Pyruvate from tryptophan, 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. 

In addition to the pathway just outlined, tryptophan can result from the transamination of indole-pyruvic acid, but it seems unlikely that this reaction makes any important contribution to the biosynthesis. A trypto-phan-kynurenine-anthranilate-indole-tryptophan cycle has also been proposed i.e. the reverse of the catabolic pathway described in Fig. 58. However this sequence of reactions is only traversed if an excess of tryptophan is present and its function appears to be purely degradative. 

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