Y-Aminobutyrate cycle

The two transamination steps in the pathways may be linked, as indicated in Fig. 17-5, to form a complete cycle that parallels the citric acid cycle but in which 2-oxoglutarate is oxidized to succinate via glutamate and y-aminobutyrate. No thiamin diphosphate is required, but 2-oxoglutarate is reductively aminated to glutamate. The cycle is sometimes called the y-aminobutyrate shunt, and it plays a significant role in the overall oxidative processes of brain tissue. 

Ornithine (a) and hydroxyproline (d) are both a-amino acids, because an amino group is attached to the same carbon atom as that which carries a carboxyl. Although ornithine is not used in protein synthesis, it is an intermediate in the urea cycle (Chap. 15). /3-Alanine (b) and y-aminobutyrate (c) have their amino group attached to a carbon atom different from that bearing the carboxyl, and are a /3- and a y-amino acid, respectively. Strictly speaking proline is a secondary amino acid, because the amino nitrogen is covalently connected to the side chain. It is sometimes referred to as an imino acid. 

Sawai, Y., Yamaguchi, Y, Miyama, D., and Yoshitomi, H. 2001. Cycling treatment of anaerobic and aerobic incubation increases the content of y-aminobutyric acid in tea shoots. Amino Acids 20 331-34. 

Balazs, R., Machiyama, Y., Hammond, B. J., Julian, T. and Richter, D. (1970) The operation of the y-aminobutyrate bypath of the tricarboxylic acid cycle in brain tissue in vitro. Biochem. J., 116, 445-460. 

There are no studies that indicate that metabolism of ammonia differs between children and adults. Ammonia is eliminated from the body mainly by processing through the urea cycle in the liver, and urea is then eliminated in the urine and feces. The urea cycle is fully functional in infants at birth therefore, it is not expected that infants or children are at greater risk of hyperammonemia. Neurotoxicity resulting from h q)erammonemia involves alteration of levels of some components of the citric acid cycle, which leads to depletion of ATP, and starvation of brain cells, and depletion of glutamate, a precursor to the neurotransmitter y-aminobutyrate (GABA). It is not expected that children are more susceptible than adults to ATP depletion via this mechanism. 

Although there are over 200 amino acids found in nature, genes (DNA) only contain codons for 20 amino acids (see Table 9.1 for a list of these amino acids and selected properties). These 20 amino acids are the foundation for all proteins, including all enzymes and structural protein. Some of these amino acids are then modihed during and/or after the protein is synthesized. Other nonprotein amino acids are synthesized in the body. Many of the above-mentioned 200 amino acids do not occur in mammalian tissue but can be present in plant tissue. Some are intermediates in various metabolic pathways for example, ornithine is part of the urea cycle. Other nonprotein amino acids such as y-aminobutyric acid (GABA) are neurotransmitters. 

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