BIOPOLYMERS

Fig. 1. Formation of the quaternary structure of globular proteins. Sparsely shaded are polar (hydrophilic) parts of protein globules, densely nonpolar (hydrophobic) regions.

For example, the structure of native DNA is formed by two complementary strands and is a Crick-Watson double helix; in it, opposite nitrogenous bases are paired with hydrogen bonds - adenine with thymine and guanine with cytosine. The stability of the double helix is ensured, along with hydrogen bonds, also by the hydrophobic interaction between flat rings of nitrogenous bases arranged in a stack (stack interaction, or stacking). The RNA strands are only partially spiralized. The DNA of viruses, bacteriophages, bacteria, as well as mitochondrial DNA, in some cases, is a closed ring; at the same time, along with the Crick-Watson spiral, an additional so-called superspiralization is observed.

K o n f o r m a c i I, i.e. one or another spatial shape of the molecules of B., is determined by their primary structure. Depending on the chemical. buildings and exterior. the conditions of the molecule B. can be either in one or in several. predominant conformations (usually occurring in natural conditions, the native states of B.: for example, the globular structure of proteins, the double helix of DNA), or take many b. or m, equally probable conformations. Proteins are divided according to their spatial structure into fibrillar (filamentous) and globular; proteins are enzymes, carrier proteins, immune cells, etc. they have, as a rule, a globular structure. For a number of proteins - hemoglobin, myoglobin, lysozyme, ribonuclease, etc. - this structure is established in all details (with the determination of the location of each atom using X-ray diffraction analysis). It is determined by the sequence of amino acid residues and is formed and maintained by relatively weak interactions between monomeric links of polypeptide chains in an aqueous salt solution (Coulomb and dipole forces, hydrogen bonds, hydrophobic interactions), as well as disulfide bonds. The protein globule is formed in such a way that most of the polar hydrophilic amino acid residues are outside and in contact with the solvent, and most of the nonpolar (hydrophobic) residues are inside and isolated from interaction with water. Protein molecules with an excess of nonpolar groups, when some of them appear on the surface of the globule, form a higher, i.e. quaternary structure, with a few. globules aggregate, interacting with each other mainly by non-polar regions (Fig. 1). Of course, the structure of each protein-enzyme is unique and provides the location necessary for its functioning in the space of all the links of the protein, especially the so-called active centers. At the same time, it is not absolutely rigid and allows for conformational shifts and changes necessary in the process of functioning (when interacting with substrates, inhibitors, etc. substances).

The primary structure of B. The composition and sequence of monomeric units of B. determine their so-called primary structure. All nucleic acids are linear heteropolymers-sugar phosphate chains, to the links of which are attached side groups -nitrogenous bases: adenine and thymine (in RNA - uracil), guanine and cytosine; in some cases (mainly in t-RNA), side groups can be represented by other nitrogenous bases. Proteins are also heteropolymers; their molecules are formed by one or more. polypeptide chains connected by disulfide bridges. The composition of polypeptide chains includes 20 types of various monomeric links - amino acid residues. Mol. the mass of DNA varies from several million (in small viruses and bacteriophages) to one hundred million or more (in larger phages); bacterial cells contain one DNA molecule with a mol. mass in nesk. billion The DNA of higher organisms can have a large mole. mass, but it has not yet been possible to measure it due to breaks in DNA molecules that occur during their isolation. Ribosomal RNAs have a mol. mass from 600 thousand to 1.1 million, information (i-RNA) - from hundreds of thousands to several. millions, transport (t-RNA) - about 25 thousand. The mass of proteins varies from 10 thousand (or less) to millions; in the latter case, however, it is usually possible to divide a protein particle into subunits interconnected by weak, b. h. hydrophobic, bonds.

BIOPOLYMERS,

BIOPOLYMERS, high-molecular natural compounds that are the structural basis of all living organisms and play a decisive role in the processes of vital activity. B. include proteins, nucleic acids and polysaccharides; there are also mixed B.- glycoproteins, lipoproteins, glycolipids, etc.

Proteins perform a number of important functions in the cell. Proteins-enzymes carry out all chemical reactions of

metabolism in the cell, conducting them in the necessary sequence and at the right speed. Proteins of muscles, flagella of microbes, cellular villi, etc. they perform a reduced function, turning chemical energy into mechanical. work and ensuring the mobility of the body as a whole or its parts. Proteins - osn. the material of most cellular structures (including in special types of tissues) of all living organisms, virus shells and phages. Cell membranes are lipoprotein membranes, ribosomes are constructed of protein and RNA, etc. The structural function of proteins is closely related to the regulation of the intake of various substances into subcellular organelles (active ion transport, etc.) and with enzymatic catalysis. Proteins also perform regulatory functions (repressors), "prohibiting" or "allowing" the manifestation of a particular gene. In higher organisms, there are proteins that carry certain substances (for example, hemoglobin, a carrier of molecular oxygen) and immune proteins that protect the body from foreign substances that enter the body (see Immunity). Polysaccharides perform structural, reserve and some other functions. Proteins and nucleic acids are formed in living organisms by matrix enzymatic biosynthesis. There are now biochem. extracellular synthesis systems are developed with the help of enzymes isolated from cells. Chemical methods have been developed. synthesis of proteins and nucleic k-T.

Biological functions B. Nucleic acids perform genetic functions in the cell. The sequence of monomeric units (nucleotides) in deoxyribonucleic acid - DNA (sometimes in ribonucleic acid - RNA) determines (in the form of a genetic code) the sequence of monomeric units (amino acid residues) in all synthesized proteins and, thus, the structure of the organism and the chemical processes occurring in it. During the division of each cell, both daughter cells receive a complete set of genes due to the previous self-duplication (replication) of DNA molecules. Genetic. information from DNA is transferred to RNA synthesized on DNA as a matrix (transcription). This so-called informational RNA (i-RNA) serves as a matrix for protein synthesis occurring on special cell organoids - ribosomes (translation) with the participation of transport RNA (t-RNA). The biological variability necessary for evolution is carried out at the molecular level due to changes in DNA (see Mutation).

BIOPOLYMERS,

high-molecular natural compounds that are the structural basis of all living organisms and play a decisive role in the processes of vital activity. B. include proteins, nucleic acids and polysaccharides; there are also mixed B.- glycoproteins, lipoproteins, glycolipids, etc.