To imagine the importance of proteins, it is enough to recall the well-known phrase of Friedrich Engels: “Life is a way of existence of protein bodies”. In fact, on Earth, these substances, along with nucleic acids, determine all manifestations of living matter. In this paper, we will find out what the protein consists of, study what function it performs, and also determine the structural features of various species.
Peptides - Highly Organized Polymers
Indeed, in a living cell of both plant and animal, proteins quantitatively prevail over other organic substances, and also perform the greatest number of various functions. They participate in many different very important cellular processes, such as movement, defense, signaling function and so on. For example, in the muscle tissue of animals and humans, peptides make up 85% of the dry matter mass, and in the bone and dermis, from 15-50%.
All cellular and tissue proteins are composed of amino acids (20 species). Their number in living organisms is always equal to twenty species. Different combinations of peptide monomers form a variety of proteins in nature. It is calculated by an astronomical number of 2x10 18 possible types. In biochemistry, polypeptides are called high molecular weight biological polymers - macromolecules.
Amino Acids - Protein Monomers
All 20 types of these chemical compounds are structural units of proteins and have the general formula NH 2 -R-COOH. They are amphoteric organic substances capable of exhibiting both basic and acidic properties. Not only simple proteins, but also complex ones, contain the so-called replaceable amino acids. But irreplaceable monomers, for example, such as valine, lysine, methionine can be found only in certain types of proteins. Such proteins are called complete.
Therefore, when characterizing a polymer, not only how many amino acids does a protein consist of, but also which monomers are connected by peptide bonds to a macromolecule. We also add that non-essential amino acids, such as asparagine, glutamic acid, cysteine, can be independently synthesized in human and animal cells. Essential protein monomers are formed in the cells of bacteria, plants and fungi. They enter heterotrophic organisms only with food.
How is a polypeptide formed?
As you know, 20 different amino acids can be combined into many different protein molecules. How is the binding of monomers to each other? It turns out that carboxyl and amine groups of adjacent amino acids interact with each other. So-called peptide bonds are formed, and water molecules stand out as a by-product of the polycondensation reaction. The resulting protein molecules are composed of amino acid residues and repeating peptide bonds. Therefore, they are also called polypeptides.
Often, proteins can contain not one, but several polypeptide chains at once and consist of many thousands of amino acid residues. Moreover, simple proteins, as well as proteids, can complicate their spatial configuration. This creates not only a primary, but also a secondary, tertiary and even quaternary structure. Consider this process in more detail. Continuing to study the question: what does a protein consist of, let us find out what configuration this macromolecule has. We have established above that the polypeptide chain contains many covalent chemical bonds. It is such a structure that is called primary.
An important role is played by the quantitative and qualitative composition of amino acids, as well as the sequences of their compounds. The secondary structure occurs at the time of the formation of the spiral. It is stabilized by many newly emerging hydrogen bonds.
Higher levels of protein organization
The tertiary structure appears as a result of packing a spiral in the form of a ball - a globule, for example, myoglobin protein of muscle tissue has just such a spatial structure. It is supported by both newly formed hydrogen bonds and disulfide bridges (if several cysteine residues enter the protein molecule). The quaternary form is the result of combining several protein globules into a single structure through new types of interactions, for example, hydrophobic or electrostatic. Along with peptides, the non-protein parts also enter the quaternary structure. They can be magnesium, iron, copper ions, or the remains of orthophosphate or nucleic acids, as well as lipids.
Features of protein biosynthesis
Earlier, we found out what protein consists of. It is built from a sequence of amino acids. Their assembly into the polypeptide chain occurs in ribosomes - non-membrane organelles of plant and animal cells. Molecules of information and transport RNA also take part in the biosynthesis process itself . The former are the matrix for protein assembly, and the latter transport various amino acids. A dilemma arises in the process of cell biosynthesis, namely, a protein consists of nucleotides or amino acids? The answer is unequivocal - polypeptides, both simple and complex, consist of amphoteric organic compounds - amino acids. In the life cycle of a cell, there are periods of its activity when protein synthesis is especially active. These are the so-called J1 and J2 interphase stages. At this time, the cell is actively growing and needs a large amount of building material, which is protein. In addition, as a result of mitosis, which ends with the formation of two daughter cells, each of them needs a large amount of organic substances, therefore, active synthesis of lipids and carbohydrates occurs on the channels of the smooth endoplasmic reticulum, and protein biosynthesis occurs on the granular EPS.
Protein functions
Knowing what the protein consists of, one can explain both the huge variety of their species and the unique properties inherent in these substances. Proteins perform a variety of functions in a cell, for example, building, as they are part of the membranes of all cells and organoids: mitochondria, chloroplasts, lysosomes, Golgi complex, and so on. Peptides such as gamoglobulins or antibodies are examples of simple proteins that have a protective function. In other words, cellular immunity is the result of the action of these substances. Complex protein - hemocyanin, along with hemoglobin, performs a transport function in animals, that is, it carries oxygen in the blood. The signaling proteins that make up the cell membranes provide the cell with information about substances trying to get into its cytoplasm. The albumin peptide is responsible for the main indicators of blood, for example, for its ability to coagulate. The egg protein ovalbumin is stored in the cell and serves as the main source of nutrients.
Proteins - the basis of the cytoskeletal cell
One of the important functions of peptides is support. It is very important for maintaining the shape and volume of living cells. The so-called submembrane structures - microtubules and microfilaments intertwined form the inner skeleton of the cell. Proteins in their composition, for example, tubulin, are able to easily compress and stretch. This helps the cell maintain its shape under various mechanical deformations.
In plant cells, along with hyaloplasmic proteins, the supporting function is also performed by cytoplasm strands - plasmodesmata. Passing through the pores in the cell wall, they determine the relationship between the adjacent cellular structures that form the plant tissue.
Enzymes - substances of protein nature
One of the most important properties of proteins is their effect on the rate of chemical reactions. Basic proteins are capable of partial denaturation - the process of unwinding a macromolecule in a tertiary or quaternary structure. The polypeptide chain itself is not destroyed. Partial denaturation underlies both the signal and catalytic functions of the protein. The latter property is the ability of enzymes to influence the rate of biochemical reactions in the nucleus and cytoplasm of a cell. Peptides, which, on the contrary, reduce the speed of chemical processes, are usually called inhibitors rather than enzymes. For example, a simple catalase protein is an enzyme that speeds up the process of splitting a toxic substance of hydrogen peroxide. It is formed as the end product of many chemical reactions. Catalase accelerates its utilization to neutral substances: water and oxygen.
Protein Properties
Peptides are classified in many ways. For example, in relation to water, they can be divided into hydrophilic and hydrophobic. Temperature also affects the structure and properties of protein molecules in different ways. For example, keratin protein, a component of nails and hair, can withstand both low and high temperatures, that is, it is thermolabile. But the ovalbumin protein, already mentioned earlier, is completely destroyed when heated to 80-100 ° C. This means that its primary structure is split into amino acid residues. Such a process is called destruction. Whatever conditions we create, the protein cannot return to its native form. Motor proteins - actin and milosin are present in muscle fibers. Their alternate contraction and relaxation underlies the work of muscle tissue.