3. Read and translate the text. Proteins

Proteins are nitrogenous organic substances produced by and associated with living matter. They occur in both plants and animals, those from plants are more easily isolated in crystalline form. Plants usually store proteins in the form of aleuronic grains. Man derives part of his protein from such vegetables as beans, peas, nuts, and the seeds of grains (cereals), particularly wheat arid soyabeans, In animals, proteins occur as living matter, thus making thein difficult to obtain in the individual state. Meat, fish, eggs, milk products are important sources of animal protein foods.

The word "protein" is derived from the Greek word meaning "to take first place”, and the protein group ranks first among the organic compounds. They form the most important and the most characteristic constituents of all living cells, and are present in all fluids of the body, except urine and bile. They account for many of mechanical ele­ments of living things, from parts of subcellular membranes to skin, bones and tendons. Proteins are built from simpler units, monomers of appropriate types, which are called amino acids.

Since protein substances are essential constituents of all living cells, a lack of protein lowers the body's resistance to disease because, without it, the normal amount of disease- fighting antibodies is not produced by the blood. The proteins are not always of the same composition, and we must use a varied diet to get the different kinds of proteins necessary for the muscles, skin, hair, nails, blood and tissues.

A protein is simply a large polypeptide. At one end of the molecule there is a free amino group, and at the other end a free carboxylic

group, the other amino and carboxylic groups being involved in peptide linkages.

Protein structure is elegant and complex, so complex that it may be described at several levels. They may be classified into 3 groups: simple,, conjugated, and derived. The simple proteins hydrolyze entirely into amino acids; the conjugated proteins are combinations of a protein and a non-protein group (the latter is called the prosthetic group). Each of these groups has several subdivisions.

Although proteins are important in metabolism, relatively few isolated proteins are employed as therapeutic agents.

Whole glandular products, oil-bearing plant seeds, antitoxins, serums, and globulins contain proteins in combination with other biochemical substances — all these substances possess therapeutic activity. Allergens are usually pro- teinaceous by nature; however, carbohydrates and fats may also produce allergic reactions. Certain proteins are highly poisonous: the plant lectins (toxalbumines), ricin from castor beans, robin from locust bark. Among the poisonous animal proteins are hemolysins from salamanders and the various toxins, neurotoxoids, from snake venom.

The number of different proteins is enormous. And each of different possible proteins has its own distinct structure; primary, secondary, tertiary, and quaternary.

By twisting and folding, each specific kind of protein adopts a specific overall structure that distinguishes it from every other protein.