Fundamentals of Biochemistry - A. A. Anisimov 1986

Proteins
General Characteristics

One of the fundamental differences between the DEFINITION OF LIFE given by F. Engels and earlier idealistic Structure/97.html">Definitions is that it emphasizes the materiality of life and identifies its material bearer: protein. Engels calls protein "...the exclusive independent bearer of life"¹. The paramount importance of Proteins in the vital activity of organisms was also emphasized by prominent Russian materialist biologists, such as I. M. Sechenov, A. Y. Danilevsky, K. A. Timiryazev, I. P. Pavlov, and others.

It is now established that there are no non-protein organisms in living nature. At the same time, the term "proteins" should be understood in a broad sense, including complex proteins, particularly Nucleoproteins—Complexes of Proteins with Nucleic Acids. This renders completely groundless and scholastic any discussions about which is more important for life: proteins or nucleic acids.

Proteins are high-molecular-weight polymeric compounds that yield Amino Acids upon Hydrolysis. Sometimes they are called proteins (from the Greek protos—first, most important). For living organisms, they are indeed paramount both in content and, above all, in significance. In animal organisms, proteins account for up to 40–50% or more of the dry mass, and less in plants—up to 20–35%. The Functions of Proteins are diverse and highly important.

Structural function. Proteins form The basis of the protoplasm of any living Cell; in complex with Lipids, they serve as the primary structural material for all cell membranes and Organelles.

Catalytic function. All Enzymes are proteins, whether simple or complex. Thus, practically all biochemical reactions are catalyzed by protein enzymes.

Motile function. All forms of movement in living nature (Muscle contraction, the movement of Cilia and flagella in Protozoa, the movement of protoplasm within a cell, etc.) are carried out by the protein structures of Cells.¹

¹ Marx K., Engels F. Works, 2nd ed., vol. 20, p. 514.

Transport function. The Blood protein Hemoglobin transports oxygen from the Lungs to Tissues and Organs. The transport of Fatty acids through the body involves another blood protein, albumin. There are also blood proteins that transport lipids, iron, and Steroid Hormones. The transfer of many substances across cell membranes is carried out by specialized carrier proteins.

Protective function. The most critical factors of Immunity—Antibodies and The Complement System—are proteins. The blood clotting process, which protects the Organism from excessive blood loss, is based on the transformations of the blood protein fibrinogen. These transformations occur with the participation of the protein Thrombin and A large number of other clotting factors, which are also proteins. The inner walls of the Esophagus and Stomach are lined with a protective layer of mucous proteins called mucins. Toxins from many species of organisms that protect them in the Struggle for Existence are also proteins (snake venoms, Bacterial toxins). The basis of Skin, which protects the animal body from many external influences, is the protein Collagen. Keratin is the protein of the protective Hair coat.

Hormonal function. A number of hormones are structurally classified as proteins (Insulin) or Peptides (adrenocorticotropic hormone, oxytocin, vasopressin, etc.).

Reserve function. Ovalbumin in eggs, casein in milk, and many plant seed proteins are capable of forming nutrient reserves.

Supportive function. Tendons, joint connections, skeletal bones, and hooves are formed to a significant extent by proteins. Receptor function. Many proteins (especially Glycoproteins and Lectins) perform the crucial function of selective recognition and binding of specific substances.

The listed functions of proteins do not exhaust their entire diversity. For instance, the regulatory action of proteins is not limited solely to catalytic and hormonal effects. A very important group of proteins that regulate genome activity is well known. Some Polypeptides play The Role of Enzyme Inhibitors and thereby regulate their action. Of great interest are studies of so-called Brain-specific proteins (BSPs) conducted in recent years. Subtle immunochemical Methods for their analysis in CEREBROSPINAL FLUID and blood have made it possible to establish a correlation between the levels of specific groups of BSPs and behavior, as well as certain aspects of mental activity.

Proteins also hold immense economic significance. This is determined primarily by the fact that Proteins are essential components of human food and livestock feed. Mass epidemics and the low average life expectancy in some formerly colonial countries are often associated with chronic Protein deficiency (especially a lack of animal proteins). The productivity of farm animals depends significantly on the protein content in their feed. The technology of many industries is based on the Processing of proteins and the modification of their properties, such as in the tanning industry, fur dressing, and natural silk production. In baking, The properties of flour proteins play a critical role.

The paramount role of proteins in the vital activity of all organisms and their great economic significance explain the enormous interest shown in them in biochemistry, establishing proteins at the very center of biochemical research.

It is most convenient to begin examining the Structure and properties of proteins with a characterization of their structural units—amino acids.