Textbook - BIOLOGICAL CHEMISTRY - Hubskyi Yu.I. - 2000

Chapter I. BIOMOLECULES AND CELLULAR STRUCTURES

CHAPTER 3. NUCLEIC ACIDS. NUCLEOTIDES

Nucleic Acids — Deoxyribonucleic Acids (DNA) and Ribonucleic Acids (RNA) — are polynucleotides consisting of monomeric units known as Nucleotides (mononucleotides).

Nucleotides — three-component compounds composed of a purine or pyrimidine nitrogenous base, a pentose residue (ribose or deoxyribose), and a phosphate group.

Nucleic acids are high-molecular-weight compounds with molecular weights ranging from several thousand (Transfer RNAs) to several million daltons (eukaryotic DNA). They are Biopolymers that, along with Proteins, belong to the Class of informational Biomacromolecules. Nucleic acids perform A number of unique biological Functions not shared by other biopolymers: they ensure the preservation and transmission of hereditary information to progeny, and directly participate in the mechanisms of its expression by programming the template synthesis of all proteins in an individual Organism.

Nucleotides serve as structural components (monomer units) of nucleic acid molecules — DNA and RNA. In addition, certain ribonucleotides and their derivatives that are not part of nucleic acids (free nucleotides) function as Coenzymes, Cofactors, and allosteric effectors in various enzyme systems. Free nucleotides are of particular importance in enzymatic processes associated with the accumulation, storage, and intermolecular transfer of energy within Cells.

3.1. NUCLEOTIDES: STRUCTURE, BIOCHEMICAL FUNCTIONS

Structure of Nucleotides

Complete Hydrolysis of nucleic acids (acidic or alkaline) yields purine and pyrimidine nitrogenous bases, pentoses (D-ribose or 2-deoxy-D-ribose), and phosphoric acid.

The structure of the nitrogenous bases in nucleotides is based on the aromatic heterocyclic compounds purine and pyrimidine.

Purine Bases of Nucleic Acids

Nucleic acid hydrolysates consistently contain two purine bases — adenine (A) and guanine (G) — which have the following structure:

Pyrimidine Bases of Nucleic Acids

The nucleotides of nucleic acids contain three main pyrimidine bases: uracil (U), thymine (T), and cytosine (C).

Oxypurines and oxyprimidines can exist in two tautomeric forms, lactam and lactim, depending on the pH of the medium. Within nucleic acids, oxypurine and oxypyrimidine derivatives occur in the lactam form, which facilitates The formation of intermolecular Hydrogen Bonds between Purines and Pyrimidines of separate strands in the double-stranded structure of DNA molecules and in single-stranded RNAs.

Nucleosides — two-component bioorganic molecules consisting of a purine or pyrimidine nitrogenous base (from English "Base") and a pentose (D-ribose or 2-deoxy-D-ribose). Chemically, nucleosides are N-Glycosides of ribose or deoxyribose and a corresponding nitrogenous base. The Formation of the N-glycosidic bonds involves N-1 of the pyrimidine and C-1 of the pentose in pyrimidine nucleosides, and N-9 of the purine and C-1 of the pentose in purine nucleosides.

Phosphorylation (acylation by phosphoric acid) of a specific hydroxyl group in the pentose of a nucleoside leads to the formation of a nucleotide (nucleoside monophosphate). Nucleotides (and nucleosides) of DNA contain 2-deoxy-D-ribose, while those of RNA contain D-ribose:

Depending on THE POSITION OF pentose hydroxyl phosphorylation, Three types of nucleotides (nucleoside monophosphates, NMPs) are distinguished:

Hydrolysis of nucleic acids predominantly yields nucleoside-5'-phosphates (NMPs). Besides the difference in their pentose sugars, RNA and DNA nucleotides also differ in the composition of their pyrimidine bases (Table 3.1).

Table 3.1. Nomenclature of RNA and DNA Nucleosides and Nucleotides

Minor nucleotides

In addition to the five major nitrogenous bases mentioned above (two purines and three pyrimidines), certain nucleic acids contain relatively small amounts of additional (minor) nitrogenous bases and their corresponding minor nucleotides. The highest Abundance of minor nucleotides is found in Transfer RNA (tRNA) molecules, accounting for up to 5% of their total nucleotide composition. Minor nucleotides include methylated derivatives of standard nitrogenous bases, such as 1-methyladenine, 2-methyladenine, 6-dimethyladenine, 1-methylguanine, 7-methylguanine, 1-methyluracil, 5-hydroxymethyluracil, 3-methylcytosine, etc. Human DNA contains significant amounts of 5-methylcytosine, whereas messenger RNAs contain N-methylated derivatives of adenine and guanine.

An unusual nucleotide found in tRNA is pseudouridine (Ψ), in which ribose is attached to uracil at the 5-position via a carbon-carbon bond rather than the standard carbon-nitrogen bond.

The BIOLOGICAL FUNCTIONS OF minor nucleotides are not yet fully understood.

Biochemical functions of free nucleotides:

1. Participation in METABOLISM/26.html">Energy Metabolism (Oxidative Phosphorylation reactions) — a function performed by the nucleotides of the adenylic system (ATP, ADP). These same nucleotides, along with AMP, can act as allosteric modulators of certain regulatory Enzymes, particularly those involved in Glycolysis and purine nucleotide Biosynthesis.

2. Participation in metabolic reactions as coenzymes, specifically:

- NAD, NADP, FAD, FMN — in Biological Oxidation REACTIONS;

- UTP, UDP — in Glycogen biosynthesis reactions;

- CTP, CDP — in glycerophospholipid biosynthesis.