Amino acid carbon skeletons, catabolism

The liver contains all the pathways for catabolism of all of the amino acids and can oxidize most of the carbon skeletons to carbon dioxide. A small proportion of the carbon skeletons are converted to ketone bodies. The liver also contains the pathways for converting amino acid carbon skeletons to glucose (gluconeo-genesis) that can be released into the blood. 

Amino acid catabolism occurs in three stages (1) removal of the a amino group as ammonia, (2) conversion of the ammonia into urea, and (3) conversion of the remaining amino acid carbon skeleton, usually an a-keto acid, into an intermediate that can enter the citric acid cycle. 

We can also make some generalizations about amino acid metabolism in terms of the relationship of the carbon skeleton to the citric acid cycle and the related reactions of pyruvate and acetyl-GoA (Figure 23.7). The citric acid cycle is amphibolic it has a part in both catabohsm and anabolism. The anabolic aspect of the citric acid cycle is of interest in amino acid biosynthesis. The catabolic aspect is apparent in the breakdown of amino acids, leading to their eventual excretion, which takes place in reactions related to the citric acid cycle. 

The calorific capacity of amino acids is comparable to that of carbohydrates so despite their prime importance in maintaining structural integrity of cells as proteins, amino acids may be used as fuels especially during times when carbohydrate metabolism is compromised, for example, starvation or prolonged vigorous exercise. Muscle and liver are particularly important in the metabolism of amino acids as both have transaminase enzymes (see Figures 6.2 and 6.3 and Section 6.4.2) which convert the carbon skeletons of several different amino acids into intermediates of glycolysis (e.g. pyruvate) or the TCA cycle (e.g. oxaloacetate). Not all amino acids are catabolized to the same extent... 

The oxidation of pyruvate is an important catabolic process, but pyruvate has anabolic fates as well. It can, for example, provide the carbon skeleton for the synthesis of the amino acid alanine. We return to these anabolic reactions of pyruvate in later chapters. 

The pathways of amino acid catabolism are quite similar in most organisms. The focus of this chapter is on the pathways in vertebrates, because these have received the most research attention. As in carbohydrate and fatty acid catabolism, the processes of amino acid degradation converge on the central catabolic pathways, with the carbon skeletons of most amino acids finding their way to the citric acid cycle. In some cases the reaction pathways of amino acid breakdown closely parallel steps in the catabolism of fatty acids (Chapter 17). 

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. 

The carbon skeletons of six amino acids are converted in whole or in part to pyruvate. The pyruvate can then be converted to either acetyl-CoA (a ketone body precursor) or oxaloacetate (a precursor for gluconeogenesis). Thus amino acids catabolized to pyruvate are both ke-togenic and glucogenic. The six are alanine, tryptophan, cysteine, serine, glycine, and threonine (Fig. 18-19). Alanine yields pyruvate directly on transamination with... 

More Than One Carrier Exists for Transporting Ammonia from the Muscle to the Liver Amino Acid Catabolism Can Serve as a Major Source of Carbon Skeletons and Energy For Many Genetic Diseases the Defect Is in Amino Acid Catabolism... 

Amino Acid Catabolism Can Serve as a Major Source of Carbon Skeletons and Energy... 

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