Catabolism of amino acids

The catabolism of amino acids provides pyruvate, acetyl-CoA, oxaloacetate, fumarate, a-ketoglutarate, and succinate, ail of which may be oxidized by the TCA cycle. In this way, proteins may serve as excellent sources of nutrient energy, as seen in Chapter 26. 

The reaction shown in Figure 8.6 is also important in the liver where glutamate dehydrogenase is involved in the catabolism of amino acids and the entry of nitrogen into the urea cycle, as explained in Chapter 6. 

Table 8.9 Processes involved in catabolism of amino acids...

An early step in the catabolism of amino acids is the separation of the amino group from the carbon skeleton. In most cases, the amino group is transferred to a-ketoglutarate to form glutamate. This transamination reaction requires the coenzyme pyridoxal phosphate. 

Pernicious anemia is caused by impaired absorption of vitamin B12. What is the effect of this impairment on the catabolism of amino acids Are all amino acids equally affected (Hint See Box 17-2.)... 

An initially surprising conclusion drawn from the studies of Schoenheimer and Rittenberg was that proteins within cells are in a continuous steady state of synthesis and degradation. The initial biosynthesis, the processing, oxidative and hydrolytic degradative reactions of peptides, and further catabolism of amino acids all combine to form a series of metabolic loops as discussed in Chapter 17 and dealt with further in Chapters 12 and 29. Within cells some proteins are degraded much more rapidly than others, an important aspect of metabolic control. This is accomplished with the aid of the ubiquitin system (Box 10-C) and proteasomes (Box 7-A).107 Proteins secreted into extracellular fluids often undergo more rapid turnover than do those that remain within cells. 

Allantoin is the excretory product in most mammals other than primates. Most fish hydrolyze allantoin to allantoic acid, and some excrete that compound as an end product. However, most continue the hydrolysis to form urea and glyoxylate using peroxisomal enzymes.336 In some invertebrates the urea may be hydrolyzed further to ammonia. In organisms that hydrolyze uric acid to urea or ammonia, this pathway is used only for degradation of purines from nucleotides. Excess nitrogen from catabolism of amino acids either is excreted directly as ammonia or is converted to urea by the urea cycle (Fig. 24-10). 

Catabolism of amino acids usually entails their conversion to intermediates in the central metabolic pathways. All amino acids can be degraded to carbon dioxide and water by appropriate enzyme systems. In every case, the pathways involve the formation, directly or indirectly, of a dicarboxylic acid intermediate of the tricarboxylic acid cycle, of pyruvate, or of acetyl-CoA (fig. 22.11). 

Proteolysis is the most important of the three primary events occurring during cheese ripening. Due to the complexity of proteolysis, including the catabolism of amino acids and their contribution to cheese flavor, this topic has been the focus of several studies. A comprehensive review of the... 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

Presumably, vasoactive substances act upon smooth muscle cells by intimate association with them, and are destroyed in the process of stimulation or depression. Epinephrine, when injected intravenously, can be recovered in much larger amounts from arterial than from venous blood minute doses given intraarterially may affect only the local circulation. If amines formed from the incomplete catabolism of amino acids are active in hypertension, one must postulate their formation by ischemic organs in direct venous connection with the heart (kidneys, brain, liver, adrenals, etc.) or in direct arterial connection with the arteriolar bed (heart and lungs). If the former, they must not be destroyed in large amounts by the lungs. Furthermore, arterial blood could be expected to contain larger quantities than venous. Absorption from or formation in the intestinal tract or spleen of amines would not produce vascular effects, as these substances probably would be metabolized by the liver. 

Glutamate dehydrogenase catalyzes the reductive amination of the citric acid cycle intermediate a-ketoglutarate (Fig. 3a) (see Topic LI). Although the reaction is reversible, the reductant used in the biosynthetic reaction is NADPH. This enzyme is also involved in the catabolism of amino acids (see Topic M2). 

Figure 2.3(B). Sites of entry of amino acids into the Krebs cycle these are requisite steps for the complete catabolism of amino acids and proteins. (Modified from Hochachka, 1994.)...

Table 20.2 Net ATP Yields from the Catabolism of Amino Acids...

Curtin, A. C., McSweeney, P.L.H. 2004. Catabolism of amino acids in cheese during ripening. In Cheese Chemistry, Physics and Microbiology, Vol. 1, General Aspects, 3rd edn (P.F. Fox, P.LH. McSweeney, T.M. Cogan, T.P. Guinee, eds.), pp. 435-454, Elsevier Academic Press, Amsterdam. 

Although the liver is crucial in protein synthesis, it is of equal importance in amino acid metabolism and degradation. This is evidenced by the high daily turnover of amino acids, and the high proportion of amino acids that are recycled and reconstituted into new protein molecules. Over 30 g of protein are irreversibly catabolised (and hence lost) daily. The nitrogen released from the complete catabolism of amino acids can be removed by a variety of routes, but the principal... 

Catabolism of amino acids in the liver yields high volumes of ammonia. This can be used in the synthesis of other nitrogenous... 

Vitamin Be has a central role in the metabolism of amino acids in transaminase reactions (and hence the interconversion and catabolism of amino acids and the synthesis of nonessential amino acids), in decarboxylation to yield biologically active amines, and in a variety of elimination and replacement reactions. It is also the cofactor for glycogen phosphorylase and a variety of other enzymes. In addition, pyridoxal phosphate, the metabolically active vitamer, has a role in the modulation of steroid hormone action and the regulation of gene expression. 

Estimation of the vitamm Be requirements of infants presents a problem, and there is a clear need for further research. Human mUk, which must be assumed to be adequate for infant nutrition, provides only 2.5 to 3.5 //g of vitamin Be per g of protein-lower than the requirement for adults. Although their requirement for catabolism of amino acids may be lower than in adults (because they have net new protein synthesis), they must also increase their body content of the vitamin as they grow. Coburn (1994) noted that the requirement for growth in a number of animal species was less than that to maintain saturation of transaminases or rniriimum excretion of tryptophan metabolites after a test dose and was about 15 nmol per g of body weight gain across a range of species. 

The other reactions involved in the catabolism of amino acids are decarboxylation, transulfuration, desulfuration, dehydration etc. The decarboxylation process is important since the products of decarboxylation reactions give rise to physiologically active amines. 

Let begin by looking at the catabolism of amino acids by groups 3-C (feed into pyruvate), 4-C (feed into oxalacetate), and 5-C (feed into glutamate). 

Removal of the a-amino group is the first step in catabolism of amino acids. It may be accomplished oxidatively or nonoxidatively. 

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