Classification and chemical properties of proteins in plant agrochemistry

Plant Protein Fractions and Their Properties

Plant proteins determine the nutritional value and technological properties of a harvest. They are divided into simple proteins, consisting solely of amino acids, and complex proteins (proteids), which contain additional non-protein groups. In agrochemistry, simple proteins are classified according to their solubility. This characteristic indicates how easily a protein is digested and how it behaves during the technological processing of raw materials.

• Mass of albumins — 15–17 thousand Da
• Mass of globulins — up to 100 thousand Da
• Mass of prolamins — 26–40 thousand Da
• Alcohol for extraction of prolamins — 70%
• Alkali for extraction of glutelins — 0.2%

Each group of proteins has unique properties and is distributed differently in tissues. Albumins are easily soluble in water; globulins require weak solutions of neutral salts (a 10% solution of sodium or potassium chloride is used for extraction); prolamins are extracted only with alcohol, and glutelins with diluted alkalis. Depending on the crop, specific proteins have their own names reflecting their botanical origin.

• Albumins: leucosins (in wheat, rye, and barley), legumelins (in peas and soybean), ricins (in castor bean).
• Globulins: hemp edestin, bean phaseolin, soybean glycinin, lupine conglutin, peanut arachin, pea vicilin, pumpkin cucurbitin, potato tuberin.
• Prolamins: storage proteins of cereals — wheat and rye gliadin, barley hordein, corn zein, oat avenin.
• Glutelins: complete proteins — wheat glutenin, rice oryzenin.

The distribution of these fractions in plants is extremely uneven. Globulins predominate in the seeds of legumes and oil crops, whereas in cereal grains, prolamins and glutelins make up the main share. For example, in soybean seeds, globulins account for up to 95% of total proteins, while in rice grain, up to 70% consists of glutelins.

Regulating Protein Composition with Top Dressing

The fractional composition of proteins in the marketable part of a harvest can be influenced by agronomic practices. Rice glutelins (oryzenins) contain all essential amino acids, which makes them the most valuable part of the grain. The nutritional completeness of the rice protein complex can be improved through the targeted application of micronutrient fertilizers.

According to field trials, foliar treatment of rice crops with micronutrient fertilizers during the tillering stage noticeably changes the ratio of nitrogenous fractions in ripe grain. The application of cobalt and manganese increases the oryzenin content to 71.2% and 71.4%, respectively (compared to 64.0% in the control). At the same time, the share of hard-to-digest nitrogen in the non-extractable residue decreases, which significantly improves the nutritional value of the cereal.

Next in quantity is the salt-soluble fraction – globulins, which account for 10% of total protein. Approximately 7% of the rice grain protein complex is represented by the water-soluble fraction – albumins. The most insignificant part consists of alcohol-soluble proteins, prolamins, which account for approximately 4%. In addition to the above fractions, the rice protein complex contains 11% nitrogenous substances that cannot be extracted by exchange solvents.

Fertilizers have a significant impact on the fractional composition of protein. Thus, top dressing foliar top dressing of growing plants with cobalt, copper, and molybdenum fertilizers significantly stimulates the process of oryzenin accumulation and reduces the share of albumins and insoluble protein substances in rice grain. These micronutrient fertilizers do not have a noticeable effect on the content of globulins and prolamins. Boron and manganese fertilizers increase the amount of globulins in the rice protein complex and decrease the content of albumins and prolamins, but do not significantly affect the alkali-soluble fraction. Of the studied micronutrient fertilizers, only zinc stimulates the accumulation of prolamins in rice grain, and this occurs against the backdrop of an increasing oryzenin content and a decrease in the share of albumins, globulins, and insoluble organic substances.

Protamines are highly basic, low-molecular-weight (molecular mass does not exceed 12,000 Da) proteins. They were discovered by Miescher in 1868 in spermatozoa, and their protein nature was deciphered by Kossel in 1886.

The high alkalinity of protamines is explained by the large amount of arginine, histidine, and lysine they contain, which account for up to 80% of the total amino acid content in the protein molecules. They lack sulfur-containing amino acids. Protamines are highly soluble in water and weak acids. They are found in germ cells and constitute the bulk of chromatin protein. Like histones, protamines form a complex with DNA, providing them with chemical stability. In plants, protamines have been found in pollen.

Histones are low-molecular-weight alkaline proteins that do not have a quaternary structure. However, their alkalinity is less pronounced than that of protamines, as they contain only 20-30% of basic amino acids. Histones are soluble in weak acids (0.2 N HCl) and are precipitated by ammonia and alcohol. In plants, they are localized in the cell nucleus, ribosomes, and mitochondria. Quantitatively predominating among chromosomal proteins, these proteins play an important role in the structure of chromatins. Histones are firmly bound to DNA and are part of nucleoproteins. Their main function is the regulation of genetic information transfer from DNA to RNA. Histones isolated from plant tissue can be divided into five fractions: H1, H2a, H2b, H3, and H4. Histones designated as H2a have a molecular mass from 16,600 to 19,000 Da, and those designated as H2b from 15,300 to 19,000 Da.