Biochemistry

from the deamination of amino acids is a toxic substance, its high concentration in the blood and other tissues has an adverse action, especially on the nervous system. The ammonia toxicity is due to the fact that it contributes to the reductive amination of a-ketoglutaric acid in mitochondria. This leads to its removal from the Krebs cycle and, as a consequence, to a decrease in tissue respiration and excessive formation of ketone bodies from acetyl-CoA. In the process of evolution, living organisms have developed various effective mechanisms to neutralize the toxic effects of ammonia. They are: the formation of amines - glutamine or asparagine, reductive amination, neutralization of acids, and urea synthesis. The synthesis of glutamine or asparagine has great importance to the body. It occurs in places of direct ammonia formation (for example, in the liver, brain), and there is also an enzyme that catalyzes this process - glutamine synthetase, which belongs to the ligases class. The synthesis of amides requires a delivery of energy in the form of ATP, the presence of glutamic or aspartic acids, free ammonia, and the corresponding specific enzymes. The amide synthesis reaction is coupled with the decomposition of ATP. Asparagine is formed similarly. i ADP + P COOH CO Glutamic acid NH 2 Glutamine As a result of the interaction of ammonia with glutamic and aspartic acids, its binding occurs, and thus ammonia is neutralized. Bound ammonia can be used as a nitrogen source (for example, for the synthesis of purine and pyrimidine bases, mucopolysaccharides). Glutamine and asparagine not only neutralize ammonia, but also act as its transport form. Bound ammonia is delivered to the place of final disposal - to the liver, where urea is synthesizedfrom it. Reductive amination. This process is an opposite process to deamination. It provides the binding of ammonia with keto acids for the formation of corresponding amino acids. 170