Proteins are one of the important organic elements of any living cell in the body. They perform many functions: supporting, signaling, enzymatic, transport, structural, receptor, etc. The primary, secondary, tertiary and quaternary structures of proteins have become an important evolutionary device. What do these molecules consist of? Why is the correct conformation of proteins in the cells of the body so important?
Structural components of proteins
Monomers of any polypeptide chain are amino acids (AK). These low molecular weight organic compounds are quite common in nature and can exist as independent molecules that perform their inherent functions. Among them are transport of substances, reception, inhibition or activation of enzymes.
In total, there are about 200 biogenic amino acids, but only 20 of them can be protein monomers. They are easily soluble in water, have a crystalline structure and many of them are sweet in taste.
From a chemical point of view, AKs are molecules in which two functional groups are necessarily present: —COOH and –NH2. Using 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 differ. By the composition of such radicals, all AKs are classified into several groups.
- Non-polar: 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 AK. The only difference is how this sequence develops in space and with what chemical bonds such a conformation is maintained.
Primary protein structure
Any protein is formed on ribosomes - non-membrane organelles of the cell that are involved in the synthesis of the polypeptide chain. Here, amino acids combine with each other via a strong peptide bond, forming the primary structure. However, this primary structure of the protein from the Quaternary is extremely different, therefore, further ripening of the molecule is necessary.
Proteins such as elastin, 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 stage of maturation of most proteins. In order for them to fulfill their functions, their local conformation must undergo some changes. This is achieved using hydrogen bonds - fragile, but at the same time, numerous compounds between the main and acid centers of amino acid molecules.
Thus, the secondary structure of the protein is formed, which differs from the quaternary in simplicity of assembly and local conformation. The latter means that not the entire chain undergoes a 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 having 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 may differ from each other. Several dozen of them have already been studied, and three of them are the most common. Among them are the alpha helix, beta layers and beta rotation.
- The alpha helix is 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. The amino acid radicals are directed outward.
Right-handed spirals are most common, and sometimes left-handed counterparts can be found. The formative 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 rotation is isolated in a separate conformation, although this 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 bend site itself consists of 4-5 amino acids, among which the presence of proline is mandatory. This AK is the only one that has a hard and short skeleton, which allows you to form the turn itself.
- 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 in an accordion. Most often, aggressive proteins have this form, but there are many exceptions.
Distinguish between parallel and antiparallel beta layer. In the first case, the C- and N-ends coincide at the points of bending and at the ends of the chain, while in the second case they do not.
Tertiary structure
Further protein packaging leads to the formation of a tertiary structure. Such a conformation is stabilized by hydrogen, disulfide, hydrophobic and ionic bonds. Their large number allows you to twist the secondary structure into a more complex form and stabilize it.
Globular and fibrillar proteins are separated . The globular peptide molecule is a spherical structure. Examples: albumin, globulin, histones in the tertiary structure.
Fibrillar proteins form strong cords whose length 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 are combined into one complex, the so-called quaternary structure is formed. This conformation is not characteristic of 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. Taken together, the structure of the proteins of the quaternary structure of the molecule is called the oligomer.
Typically, such a protein has several stable conformations that constantly replace each other, either depending on the influence of any external factors, or, if necessary, to perform different functions.
An important difference between the tertiary structure of the protein and the quaternary are intermolecular bonds, which are responsible for the connection of several globules. In the center of the whole molecule, a metal ion is often located, which directly affects the formation of intermolecular bonds.
Additional protein structures
Not always a chain of amino acids is enough to fulfill the functions of a protein. In most cases, other substances of an organic and inorganic nature join such molecules. Since this feature is characteristic of the overwhelming number of enzymes, the composition of complex proteides 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 may include various types of lipids, carbohydrates, or even nucleic acids. This includes representatives of biologically active compounds, among which there are vitamins.
- Cofactor is an inorganic part, represented in the vast majority of cases by metal ions.
The structure of proteins in the quaternary structure of the molecule requires the participation of several molecules of different origin, therefore, many enzymes have three components at once. An example is phosphokinase, an enzyme that transfers a phosphate group from an ATP molecule.
Where is the quaternary structure of the protein molecule formed?
The polypeptide chain begins to be synthesized on the ribosomes of the cell, however, further maturation of the protein occurs already in other organelles. The newly formed molecule must enter the transport system, which consists of a nuclear membrane, EPS, 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 coenzyme and cofactor also join. This forms the quaternary structure of the protein.
When the molecule is completely ready for work, it enters either the cytoplasm of the cell or the Golgi apparatus. In the latter case, these peptides are packed into lysosomes and transported to other compartments of the cell.
Examples of Oligomeric Proteins
The quaternary structure is the structure of proteins, which is designed to contribute to the implementation 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 is 4 globules connected by intermolecular bonds. In the center is porphin with a ferrous ion. Protein is carried in the cytoplasm of red blood cells, where they occupy about 80% of the total cytoplasm.
The basis of the molecule is heme, which is more inorganic in 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 through the human body. The complex conformation of the protein molecule forms special active centers, which are capable of binding the corresponding gases with hemoglobin.
When a protein-gas complex is formed, the so-called oxyhemoglobin and carbohemoglobin are formed. However, there is another variety of such associations that is quite stable: carboxyhemoglobin. It is a complex of protein and carbon monoxide, the stability of which explains asthma attacks with excessive toxicity.
Chlorophyll
Another representative of quaternary structure proteins, the domain bonds of which are supported by a magnesium ion. The main function of the whole molecule is to participate in the processes of photosynthesis in plants.
There are various types of chlorophylls that differ from each other by the radicals of the porphyrin ring. Each of these varieties is marked with a separate letter of the Latin alphabet. For example, the presence of chlorophyll a or chlorophyll b is characteristic of terrestrial plants, while other types of this protein are also found in algae.
Hemocyanin
This molecule is an analogue of hemoglobin in many lower animals (arthropods, mollusks, etc.). The main difference in the structure of the protein with the quaternary structure of the molecule is the presence of a zinc ion instead of an iron ion. Hemocyanin has a bluish color.
Sometimes people wonder what would happen if human hemoglobin is replaced with hemocyanin. In this case, the habitual content of substances in the blood, in particular amino acids, is violated. Also, hemocyanin unstably forms a complex with carbon dioxide, so blue blood would have a tendency to form blood clots.