University:
Capital University
Course:
BIOL 250 | Environmental Science
Academic year:

2019
Views:

54

Pages:

4
Author:

gewoekerdodx

Amino Acids: Nitrogen Disposal I. 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 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 a-amino groups (usually by transamination and subsequent oxidative deamination), forming ammonia and the corresponding a-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 a-keto acids are converted to common intermediates of energy-producing metabolic pathways. These compounds can be metabolized to carbon dioxide (C02) and water (H2'>), 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, .... HCC>a- T Cutlamoyl phoephala r l. Otnlthlne A..-1* CltNllln~ Arglnlnosucclnate ) u~tnlne Figure 19.1 Urea cycle shown as part of the essential pathways of energy metabolism. [Note: See Fig. 8.2, p. 92, for a more detalled map of metabollsm.] NHa =ammonia; 002 =carbon dioxide. 19. Amino Acids: Nitrogen Disposal 246 TURNOVER Protein turnover I• the 1lmultaneou• syntheale and degradation of protein molecules. In h•lthy, fed adults, the total amount of protein In the body remalne conetant becauae the rate of protein synth881a le Juet sufficient to replace the protein that la degraded. DleWy problln can vary from none (for example, faatlng) to >800 g/day (high-protein dim). but 100 g/day le typical of the U.S. diet. Synlheels of noneuentlal amlnoaclda 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 C

Amino Acids Nitrogen Disposal

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