Aspartic acid carbon catabolism

In considering amino acid catabolism, one must distinguish the catabolism of the carbon chain from that of the nitrogen moiety. The breakdown of the carbon chain of the amino acids yields carbon units that can be used in carbohydrate metabolism, acetate metabolism, or the metabolism of single carbon units. The fate of the carbon units of the individual amino acids has been discussed in other sections of this book, and only a synopsis of the results will be presented here. The carbon skeletons of isoleucine, phenylalanine, threonine, tryptophan, valine, histidine, alanine, arginine, aspartic acid, glycine, proline, glutamic acid, and hydroxyproline are ultimately converted to pyruvic acid. 

The catabolic pathways of the carbon chains of the amino acids, alanine, glutamic, and aspartic acids, appear to be readily apparent once these amino acids lose their amino groups. When this occurs, alanine is converted to pyruvic acid, glutamic acid to a-ketoglutaric acid, and aspartic acid to either oxalacetic or fumaric acid. All of the above acids are integral members of the citric acid cycle, and the subsequent degradation of each one has been adequately explained in terms of the operation of the citric acid cycle (see the chapter. The Tricarboxylic Acid Cycle). 

Homoserine also has been detected in filtrates of liver preparations incubated with methionine. Cantoni provisionally identified homoserine as a product of the acid hydrolysis of active methionine (S-adeno-sylmethionine). Beyond homoserine, the postulated reactions 2 and 3 are still more speculative. It might be presumed that homoserine is oxidized to aspartic acid, in analogy to the observations on the catabolism of lysine, in which the analogous a-amino adipic acid is an intermediate. If aspartic acid is formed, the subsequent reaction sequence is readily apparent. Evidence favorable to the proposed reaction pathway is the finding of Marshall and Friedberg, of the occurrence of a small amount of fumaric acid, labeled in the methine carbons, from the livers of mice injected with DL-methionine-2-C. ... 

Three of the amino acids, alanine, aspartic acid, and glutamic acid are readily formed by transamination from products of the citric acid cycle. This has been explained in the chapter. Carbon Catabolism of Amino Acids. Glutamic acid is the probable precursor of a considerable number of the other nonessential amino acids, namely, proline, hydroxy-proline, ornithine, and from it arginine. 

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