Isoleucine transamination

Amino acids are metabolized by a transamination reaction in which the —NH2 group of the amino acid changes places with the keto group of an a-keto acid. The products are a new amino acid and a new a-keto acid. Show the product from transamination of isoleucine. 

The wild type ilvA gene was modified to target the protein to the plastid and expressed in A. thaliana. Transgenic plants showed a 20-fold increase in levels of 2-ketobutyrate as well as a large increase in 2-aminobutyrate, the transaminated product of 2-ketobutyrate [27, 41]. The levels of threonine remained stable whereas isoleucine concentration increased. Constitutive expression of the ilvA protein along with bktB, phaA, and phaC proteins in the plastids of A. thaliana led to the synthesis of poly(3HB-co-3HV) in the range of 0.2 - 0.8 % dry weight, with a HV level between 4-17 mol % [27,41]. Co-expression of the iso-... 

In a muscle at rest, most of the 2-oxo acids produced from transamination of branched chain amino acids are transported to the liver and become subject to oxidation in reactions catalysed by branched-chain 2-oxo acid dehydrogenase complex. During periods of exercise, however, the skeletal muscle itself is able to utilize the oxo-acids by conversion into either acetyl-CoA (leucine and isoleucine) or succinyl-CoA (valine and isoleucine). 

Figure 9-4. Metabolism of the branched-chain amino acids. The first two reactions, transamination and oxidative decarboxylation, are catalyzed by the same enzyme in all cases. Details are provided only for isoleucine. Further metabolism of isoleucine and valine follows a common pathway to propionyl CoA. Subsequent steps in the leucine degradative pathway diverge to yield acetoacetate. An intermediate in the pathway is 3-hydroxy-3-methylglutaryl CoA (HMG-CoA), which is a precursor for cytosolic cholesterol biosynthesis.

The third type of carbon-branched unit is 2-oxoisovalerate, from which valine is formed by transamination. The starting units are two molecules of pyruvate which combine in a thiamin diphosphate-dependent a condensation with decarboxylation. The resulting a-acetolactate contains a branched chain but is quite unsuitable for formation of an a amino acid. A rearrangement moves the methyl group to the (3 position (Fig. 24-17), and elimination of water from the diol forms the enol of the desired a-oxo acid (Fig. 17-19). The precursor of isoleucine is formed in an analogous way by condensation, with decarboxylation of one molecule of pyruvate with one of 2-oxobutyrate. 

In a rare autosomal recessive condition (discovered in 1954) the urine and perspiration has a maple syrup odor/ High concentrations of the branched-chain 2-oxoacids formed by transamination of valine, leucine, and isoleucine are present, and the odor arises from decomposition products of these acids. The branched-chain amino acids as well as the related alcohols also accumulate in the blood and are found in the urine. The biochemical defect lies in the enzyme catalyzing oxidative decarboxylation of the oxoacids, as is indicated in Fig. 24-18. Insertions, deletions, and substitutions may be present in any of the subunits (Figs. 15-14,15-15). The disease which may affect one person in 200,000, is usually fatal in early childhood if untreated. Children suffer seizures, mental retardation, and coma. They may survive on a low-protein (gelatin) diet supplemented with essential amino acids, but treatment is difficult and a sudden relapse is apt to prove fatal. Some patients respond to administration of thiamin at 20 times the normal daily requirement. The branched-chain oxoacid dehydrogenase from some of these children shows a reduced affinity for the essential coenzyme thiamin diphosphate.d... 

Finally, a pyridoxal transamination converts the two keto-acids stereospecifically to the corresponding amino acids, valine (R = Me) and isoleucine (R = Et). The donor amino acid is probably glutamate—-it usually is in amino acid synthesis. 

E. coli (107, 125). The complexes have recently been reviewed (126). It is possible that lipoamide dehydrogenase also functions in the complexes that oxidatively decarboxylate the a-keto acids resulting from the transamination of valine, isoleucine, and leucine but these have proved difficult to resolve (127). Lipoamide dehydrogenase also functions in the pyridoxal phosphate and tetrahydrofolate-dependent oxidative decarboxylation of glycine in the anaerobic bacterium Peptococcus glyci-nophilus. The reaction in which the protein-bound lipoic acid is reduced is very complex and not yet fully understood the ultimate electron acceptor is NAD+ (112,113,128). 

The degradative pathways of valine and isoleucine resemble that of leucine. After transamination and oxidative decarboxylation to yield a CoA derivative, the subsequent reactions are like those of fatty acid oxidation. Isoleucine yields acetyl CoA and propionyl CoA, whereas valine yields CO2 and propionyl CoA. The degradation of leucine, valine, and isoleucine validate a point made earlier (Chapter 14) the number of reactions in metabolism is large, but the number of kinds of reactions is relatively small. The degradation of leucine, valine, and isoleucine provides a striking illustration of the underlying simplicity and elegance of metabolism. 

The liver also plays an essential role in dietary amino acid metabolism. The liver absorbs the majority of amino acids, leaving some in the blood for peripheral tissues. The priority use of amino acids is for protein synthesis rather than catabolism. By what means are amino acids directed to protein synthesis in preference to use as a fuel The K jyj value for the aminoacyl-tRNA synthetases is lower than that of the enzymes taking part in amino acid catabolism. Thus, amino acids are used to synthesize aminoacyl-tRNAs before they are catabolized. When catabolism does take place, the first step is the removal of nitrogen, which is subsequently processed to urea. The liver secretes from 20 to 30 g of urea a day. The a-ketoacids are then used for gluconeogenesis or fatty acid synthesis. Interestingly, the liver cannot remove nitrogen from the branch-chain amino acids (leucine, isoleucine, and valine). Transamination takes place in the muscle. 

Branched-Chain Oxo-acid Decarboxylase and Maple Syrup Urine Disease The third oxo-acid dehydrogenase catalyzes the oxidative decarboxylation of the hranched-chain oxo-acids that arise from the transamination of the hranched-chain amino acids, leucine, isoleucine, and valine. It has a similtu suhunit composition to pyruvate and 2-oxoglutarate dehydrogenases, and the E3 suhunit (dihydrolipoyl dehydrogenase) is the same protein as in the other two multienzyme complexes. Genetic lack of this enzyme causes maple syrup urine disease, so-called because the hranched-chain oxo-acids that are excreted in the urine have a smell reminiscent of maple syrup. 

In maple syrup urine disease, the enzyme complex that decarboxy-lates the transamination products of the branched-chain amino acids is defective (see Figure 7-11). Valine, isoleucine, and leucine accumulate. Urine has the odor of maple syrup. Mental retardation occurs. 

D. Valine, isoleucine, and leucine (the branched-chain amino acids) are transaminated and then oxidized by an a-keto acid dehydrogenase that requires lipoic acid as well as thiamine pyrophosphate, coenzyme A, FAD, and NAD+. Four of the carbons of valine and isoleucine are converted to succinyl CoA. Isoleucine also produces acetyl CoA Leucine is converted to HMG CoA, which is cleaved to acetoacetate and acetyl CoA... 

C. In maple syrup urine disease, the branched-chain amino acids (valine, leucine, and isoleucine) can be transaminated but not oxidatively decarboxylated because the a-keto acid dehydrogenase is defective. 

D. The branched-chain amino acids (valine, isoleucine, and leucine) are transaminated and then oxidatively decarboxylated by an enzyme that requires thiamine, lipoic add, coenzyme A, FAD, and NAD. 

In a similar way, other a-keto acids, e.g., a-ketoglutarate (in the TCA cycle see below) and branched-chain cc-keto acids derived by transamination from the branched-chain amino acids valine, leucine, and isoleucine (Chapter 17), undergo decarboxylation and dehydrogenation catalyzed by enzyme complexes. These enzyme complexes differ in specificity of Ei and E2, but all contain the same E3 (the dihydrolipoyl dehydrogenase). 

The degradative pathways of valine and isoleucine resemble that of leucine. After transamination and oxidative decarboxylation to yield a CoA derivative, the subsequent reactions are like those of fatty acid oxidation. Isoleucine yields acetyl CoA and propionyl CoA, whereas valine yields... 

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. 

Isoleucine and valine. The first four reactions in the degradation of isoleucine and valine are identical. Initially, both amino acids undergo transamination reactions to form a-keto-/T methyl valerate and a-ketoiso valerate, respectively. This is followed by the formation of CoA derivatives, and oxidative decarboxylation, oxidation, and dehydration reactions. The product of the isoleucine pathway is then hydrated, dehydrogenated, and cleaved to form acetyl-CoA and propionyl-CoA. In the valine degradative pathway the a-keto acid intermediate is converted into propionyl-CoA after a double bond is hydrated and CoA is removed by hydrolysis. After the formation of an aldehyde by the oxidation of the hydroxyl group, propionyl-CoA is produced as a new thioester is formed during an oxidative decarboxylation. 

---