Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Oxidative Degradation of Amino Acids. The Urea Cycle
Genetic defects affecting the urea cycle cause ammonia accumulation in the blood

In inherited genetic defects affecting any enzyme of The Urea Cycle, the body is unable to synthesize urea from ammonia. Individuals with such defects cannot tolerate protein-rich diets. If their amino acid intake exceeds the minimum daily requirement for Protein Synthesis, free ammonia, generated by the deamination of excess Amino Acids in the Liver, accumulates in their Blood. As we already know, ammonia is highly toxic. It causes psychiatric disorders, mental retardation, and, at high concentrations, coma and death. Children with such disorders are often treated by supplementing their diet with the α-keto analogs of these amino acids instead of the amino acids required for growth (Ch. 26). As is well known, the carbon skeletons of Essential Amino Acids are what matters, not their amino groups. Under the action of transaminases, the α-keto analogs of essential Amino acids can accept amino groups from the abundant nonessential amino acids (Fig. 19-24). This prevents these amino groups from entering the blood as ammonia.

Class="center">

Fig. 19-24. Amination of α-keto analogs of essential amino acids (subscript E) in Transamination reactions with nonessential amino acids (subscript N).