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

Biochemical Foundations of Human Vital Activity
Nucleic Acid Biochemistry
Nucleic Acid Metabolism

Degradation of Nucleic acids during Digestion. In the intestine, Nucleic Acids are broken down into mononucleotides by the action of Nucleases secreted with pancreatic juice. The mononucleotides are then acted upon by Phosphatases, resulting in the Cleavage of a phosphoric acid residue (Н₃РО₄) from the mononucleotide to form a nucleoside. Intestinal juice also contains active nucleosidase Enzymes, which cleave nucleosides into nitrogenous bases and pentoses. The products of nucleotide Hydrolysis are absorbed from the intestine into the Blood, and subsequently transported to The Liver and other Tissues of the body.

Degradation of nucleic acids in body tissues. Tissue degradation is carried out by DNases and RNases. Under their influence, nucleic acids break down into NUCLEOTIDES, which in turn are degraded into nitrogenous bases, pentoses, and Н₃РО₄. Further breakdown of nitrogenous bases and CARBOHYDRATES occurs through their oxidation. Pentoses enter the Pentose Phosphate Pathway of carbohydrate degradation, yielding СО₂ and Н₂О. Purine bases (A and G) undergo deamination and oxidation to uric acid, which is excreted from the body in the urine:

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During the tissue METABOLISM of pyrimidine bases, their cyclic Structure is disrupted to form a non-cyclic amino acid. For example, the End products of uracil degradation are The amino acid ß-Alanine, ammonia, and carbon dioxide:

The resulting ß-Amino Acids can undergo further transformations; for instance, ß-alanine participates in the synthesis of coenzyme A, while NH₃ and СО₂ are excreted from the body. Enhanced degradation of nucleic acids in tissues leads to an increase in free ammonia, which is bound in the liver during urea synthesis or excreted in the urine as salts.

Synthesis of nucleic acids. The primary Synthesis of purine bases in tissues occurs through the enzymatic transformations of Glycine, glutamine, formic and aspartic acids, as well as СО₂. It is formed with the participation of ribose phosphate via inosinic acid:

Pyrimidine bases are synthesized from carbamoyl phosphate, aspartic acid, and ribose phosphate, proceeding via orotidylic acid as an intermediate product of synthesis:

Under the action of enzymes, nucleotides are synthesized first, followed by the synthesis of nucleic acids, DNA and RNA.

Intensive DNA Synthesis occurs during Cell Division through DNA Replication. This process begins with the breaking of Hydrogen Bonds between the two complementary strands of the helical DNA molecule (see Fig. 80). The strands unwind, and each serves as a template for the synthesis of a new complementary polynucleotide strand using free nucleotides. As a result, two identical DNA molecules are formed, preserving the nucleotide arrangement characteristic of the given Organism species. These processes involve the enzyme DNA polymerase.

RNA Synthesis proceeds on one of the DNA strands functioning as a template, catalyzed by RNA polymerase enzymes. Only a small segment of DNA (a Gene or a group of interrelated genes) participates in this synthesis, being copied through The formation of a complementary polynucleotide strand.

Review Questions

1. What is The Role of nucleic acids in the body?

2. What are the main Structural components of nucleic acids?

3. How is a nucleotide structured?

4. Which nucleotides are involved in the REGULATION OF METABOLISM and energy in the body?

5. What are the structural features, properties, and Functions of DNA and RNA?

6. What is The Essence of the DNA replication process?

7. What is understood by a gene and a genome? What is The Genetic Code?

8. What are the characteristics of nucleic acid degradation and synthesis in the Digestive System and body tissues?