Proteins

Proteins are large molecules found universally in the cells of living organisms, or in such biological fluids as blood plasma. They invariably contain carbon, hydrogen, oxygen and nitrogen, almost invariably contain sulfur, and sometimes contain phosphorus. They are specifically characterized by yielding a mixture of alpha amino acids when hydrolyzed by means of acids, alkalies or certain enzymes.

Proteins are exceedingly diverse in properties and functions. Some are relatively inert fibers, such as the keratins of wool, hair or horn, or the collagens of tendon and connective tissue, which play an important structural role in animal organisms. Others are readily soluble in water or in dilute salt solutions, such as ovalbumin of egg white, serum albumin of blood plasma, or hemoglobin of red blood cells, and their molecules are not very far from spherical in shape. These are often called globular or corpuscular proteins in contradistinction to the fibrous proteins. Many of them can be obtained from water in crystalline form, and x-ray studies have shown that protein crystals are highly ordered systems-true crystals in every respect. All known enzymes, the essential catalysts of biological systems, are proteins, many being present in solution in cytoplasm or cellular secretions, others more or less firmly anchored to larger cellular structures. A number of hormones, such as insulin and several of the hormones of the pituitary gland, are also proteins, as are all the antibodies which are called forth in immunological reactions. Protein foods are essential to the nutrition of all animals. Proteins are thus of prime importance in the functioning of all living organisms.

Many proteins have been obtained which yield only alpha amino acids, and no other substances, on hydrolysis-for example, the hormone insulin and the enzyme pepsin. These are known as simple proteins. Others yield additional compounds beside the amino acids; these are known as conjugated proteins. There are numerous classes of conjugated proteins; they include the glyco-proteins and mucoproteins, which contain carbohydrate groups; lipoproteins, which contain fatty acids, cholesterol and phospholipids; heme proteins, such as hemoglobin and several oxidative enzymes, which contain iron-prophyrin (heme) groups; nucleoproteins, in which proteins are associated with nucleic acids; and many others. Many of the simpler viruses, such as tobacco mosaic or tomato bushy stunt virus, are nucleoproteins; and all viruses are constituted largely of proteins and nucleic acids.

In the past proteins have commonly been classified according to their solubility in various solvents; for instance, albumins, which are readily soluble in pure water; globulins, which are insoluble in water but dissolve readily in aqueous salt solutions; prolamines, which are soluble in alcohol-water mixtures but insoluble in either pure alcohol or pure water; and various other classes. This terminology is largely arbitrary, although still used in the naming of many proteins.

Proteins which are known to exert a specific activity are often given names which distinguish this activity. This is particularly true of the enzyme proteins, which catalyze specific chemical reactions, and the protein hormones, which arouse specific physiological responses.

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