll and hemocyanin. The porphyrin ring is the basis of these molecules, in the center of which is a metal ion.
Hemoglobin
The quaternary structure of the hemoglobin protein molecule consists of 4 globules connected by intermolecular bonds. In the center is a porphin with a ferrous ion. The protein is transported in the cytoplasm of erythrocytes, where they occupy about 80% of the total volume of the cytoplasm.
The basis of the molecule is heme, which has a more inorganic nature and is colored red. It is also the primary breakdown product of hemoglobin in the liver.
We all know that hemoglobin performs an important transport function - the transfer of oxygen and carbon dioxide throughout the human body. The complex conformation of the protein molecule forms special active centers that are capable of binding the corresponding gases to hemoglobin.
When a protein-gas complex is formed, so-called oxyhemoglobin and carbohemoglobin are formed. However, there is one morea variety of such associations, which is quite stable: carboxyhemoglobin. It is a complex of protein and carbon monoxide, the stability of which explains the attacks of suffocation with excessive toxicity.
Chlorophyll
Another representative of proteins with a quaternary structure, whose domain bonds are already supported by a magnesium ion. The main function of the entire molecule is participation in the processes of photosynthesis in plants.
There are different types of chlorophylls, which differ from each other in the radicals of the porphyrin ring. Each of these varieties is marked with a separate letter of the Latin alphabet. For example, land plants are characterized by the presence of chlorophyll a or chlorophyll b, and other types of this protein are found in algae.
Hemocyanin
This molecule is an analogue of hemoglobin in many lower animals (arthropods, molluscs, etc.). The main difference in the structure of a protein with a quaternary molecular structure is the presence of a zinc ion instead of an iron ion. Hemocyanin has a bluish color.
Sometimes people wonder what would happen if we replaced human hemoglobin with hemocyanin. In this case, the usual content of substances in the blood, and in particular amino acids, is disturbed. Hemocyanin is also unstable to form a complex with carbon dioxide, so "blue blood" would have a tendency to form blood clots.
