Glutamic 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). 

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. 

Evidence is slowly accumulating on the mechanism of the biosynthesis of histidine. It has been established that glutamate and formate are products of the catabolism of histidine (see the chapter. Carbon Catabolism of Amino Acids). This offered a clue to the possible approach to the problem, as it is conceivable that the reversal of at least some of the reactions of degradation may be involved in the synthesis. 

In summary, the biochemical function of folate coenzymes is to transfer and use these one-carbon units in a variety of essential reactions (Figure 2), including de novo purine biosynthesis (formylation of glycinamide ribonucleotide and 5-amino-4-imidazole carboxamide ribonucleotide), pyrimidine nucleotide biosynthesis (methylation of deoxyuridylic acid to thy-midylic acid), amino-acid interconversions (the interconversion of serine to glycine, catabolism of histidine to glutamic acid, and conversion of homocysteine to methionine (which also requires vitamin B12)), and the generation and use of formate. 

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. 

Arginine and histidine contain five adjacent carbons and a sixth carbon attached through a nitrogen atom. The catabolic conversion of these amino acids to glutamate is therefore slightly more complex than the path from proline or glutamine (Fig. 18-26). Arginine is... 

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