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

Protein Chemistry
Classification of Proteins

To date, a systematic and well-organized Nomenclature and Classification of Proteins has not yet been established. The traditional Classification of Proteins into groups—largely based on arbitrary parameters such as physicochemical properties, molecular shape, localization, origin, and Amino Acid Composition—no longer fully meets the demands of our advanced understanding of protein Structure and function. Out of the vast multitude of naturally occurring proteins, the structures and Functions have been elucidated for a relatively small number (not more than several hundred), and therefore structure and function cannot yet serve as a basis for a rational classification system. Perhaps the only group of proteins possessing The ability to catalyze Chemical Reactions—namely, Enzymes—features a coherent system of nomenclature and classification based on the types of catalyzed chemical reactions and the Chemical Nature of the reacting substances. However, fully identified enzymes still account for only a minor fraction of all proteins (over 3,000 enzymes have been described). Nevertheless, the functional principle recommended by some authors, although insufficient as a universal basis for classifying all proteins, is of considerable interest. According to this functional approach, proteins are divided into 12 main classes: 1) catalytically active proteins (enzymes); 2) hormone proteins (although Steroid Hormones also exist); 3) genome activity regulatory proteins; 4) protective proteins (Antibodies, Components of the Blood clotting and anticoagulation systems); 5) toxic proteins; 6) transport proteins; 7) Membrane Proteins; 8) contractile proteins; 9) receptor proteins; 10) enzyme inhibitor proteins; 11) viral coat proteins; and 12) proteins with other functions.

Attempts have also been made to classify proteins based on the features of their secondary and tertiary structure. Accordingly, proteins are categorized into a-, ß-, a+ß-, and a/ß-proteins. a-Proteins contain exclusively a-helices (at least 60%), ß-proteins consist solely of ß-structures (at least two antiparallel strands), a+ß-proteins incorporate both structures within a single polypeptide chain (e.g., Lysozyme molecules), while the a/ß-protein Class contains numerous a- and ß-structures alternating along the polypeptide chain or domain (see Fig. 1.19). Domains are formed by the combination and alternation of a-helices and ß-sheets, interspersed with more loosely packed structures.

Recognizing the necessity for medical students to acquire a thorough understanding of specific protein groups relevant to their future professional practice (such as blood proteins), we present the classical Protein Classification along with a Brief Overview of recent data regarding the structure, composition, and Properties of Individual representatives. According to this classification, the vast class of protein substances is divided into simple and complex proteins based on their chemical composition*.

Simple proteins are composed exclusively of amino acid residues and, upon Hydrolysis, break down exclusively into free Amino Acids.

Complex proteins are two-component systems consisting of a simple protein moiety and a non-protein component known as the prosthetic group. Upon hydrolysis of complex proteins, alongside free amino acids, the non-protein moiety or its breakdown products are released.

Simple proteins, in turn, are subdivided into several subgroups based on certain conventionally chosen criteria: protamines, Histones, albumins, globulins, prolamins, glutelins, etc. The classification of complex proteins (see Chapter 2) is based on the chemical nature of their non-protein constituent. Accordingly, they are divided into Phosphoproteins (containing phosphoric acid), Chromoproteins (containing pigments), Nucleoproteins (containing Nucleic Acids), Glycoproteins (containing CARBOHYDRATES), Lipoproteins (containing Lipids), and Metalloproteins (containing metals).

* The division of proteins into simple and complex is likely conventional, since many proteins traditionally regarded as simple (e.g., Ovalbumin) are actually complex, as they contain a non-protein component—most frequently of a carbohydrate or lipid nature—or a metal ion (see Chapter 2).