Amino acids, unlike fats and carbohydrates, cannot be stored in the body. This means that there is no specific protein whose main function is to maintain a supply of amino acids for future use. Therefore, amino acids must either be obtained from the diet, synthesized de novo, or produced through the breakdown of body protein. Any excess amino acids beyond what the cell needs for biosynthesis are quickly broken down. The initial phase of catabolism involves the removal of the α-amino groups, usually through transamination and subsequent oxidative deamination. This process results in the formation of ammonia and the corresponding α-keto acids, which are the carbon skeletons of amino acids. Some of the free ammonia is excreted in urine, while the majority is used to produce urea, which is the primary method for eliminating nitrogen from the body. In the second phase of amino acid catabolism, which is discussed in Chapter 20, the carbon skeletons of α-keto acids are converted into common intermediates of energy-producing metabolic pathways. These compounds can be metabolized into carbon dioxide and water, glucose, fatty acids, or ketone bodies through the central pathways of metabolism described in Chapters 8-13 and 16.

II. OVERALL NITROGEN METABOLISM

Amino acid catabolism is part of the broader process of nitrogen-containing molecule metabolism. Nitrogen enters the body in various compounds found in food, with the most significant source being amino acids from dietary protein. Nitrogen exits the body in the form of urea, ammonia, and other products derived from amino acid metabolism, such as creatinine. The involvement of body proteins in these transformations revolves around two key concepts: the amino acid pool and protein turnover.

A. Amino acid pool

Free amino acids can be found throughout the body, including cells, blood, and extracellular fluids. For the purpose of this discussion, imagine all these amino acids as belonging to a single entity known as the amino acid pool. This pool is supplied by three sources: 1) amino acids produced from the degradation of endogenous proteins (most of which are reused), 2) amino acids derived from dietary protein, and 3) nonessential amino acids synthesized from simple metabolic intermediates. On the other hand, the amino acid pool is depleted through three routes: 1) the synthesis of body protein, 2) the consumption of amino acids as precursors for essential nitrogen-containing small molecules, and 3) the conversion of amino acids into glucose, glycogen, fatty acids, ketone bodies, or their oxidation into carbon dioxide and water. Although the amino acid pool is relatively small (approximately 90-100 g of amino acids) compared to the total protein content in the body (about 12 kg in a 70-kg individual), it plays a central role in whole-body nitrogen metabolism conceptually.

In healthy, well-fed individuals, the input into the amino acid pool is balanced by the output. This means that the amount of amino acids in the pool remains constant. The amino acid pool is considered to be in a steady state, and the individual is said to be in nitrogen balance.

B. Protein turnover

Most proteins in the body undergo constant synthesis and subsequent degradation (turnover), which allows for the removal of abnormal or unneeded proteins. For many proteins, the concentration in the cell is primarily regulated by synthesis, with degradation playing a minor role. However, for some proteins, the rate of synthesis is constitutive and remains essentially constant, and the cellular levels of the protein are controlled through selective degradation.

1.
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 the degradation of body protein. 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 (Fig. 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 carbon dioxide (CO2) and water (H2O), glucose, fatty acids, or ketone bodies by the central pathways of metabolism described in Chapters 8–13 and 16.
II. OVERALL NITROGEN METABOLISM
Amino acid catabolism is part of the larger process of the metabolism of nitrogen-containing molecules. Nitrogen enters the body in a variety of compounds present in food, the most important being amino acids contained in dietary protein. Nitrogen leaves the body as urea, ammonia, and other products derived from amino acid metabolism (such as creatinine, see p. 287). The role of body proteins in these transformations involves two important concepts: the amino acid pool and protein turnover.
A. Amino acid pool
Free amino acids are present throughout the body, such as in cells, blood, and the extracellular fluids. For the purpose of this discussion, envision all of these amino acids as if they belonged to a single entity, called the amino acid pool. This pool is supplied by three sources: 1) amino acids provided by the degradation of endogenous (body) proteins, most of which are reutilized; 2) amino acids derived from exogenous (dietary) protein; and 3) nonessential amino acids synthesized from simple intermediates of metabolism (Fig. 19.2). Conversely, the amino acid pool is depleted by three routes: 1) synthesis of body protein, 2) consumption of amino acids as precursors of essential nitrogen-containing small molecules, and 3) conversion of amino acids to glucose, glycogen, fatty acids, and ketone bodies or oxidation to CO2 + H2O (see Fig. 19.2). Although the amino acid pool is small (comprising ~90–100 g of amino acids) in comparison with the amount of protein in the body (~12 kg in a 70-kg man), it is conceptually at the center of whole-body nitrogen metabolism.
In healthy, well-fed individuals, the input to the amino acid pool is balanced by the output. That is, the amount of amino acids contained in the pool is constant. The amino acid pool is said to be in a steady state, and the individual is said to be in nitrogen balance (see p. 367).
B. Protein turnover
Most proteins in the body are constantly being synthesized and then degraded (turned over), permitting the removal of abnormal or unneeded proteins. For many proteins, regulation of synthesis determines the concentration of protein in the cell, with protein degradation assuming a minor role. For other proteins, the rate of synthesis is constitutive (that is, essentially constant), and cellular levels of the protein are controlled by selective degradation.
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