Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Proteins
Structural Organization of Proteins

Proteins consist of A large number (at least 50) of Amino Acids linked together by peptide bonds. Therefore, they are called Polypeptides.

A peptide bond is a strong covalent bond between the amino group (—NH₂) of one Amino Acid and the carboxyl group (—COOH) of another amino acid. This process forms a dipeptide and releases a Water molecule:

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Fig. 87 Biosynthesis reactions of creatine and creatine phosphate

The names of Peptides are derived from the amino acids that make up the polypeptide sequence. In these names, with the exception of the final amino acid which retains its carboxyl group, the suffix "ine" is changed to "yl": glycylalanine, alanylglycyltyrosine.

Cell/13.html">Protein Structure, or Conformation

The Amino Acid Sequence in proteins determines their Spatial Structure (conformation). Each protein is unique in its structural Organization, which dictates its biological Functions, as will be discussed later using specific protein Examples.

Various types of chemical bonds are involved in The formation of the Spatial structure of Proteins. The primary chemical bonds in proteins are covalent Disulfide Bonds (—S—S—) formed between the —SH groups of Cysteine residues, Hydrogen Bonds formed by electrostatic forces of attraction between hydrogen and oxygen of different functional groups within the protein, and ionic bonds formed between ionized carboxyl (—COO^-) and amino (—NH₃) groups of amino acid side chains. There are four Levels of Protein structure: primary, secondary, tertiary, and quaternary (Fig. 88).

Fig. 88 Four levels of protein molecule structure

The Introduction/19.html">Primary Structure of a protein is the specific sequence of amino acids in a polypeptide chain, as well as their quantitative and qualitative composition. The arrangement of amino acids in individual proteins is genetically determined and underlies the individual and species Specificity of the protein.

Deciphering the Primary Structure of Proteins is of great practical importance as it opens up the possibility of laboratory synthesis. Following the sequencing of the hormone Insulin (Sanger, 1954) and immunoglobulin (Edelman and Porter, 1972), these proteins have been produced synthetically and are widely used in medicine. Studies of the primary structure of Hemoglobin have revealed structural alterations associated with specific diseases. Currently, the primary structure of over 1,000 proteins has been elucidated, including Enzymes such as Ribonuclease, carboxypeptidase, Myoglobin, Cytochromes, and many others.

The Secondary structure of a protein refers to the local spatial folding of the polypeptide chain. There are Three types of secondary structure: the α-Helix, the pleated sheet (or β-sheet), and the Collagen helix.

During the Formation of the α-helix, the polypeptide chain coils through hydrogen bonds in such a way that the turns of the peptide chain repeat periodically. This creates a compact and stable protein structure. The coiling of polypeptide chains was discovered by American scientists L. Pauling and R. Corey. The α-helix is characteristic of Keratins, which form the structural basis of Skin, Hair, and Nails.

The pleated sheet structure of a protein consists of linear polypeptide chains arranged parallel to one another and tightly bound by hydrogen bonds. Such a structure serves as the basis for fibrous (fibrillar) proteins (such as Myosin, hair β-keratin, etc.).

The collagen helix features a more complex folding of polypeptide chains. Individual chains are coiled and twisted around one another to form a superhelix. This structure is typical of collagen. The collagen helix possesses high elasticity and the tensile strength of a steel wire.

The Tertiary Structure of a protein is the overall spatial arrangement of the coiled and linear segments of a polypeptide chain into a compact structure, forming a globule (spherical or ovoid) or a fibril (thread-like). Tertiary structure ensures the biological activity of each protein. Disruption of this structure leads to a partial or complete loss of its biological function.

The quaternary structure of a protein is the association of multiple identical or different polypeptide chains (subunits), each with its own tertiary structure, into a complex biologically active molecule. The subunits are held together by non-covalent bonds. Many enzymes (e.g., Lactate dehydrogenase, Pyruvate dehydrogenase) as well as hemoglobin possess quaternary structure. Proteins with a quaternary structure are referred to as oligomeric or multimeric.

All four levels of protein organization are interconnected and maintain the native conformation of each protein. Tertiary and Quaternary structures play a particularly crucial role in biological activity, as they are highly sensitive to environmental changes. Since many intracellular enzymes possess quaternary structure, altering protein conformation serves as one of the mechanisms for regulating their activity.

Shape of Protein Molecules

Depending on their tertiary structure, Proteins can be thread-like (fibrillar) or spherical (globular) in shape.

Fibrillar Proteins serve as structural or contractile Materials within the Organism. For example, collagen is a component of tendons, Cartilage, and skin, contributing to the formation of the Skeletal System, while myosin functions as a Muscle contractile protein. These proteins are insoluble in water.

Globular proteins are capable of moving freely and penetrating the walls of Blood capillaries. They are soluble in water and found in the fluid environments of the body. Globular proteins include Components of the immune system (Antibodies), the muscle contractile protein Actin, all enzymes, as well as hemoglobin, myoglobin, and many other proteins.

Globular proteins can sometimes transform into fibrillar ones. Such A change in shape alters their properties (solubility, specific function). Upon transition to a fibrillar state, globular proteins become insoluble and poorly digestible, while enzymes lose their catalytic activity.