Threonine, catabolism

L-Threonine is catabolized aerobically. L-Threonine is catabolized anaerobically. 

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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. 

Three pathways of threonine catabolism have been found in heterotrophs. 

Removal of a-amino nitrogen by transamination (see Figure 28-3) is the first catabolic reaction of amino acids except in the case of proline, hydroxyproline, threonine, and lysine. The residual hydrocarbon skeleton is then degraded to amphibolic intermediates as outhned in Figure 30-1. 

Threonine. Threonine is cleaved to acetaldehyde and glycine. Oxidation of acetaldehyde to acetate is followed by formation of acetyl-CoA (Figure 30-10). Catabolism of glycine is discussed above. 

Guillouet S, Rodal AA, An G-H, Lessard PA, Sinskey AJ. (1999) Expression of the Escherichia coli catabolic threonine dehydratase in Corynebacterium glutamicum and its effect on isoleucine production. Appl Environ Microbiol 65 3100-3107. 

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... 

FIGURE 18-27 Catabolic pathways for methionine, isoleucine, threonine, and valine. 

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. 

L-Serine and L-threonine dehydratases dehydrate and subsequently deaminate the amino add to the corresponding a-keto add. These enzymes are known to require pjn-idoxal-S -phosphate as a coenzyme. They can function in a biosynthetic or catabolic marmer (99). Both enzymes can cause problems for the whole-cell-based production of L-serine (100). 

S. C. Bell and J. M. Turner, Bacterial catabolism of threonine. Threonine degradation initiated by L-threonine acetaldehyde-lyase (aldolase) in species of Pseudomonas, Bioctem. /., 166 209 (1977). 

Amino Add Catabolic Pathways Starting with Oxidation Catabolism of Serine, Threonine, and Histidine... 

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