Threonine deamination

Umbarger HE, Brown B (1957) Threonine deamination in Escherichia coli. II. Evidence for two L-threonine deaminases. J Bacteriol 73 105-112... 

Glutamate dehydrogenation is involved in deamination of most of the amino acids. The first two reactions are not involved in the overall deamination system they are included here for completeness and because they are of some general interest. The complete biochemical description of these reactions is given in Appendix 8.4. For a few amino acids, e.g. threonine and serine, other specific reactions are responsible for deamination. 

Although the nitrogen atoms of most amino acids are transferred to a-ketoglutarate before removal, the a-amino groups of serine and threonine can be directly converted into NH4 +. These direct deaminations are catalyzed by serine dehydratase and threonine dehydratase, in which PLP is the prosthetic group. 

C Amino acid s Asn is hydrolyzed in one step to aspaartate, which in turn is transaminated in one step to oxalacetate. Threonine feeds into the TCA cycle through succinyl-CoA instead of oxalacetate. Thr is first deaminated via a dehydratase as seen earlier, then decarboxylated by Pyruvate DH Complex to give propionyl-CoA, which is then transformed via a series of steps to give succinyl-CoA. 

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). 

Serine and threonine are deaminated by serine dehydratase, which requires pyridoxal phosphate. Serine is converted to pyruvate, and threonine to a-ketobutyrate NH4+ is released. 

Fig. 1.8 Asaccharolytic fermentation produces ammonia and short-chain fatty acids. This group of fermentations by oral bacteria utilizes proteins, which are converted to peptides and amino acids. The free amino acids are then deaminated to ammonia in a reaction that converts nicotinamide adenine dinucleotide (NAD) to NADH. For example, alanine is converted to pyruvate and ammonia. The pyruvate is reduced to lactate, and ammonium lactate is excreted into the environment. Unlike lactate from glucose, ammonium lactate is a neutral salt. The common end products in from plaque are ammonium acetate, ammonium propionate, and ammonium butyrate, ammonium salts of short chain fatty acids. For example, glycine is reduced to acetate and ammonia. Cysteine is reduced to propionate, hydrogen sulfide, and ammonia alanine to propionate, water, and ammonia and aspartate to propionate, carbon dioxide, and ammonia. Threonine is reduced to butyrate, water, and ammonia and glutamate is reduced to butyrate, carbon dioxide, and ammonia. Other amino acids are involved in more complicated metabolic reactions that give rise to these short-chain amino acids, sometimes with succinate, another common end product in plaque.

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

Although the tryptophan synthetase and tryptophanase reactions have been the best studied replacement and 0 eUmination-deamination reactions, others pf special interest are D-serine dehydratase [75-77] from E. coli, D-threonine dehydratase and L-threonine dehydratase from Serratia marcescens [78]. The only information available on the above enzymes is that in these cases also, the events at occur with retention of configuration. 

In addition to glutamate, a number of amino acids release their nitrogen as NH4 (see Fig. 38.5). Histidine may be directly deaminated to form NH4 and urocanate. The deaminations of serine and threonine are dehydration reactions that require pyridoxal phosphate and are catalyzed by serine dehydratase. Serine forms pyruvate, and threonine forms a-ketobutyrate. In both cases, NH4 is released. 

The D-enantiomer of 393 was obtained in an identical sequence of reactions starting frc m 2,3-0-isopropylidene-4-deoxy-D-threitol (395). This compound was prepared from L-threonine (394) in the following way the amino acid was deaminated to 25 3/ -dihydroxy-butyric acid. Esterification of the carboxyl group and protection of both hydroxyl groups with an isopropylidene grouping gave methyl 4-deoxy-2,3-0-isopropylidene-D-threonate. Reduction of the ester group afforded 395 smoothly. 

Amino acids are used by the body to form proteins, hormones, and enzymes. Transamination reactions can convert one amino acid into another to meet immediate needs. However, just as there are essential fatty acids, there are also essential amino acids. These amino acids cannot be synthesized in the body and must come from external sources. Humans require phenylalanine, valine, tryptophan, threonine, lysine, leucine, isoleucine, and methionine as essential amino acids. All other amino acids in the body can be synthesized at rates sufficient to meet body needs. If any one of the amino acids necessary to synthesize a particular protein is not available, then the other amino acids that would have gone into the protein are deaminated, and their excess nitrogen is excreted as urea (Ganong, 1963). 

L-lsoieucine, lie L-a-amino-P-methylvaleric acid, CH3-CH2-CH(CH3)-CH(NH2)-C00H, an aliphatic, neutral amino acid found in proteins. He is found in relatively large amounts in hemoglobin, edestin, casein and serum proteins, and in sugar beet molasses, from which it was first isolated in 1904 by F. Ehrlich. It is an essential dietary amino acid, and is both glu-coplastic (degradation via propionic acid) and keto-plastic (formation of acetate) (see Leucine), The biosynthesis of He starts with oxobutyrate and pyruvate. Oxobutyrate is synthesized by deamination of L-threonine by threonine dehydratase (threonine de-... 

Mammalian tissues contain enzymes that catalyze the nonoxidative deamination of serine, threonine, and homoserine. Since the postulated reaction mechanism involves a dehydration before the deamination, these enzymes are called dehydrases. L-Serine, L-threonine, and L-homoserine dehydrases have been partially purified and all are specific for the L-amino acid. Serine and threonine dehydrases require pyridoxal phosphate, ATP, and glutathione for activity. Pyridoxal phosphate requires the homoserine enzyme, but the need for ATP and glutathione has not been demonstrated. The reaction is likely to involve the formation of a Schiff base. The homoserine dehydrase has been... 

One of the pathways to propanoyl-CoA is from catabolism of the amino acid threonine (Chapter 12). Thus, threonine (threonine dehydratase, EC 4.3.1.19, cofactor pyridoxal phosphate) undergoes deamination to give 2-oxobutanoate (a-ketobutyrate) as shown below. Then, 2-oxobutanoate (a-ketobutyrate) undergoes decarboxylation (perhaps as shown in Scheme 11.30) with formation of propanoyl dihydro-lipoamide in a (cofactor) thiamine diphosphate mediated step. Finally, as in Scheme 11.31, propanoyl-CoA is formed. An alternative pathway uses aferrodoxin to effect the decarboxylation of 2-oxobutanoate (a-ketobutyrate) Ferredoxins are small proteins containing iron and sulfur atoms in iron-sulfur clusters. 

Scheme IZl. A cartoon representation of a pathway for the deamination of L-threonine (Thr, T) using pyridoxal as a cofactor. Note that 2-oxobutanoic acid and pyridoxal form (along with, in principle, ammonia, NHj).The ammonia may actually be retained by the pyridoxal fragment to produce pyridoxamine (after reduction), which can then be used for further transaminations.

Serine and Threonine Dehydration. The reaction that is now spoken of as dehydration of a-amino, 8-hydroxy acids was originally considered as a deamination. The reaction observed is ... 

This activity was measured anaerobically by Gale and Stephenson as release of ammonia by bacterial cells. The system was not stable, but was protected by low concentrations of phosphate and reducing agents. At 0 C. inactivation occurred that was reversed by a boiled bacterial extract plus phosphate and a reducing agent. The reaction was better defined by Chargaff and Sprinson, who also studied the reaction in liver. Several bacterial species were found to deaminate both n- and L-serine and threonine. Phosphate-, methyl-, and ethyl-substituted hydroxyl groups prevented the reaction. 

Serine, Threonine, and Homoserine Dehydrases. This group of enzymes catalyzes a nonoxidative deamination reaction resulting from a primary dehydration of the substrate. Serine, threonine, and homoserine have been shown to be deaminated by bacteria " " and animal tissues (liver) 101. 

The reactions for serine, threonine, and homoserine deamination have been postulated to occur as follows ... 

Brief reference will be made to the following papers which have appeared since this chapter was written. Partially purified Neurospora L-serine dehydrase has been studied. Both L-serine and L-threonine appear to be deaminated by the same enzyme. The enzymatic pathway of histidine degradation in liver has been investigated. Soluble gluta-minase I has been prepared. Additional eiddence for the Avide scope of transamination has appeared. The presence of an ornithine-a-keto-glutarate transaminase in neurospora has been demonstrated. Transamination of non-a-amino acids has been demonstrated in brain in... 

The anaerobic deamination of L-threonine with the formation of a-ketobutyric acid is induced by the same bacterial preparations that deaminate L-serine, most probably by the same enzyme. "... 

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