Threonine carbon catabolism

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... 

During the catabolism of fatty acids with an odd number of carbon atoms and the amino acids valine, isoleucine and threonine the resultant propionyl-CoA is converted to succinyl-CoA for oxidation in the TCA cycle. One of the enzymes in this pathway, methylmalonyl-CoA miitase, requires vitamin B12 as a cofactor in the conve sion of methylmalonyl-CoA to succinyl-CoA. The 5 -deoxyadenosine derivative of cobalamin is required for this reaction. 

Threonine can be broken down by tw o separate pathways. Serine dehydratase catalyzes the conv ersion of threonine to 2-ketobutyrate plus an ammonium ion 2-ketobutyrate is then converted by branched-chaln keto acid (BCKA) dehydrogenase to propionyl-CoA plus carbon dioxide. Propionyl-CoA catabolism is described later in this chapter. Threonine can also be broken down by a complex that has been suggested to be composed of threonine dehydrogerraseand acetoacetone synthase (Tressel ef al., 1986). Here, threonine catabolism results in the production of acetyl CoA plus glycure. 

Serine is one of the two hydroxyamino acids, the other being threonine. Serine has two major pathways of catabolism. The first, and apparently predominant, direction in many mammals is catalyzed by serine dehydratase, where water is removed between the alpha and beta carbons of serine. A rearrangement of the double bond forms an amino acid with spontaneous hydrolysis to form pyruvate and ammonia. Pyruvate then can be metabolized as discussed in previous chapters. This enzyme is primarily active in the liver, where the ammo-... 

Serine - Serine has many important biological roles, including the biosynthesis of phosphopholipids and cysteine. Serine also contributes activated one-carbon units to the pool of tetrahydrofolate coenzymes. Serine can be made in a variety of ways, including the way shown here and Figure 21.24. Serine is catabolized by conversion to glycine or by action of serine-threonine dehydratase (Figure 21.25). 

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. 

Propionic acid fermentation is not limited to propionibacteria it functions in vertebrates, in many species of arthropods, in some invertebrates imder anaerobic conditions (Halanker and Blomquist, 1989). In eukaryotes the propionic acid fermentation operates in reverse, providing a pathway for the catabolism of propionate formed via p-oxidation of odd-numbered fatty acids, by degradation of branched-chain amino acids (valine, isoleucine) and also produced from the carbon backbones of methionine, threonine, thymine and cholesterol (Rosenberg, 1983). The key reaction of propionic acid fermentation is the transformation of L-methylmalonyl-CoA(b) to succinyl-CoA, which requires coenzyme B12 (AdoCbl). In humans vitamin B deficit provokes a disease called pernicious anemia. 

---