Overview of Amino Acids

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Chapter: Biochemistry : Amino Acids: Disposal of Nitrogen

Unlike fats and carbohydrates, amino acids are not stored by the body. That is, no protein exists whose sole function is to maintain a supply of amino acids for future use.


Amino Acids: Disposal of Nitrogen

OVERVIEW

Unlike fats and carbohydrates, amino acids are not stored by the body. That is, no protein exists whose sole function is to maintain a supply of amino acids for future use. Therefore, amino acids must be obtained from the diet, synthesized de novo, or produced from normal protein degradation. Any amino acids in excess of the biosynthetic needs of the cell are rapidly degraded. The first phase of catabolism involves the removal of the α-amino groups (usually by transamination and subsequent oxidative deamination), forming ammonia and the corresponding α-keto acids, the “carbon skeletons” of amino acids. A portion of the free ammonia is excreted in the urine, but most is used in the synthesis of urea (Figure 19.1), which is quantitatively the most important route for disposing of nitrogen from the body. In the second phase of amino acid catabolism, described in Chapter 20, the carbon skeletons of the α-keto acids are converted to common intermediates of energy-producing metabolic pathways. These compounds can be metabolized to CO2 and water, glucose, fatty acids, or ketone bodies by the central pathways of metabolism described in Chapters 8–13 and 16.


Figure 19.1 Urea cycle shown as part of the essential pathways of energy metabolism. (See Figure 8.2, , for a more detailed view of intermediary metabolism.)

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