The role and functional significance of proteins in plant life activity

The functions of proteins in an organism are extremely important and diverse. Each protein, as a substance with a specific chemical structure, performs one highly specialized function and, in rare cases, several interrelated ones. The most important functions of proteins in plants include: 1) catalytic; 2) storage; 3) structural; 4) protective; 5) transport; 6) regulatory.

Catalytic or enzymatic function. All chemical transformations in a cell occur with the participation of catalytically active proteins called enzymes. They constitute the most diverse and highly specialized class of proteins. To date, more than 2,000 different enzymes have been discovered, each possessing a unique structure and containing an active site capable of "recognizing its own" molecule and selectively interacting with it.

Storage function. Protein is a highly reduced compound with a large energy reserve. The oxidation of 1 g of protein releases 23 kJ of energy. For comparison, here is the caloric content of other substances that are also stored in plant cells: glucose – 15.5 kJ/g, starch – 17.6, fat – 38.2 kJ/g. The high quality of protein as a nutrient material is also due to the fact that, unlike other storage substances, its molecule contains nitrogen. Storage proteins accumulate in large quantities in seed cells during maturation on the plant, and then, during seed germination, they are hydrolyzed into amino acids or low-molecular-weight peptides, which are then used by the cell to form a new plant at early stages of development. The most well-known examples of such proteins are the proteins of wheat, corn, and rice caryopses. The quality of agricultural produce is assessed not only by protein content but also by protein digestibility. The biological value of proteins is determined primarily by their amino acid composition. If we take the value of milk or egg protein as 100%, then the biological value of rice grain proteins will be 83-86%, oats – 70-78%, rye – 68-75%, wheat – 62-68%, corn – 52-58%. The lower biological value of corn grain protein is due to the fact that it is poor in the essential amino acids lysine and tryptophan.

Protective function. Many proteins protect the plant from the invasion of other organisms or shield it from damage. The protective (immune) function of proteins is realized by antibodies, interferons, and also by toxic proteins produced by the plant. Antibodies are compounds of a protein nature, the synthesis of which increases during the immune response – the organism's reaction to the penetration of foreign proteins or other alien components, such as high-molecular-weight carbohydrates, into the internal environment. By binding to an antigen, antibodies form an insoluble complex, making the antigen harmless to the organism. Interferons are glycoproteins synthesized by a cell after a virus penetrates it. Unlike antibodies, interferons do not interact with the antigen but trigger the formation of intracellular enzymes. They block the synthesis of internal proteins, preventing the copying of viral information. This suspends the multiplication of the virus.

Extremely interesting are lectin proteins, which possess agglutinating (sticking) properties in relation to many carbohydrate-nature substances, including glycoproteins of pathogenic fungi and bacteria. These proteins are located on the surface of the cell membrane and even penetrate the cell wall. They are very specific regarding various types of infection: apple tree leaves contain proteins that "recognize" even individual strains of bacteria. Lectins specifically bind pathogenic bacteria, forming a kind of thrombus, which prevents the further spread of the disease. The ability of proteins from wheat germs, specific to chitin oligomers, to bind spores and the tips of hyphae is well-known. Lectins from soybeans and peanuts specifically bind and prevent the elongation of their mycelium. Protective proteins shield a living organism from destruction or contribute to its survival when damaged. The protective function of toxin proteins produced by plants lies in the fact that they protect plants from being eaten by animals and also inhibit the proteolytic enzymes of insect pests that damage the seeds of many plants. Objects differing in immune properties vary both in the quantity and quality of proteins present in a healthy organism and in the changes that occur in the protein spectrum after infection. The high nutritional value of proteins partially explains the fact that plants containing a large amount of protein nitrogen often better withstand diseases. For example, during the storage of agricultural products, losses in such cultivars are lower than in those containing less protein (Table 5; Rubin B.A., Artsikhovskaya E.V., Aksenova V.A., 1975).

Table 5 – Distribution of nitrogen forms in the leaves of stored cabbage heads, % of total nitrogen

 Cultivar Date Protein nitrogen Amine nitrogen Amager (resistant) 03.12 47.0 32.7 13.04 37.5 42.3 Nomer pervy (non-resistant) 03.12 39.4 39.0 13.04 10.2 58.3

In qualitative terms, the proteins of resistant forms often contain many amino acids that are poorly tolerated by a particular pathogen. For example, *Phytophthora* "does not like" proteins with a high arginine content. At the same time, grapevine cultivars easily susceptible to anthracnose are distinguished by an increased asparagine content compared to resistant ones. It is interesting that not only the composition but also the structure of protein forms resistant and non-resistant to infection differs. Thus, in the cells of potatoes resistant to cancer, proteins with an unusually dense, rigid globule have been discovered. The most active resistance to infection is provided by so-called anti-enzymes capable of inhibiting parasite enzymes: inhibitors of fungal α-amylases have been isolated from cereal seeds; gluconase and chitinase enzymes cause maceration, i.e., the softening (disintegration) of the mycelium of many parasitic fungi.

The set of proteins responsible for the immune properties of organisms changes during the course of evolution. This occurs in response to the appearance of new proteins in pathogens that cause harm to the plant. Thus, a parallel development of virulence properties in the pathogen and resistance in the plant takes place, determined primarily by the specificity of their proteins.

Structural (plastic) function. More than half of the dry mass of the cytoplasm and its organelles belongs to proteins. Structural proteins are part of plant integumentary tissues, including those of seeds and fruits. In complex with lipids, they also form the structural basis of cell biomembranes, ensuring that cell organelles maintain the necessary sequence of biochemical reactions.

Transport function. This group includes proteins that perform the binding and transport of substances between tissues across cell membranes. According to the carrier concept, the passage of substances through membranes is carried out with the help of special protein carrier molecules located directly in the membranes. An ion passes through the membrane not in a free form, but by binding to a carrier on its outer surface. Neither the carrier itself nor its complex with the ion can move into the external environment. However, the carrier-ion complex is mobile within the membrane itself and moves to its opposite side. Here, this complex breaks down and releases the ion into the internal environment, and the carrier itself moves back to the outer side of the membrane, where it once again binds to another ion. Carriers are specific, i.e., they participate in the transport of certain ions and thereby ensure the selectivity of substance uptake into the cell.

Regarding the mechanism of carrier movement in the membrane, there are three hypotheses: 1) the carrier diffuses; 2) the carrier slides (assuming the presence of pores in the membrane; in this case, it migrates along the pore walls); 3) the carrier rotates in the membrane (Fig. 8; Yakushkina N.I., 1980).

Regulatory function. Some proteins participate in the system of regulating cellular or physiological activity. These include many hormones that regulate glucose metabolism, the transport of calcium ions, and phosphate ions.

Fig. 8. Mechanisms of membrane carrier action:

A – diffusing; B – rotating; C – sliding

Hormones regulate metabolism within cells and integrate metabolism in various cells of the organism as a whole. Monellin, synthesized by one of the African plants and used for sweetening food, can be attributed to regulatory proteins. The consumption of monellin prevents obesity in humans and livestock animals.