Metabolism of aromatic amino acids and histidine

See also Table 5.1, Genetic Code, Metabolism of Aromatic Amino Acids and Histidine, Essential... 

See also Metabolism of Aromatic Amino Acids and Histidine... 

In this chapter, the metabolism of aromatic amino acids, including histidine, is examined. The aromatic amino acids are dietary essentials, with the exception of tyrosine, which can be formed from phenylalanine. Each of the amino acids has a unique metabolism, including the formation of important compounds other than proteins as neurotransmitters, pigments, and a vitamin. A number of metabolic diseases are also associated with aromatic amino-acid metabolism. An overall view of nitrogen economy and protein metabolism is discussed at the end of the chapter. 

Carbon Metabolism of Amino Acids. Nine of the 18 common amino acids are related directly or indirectly to the dicarboxylic acids of the citrate cycle. Proline, histidine, and arginine (or ornithine) produce glutamate and thence a-ketoglu-tarate (on the chart, below the citrate cycle). The aromatic amino acids are broken down to fumarate, which is also formed from aspartate in the urea cycle. Lastly, alanine should be included, since it can enter the citrate cycle via pyruvate and acetyl-CoA. The fact that the carbon chains of amino acids enter the citrate cycle is important both for their complete degradation and for their conversion to carbohydrates (as mentioned already, cf. Chapt. XV-10). The synthesis of amino acids cannot always proceed by a reversal of the breakdown many of them are essential components of the diet (cf. Chapt. XXII-2). 

On the basis of the similarities in their synthetic pathways, the amino acids can be grouped into six families glutamate, serine, aspartate, pyruvate, the aromatics, and histidine. The amino acids in each family are ultimately derived from one precursor molecule. In the discussions of amino acid synthesis that follow, the intimate relationship between amino acid metabolism and several other metabolic pathways is apparent. Amino acid biosynthesis is outlined in Figure 14.4. 

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