Proteins are biological polymers with a complex structure. They have a high molecular weight and consist of amino acids, prosthetic groups represented by vitamins, lipid and carbohydrate inclusions. Proteins containing carbohydrates, vitamins, metals or lipids are called complex. Simple proteins consist only of amino acids linked together by a peptide bond.
Peptides
No matter what structure the substance has, amino acids are the monomers of proteins. They form the base polypeptide chain, from which the fibrillar or globular structure of the protein is then formed. At the same time, protein can be synthesized only in living tissue - in plant, bacterial, fungal, animal and other cells.
The only organisms that cannot connect protein monomers are viruses and simple bacteria. All others are able to form structural proteins. But what substances are monomers of proteins, and how are they formed? Read about this and protein biosynthesis, polypeptides and the formation of complex protein structures, amino acids and their properties below.
The only monomer of a protein molecule is any alpha amino acid. In this case, a protein is a polypeptide, a chain of connected amino acids. Depending on the number of amino acids involved in its formation, dipeptides (2 residues), tripeptides (3), oligopeptides (containing from 2-10 amino acids) and polypeptides (many amino acids) are isolated.
Protein Structure Overview
The protein structure can be primary, slightly more complex - secondary, even more complex - tertiary, and the most complex - quaternary.
The primary structure is a simple chain into which protein monomers (amino acids) are connected through a peptide bond (CO-NH). The secondary structure is an alpha helix or beta folds. Tertiary is an even more complicated three-dimensional structure of the protein, which was formed from the secondary due to the formation of covalent, ionic and hydrogen bonds, as well as hydrophobic interactions.
The quaternary structure is the most complex and characteristic of receptor proteins located on cell membranes. This is a supramolecular (domain) structure formed by combining several molecules with a tertiary structure, supplemented by carbohydrate, lipid or vitamin groups. In this case, as with the primary, secondary and tertiary structures, alpha amino acids are monomers of proteins. They are also linked by peptide bonds. The only difference is the complexity of the structure.
Amino acids
The only monomers of protein molecules are alpha amino acids. There are only 20 of them, and they are almost the basis of life. Due to the appearance of the peptide bond, protein synthesis has become possible. After that, the protein itself began to perform structure-forming, receptor, enzymatic, transport, mediator and other functions. Thanks to this, a living organism functions and is able to reproduce.
The alpha amino acid itself is an organic carboxylic acid with an amino group attached to an alpha carbon atom. The latter is located next to the carboxyl group. In this case, protein monomers are considered as organic substances in which the terminal carbon atom carries both an amine and a carboxyl group.
The combination of amino acids in peptides and proteins
Amino acids combine into dimers, trimers and polymers via a peptide bond. It is formed by cleavage of the hydroxyl (—OH) group from the carboxyl region of one alpha amino acid and hydrogen (—H) from the amino group of another alpha amino acid. As a result of the interaction, water is split off, and at the carboxyl end there remains a C = O site with a free electron near the carbon of the carboxyl residue. In the amino group of another acid there is a (NH) residue with a free radical present at the nitrogen atom. This allows you to connect two radicals with the formation of bonds (CONH). It is called peptide.
Alpha Amino Acid Options
In total, 23 alpha amino acids are known. They are presented in the form of a list: glycine, valine, alanine, isolecin, leucine, glutamate, asparaginate, ornithine, threonine, serine, lysine, cystine, cysteine, phenylalanine, methionine, tyrosine, proline, tryptophan, oxyproline, arginine, histidine, asparagine and glutamine. Depending on whether they can be synthesized by the human body, these amino acids are divided into interchangeable and irreplaceable.
The concept of interchangeable and essential amino acids
The interchangeable human body can synthesize, while the irreplaceable should only come with food. At the same time, both irreplaceable and replaceable acids are important for protein biosynthesis, because without them the synthesis cannot be completed. Without one amino acid, even if all the others are present, it is impossible to build exactly the protein that the cell needs to perform its functions.
One mistake at any stage of biosynthesis is that the protein is no longer suitable, because it cannot be assembled into the desired structure due to violation of electron densities and interatomic interactions. Therefore, it is important for a person (and other organisms) to consume protein products that contain essential amino acids. Their absence in food leads to a number of disorders of protein metabolism.
Peptide bond formation process
The only monomers of proteins are alpha amino acids. They gradually merge into a chain of polypetid, the structure of which is previously stored in the genetic code of DNA (or RNA, if bacterial biosynthesis is considered). Moreover, protein is a strict sequence of amino acid residues. This is a chain ordered into a specific structure that performs a pre-programmed function in a cell.
Phased sequence of protein biosynthesis
The process of protein formation consists of a chain of steps: replication of a portion of DNA (or RNA), synthesis of informational RNA, its exit into the cell cytoplasm from the nucleus, attachment to the ribosome, and gradual attachment of amino acid residues that are delivered by transport RNA. The substance, which is a protein monomer, participates in the enzymatic reaction of the elimination of the hydroxyl group and the hydrogen proton, and then joins the expandable polypetid chain.
Thus, a polypeptide chain is obtained, which is already ordered in a certain predetermined structure already in the cellular endoplasmic reticulum and is supplemented with a carbohydrate or lipid residue, if necessary. This is called the process of "maturation" of the protein, after which it is sent by the cellular transport system to its destination.
The functions of synthesized proteins
Protein monomers are amino acids necessary to build their primary structure. The secondary, tertiary and quaternary structure is already formed itself, although sometimes it also requires the participation of enzymes and other substances. However, they are no longer the main ones, although they are essential for the proteins to fulfill their function.
An amino acid, which is a monomer of a protein, can have attachment sites for carbohydrates, metals, or vitamins. The formation of a tertiary or quaternary structure makes it possible to find even more places for the location of the insertion groups. This allows you to create a protein derivative that plays the role of an enzyme, a receptor, a carrier of substances into or out of a cell, immunoglobulin, a structural component of a membrane or cellular organelle, muscle protein.
Proteins formed from amino acids are the only basis for life. And today it is believed that life was just born after the appearance of the amino acid and due to its polymerization. Indeed, it is the intermolecular interaction of proteins that is the beginning of life, including the rational one. All other biochemical processes, including energy, are necessary for the implementation of protein biosynthesis, and as a result, the further continuation of life.