Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Protein Chemistry

The world of the most complex is life itself.

N.N. Semenov

Living organisms are characterized by the highest degree of order in their constituent ingredients and a unique structural Organization that ensures both their phenotypic traits and a wide diversity of biological Functions. Within this Structural and functional unity of organisms, which constitutes the very essence of life, Proteins play a crucial role that cannot be replaced by any other Organic compounds.

Proteins are high-molecular-weight nitrogen-containing organic substances whose molecules are built from amino acid residues. The name "proteins" (from the Greek protos — first, primary) apparently reflects more accurately the paramount Biological Significance of this Class of substances. The terms "proteins" and "protein substances" traditionally used in the domestic literature originated from the discovery in animal and plant Tissues of substances resembling the protein of chicken egg white. Nowadays, with the absolute certainty that hereditary information is localized in the DNA molecules of Cells across All living organisms, there is no doubt that proteins are the sole molecular instruments through which Genetic information is realized. Without proteins, Enzymes in particular, DNA cannot replicate or self-reproduce, i.e., it is deprived of The ability to transmit genetic information.

Living nature is characterized by A number of properties that distinguish it from non-living matter, and almost all of these properties are associated with proteins. First and foremost, living organisms exhibit A wide variety of protein structures and their high degree of order, which persists across both time and space. The remarkable ability of living organisms to reproduce their own kind is likewise linked to proteins. Contractility and movement—indispensable attributes of living systems—are directly related to the protein structures of the muscular apparatus. Finally, life is unthinkable without METABOLISM, the continuous renewal of the constituent parts of a living Organism, i.e., without the processes of anabolism and Catabolism (that striking unity of biological opposites), which are underpinned by The activity of catalytically active protein enzymes.

Thus, proteins constitute The basis of both the Structure and function of living organisms. As one of the founding fathers of molecular biology, F. Crick, figuratively put it, proteins are important primarily because they can perform the most diverse functions with extraordinary ease and elegance. It has been estimated that there are approximately 10¹⁰-10¹² different proteins in nature, ensuring the existence of about 10⁶ species of living organisms varying in organizational complexity, from Viruses to humans. Out of this vast number of natural proteins, the precise structure and conformation of only a negligible fraction are currently known (see below). Each organism is characterized by a unique set of proteins. Phenotypic traits and The Diversity of functions are driven by the specific assembly of these proteins, frequently into supra- and multi-molecular structures that in turn determine the ultrastructure of cells and their Organelles.

An E. coli Cell contains about 3,000 different proteins, whereas The Human Body boasts over 100,000 diverse proteins. Most remarkably, all natural proteins are composed of a relatively small number of simple structural blocks represented by monomeric molecules—Amino Acids linked together into polypeptide chains. Natural Proteins are built from 20 different amino acids. Because these Amino acids can combine in a vast variety of sequences, they are capable of forming an immense number of diverse proteins. The number of isomers obtainable through all possible permutations of a given number of amino acids in a polypeptide reaches astronomical magnitudes. For instance, while just 2 amino acids allow for The formation of only two isomers, 4 amino acids theoretically permit 24 isomers, and 20 amino acids can yield 2.4 ∙ 10¹⁸ diverse proteins.

It is easy to foresee that as the number of repeating amino acid residues in a protein molecule increases, the number of possible isomers surges to astronomical scales. Obviously, nature cannot afford random combinations of Amino acid sequences; rather, each species possesses its own specific set of proteins determined, as is now well established, by the hereditary information encoded in the DNA molecules of living organisms. It is precisely the information contained in the linear sequence of DNA NUCLEOTIDES that dictates the linear sequence of amino acid residues in the polypeptide chain of the synthesized protein. The resulting linear polypeptide chain is now endowed with functional information, in accordance with which it spontaneously folds into a definite, stable three-dimensional structure. Thus, the labile polypeptide chain folds and twists into the spatial architecture of the protein molecule—not chaotically, but in strict accordance with the information contained in the Amino Acid Sequence. Given the leading role of proteins in living nature and the fact that proteins comprise nearly half the dry mass of a living organism while possessing an astonishing variety of functions, The Study of biochemistry in medical universities traditionally begins with this class of organic substances.