Leucine transamination reactions

Problem 21.5 (a) What is the relationship between reactants and products in the transamination reaction (b) Which ketoacid is needed to give (i) alanine (ii) leucine (iii) serine (iv) glutamine (c) Which amino acids cannot be made by transamination 4... 

A further method to induce chirality in the pyridoxamine-mediated transamination reactions was developed by Kuzuhara et al. [13]. They synthesized optically resolved pyridinophanes (21, 22) having a nonbranched ansa chain" between the 2 - and 5 -positions of pyridoxamine. With the five-carbon chain in 21 and 22, the two isomers do not interconvert readily. In the presence of zinc(n) in organic solvents such as methanol, tert-butanol, acetonitrile, and nitromethane, they observed stereoselective transamination between pyridinophanes and keto acids. The highest ee%s are 95 % for d-and L-leucine by reaction of the corresponding a-keto acid with (S)- and (R)- 22, respectively. On the basis of kinetic analysis of the transamination reactions, Kuzuhara et al. originally proposed a mechanism for the asymmetric induction through kinetically controlled stereoselective protonation to the carboanion attached to an octahedral Zn(n) chelate intermediate. However, they subsequently raised some questions about this proposal [14]. 

Position leucine and a-ketoglutarate so that the groups to be exchanged are aligned. This arrangement makes it easy to predict the products of transamination reactions. 

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. 

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). 

In addition to the three most active transamination reactions 1,2, and 3 (page 6) the following relatively slow reactions occur in minced pigeon muscle (4) GL tI-Aminobutyiic acid, (6)GL Valine (6) GL Leucine ... 

Rudman and Meister IJ ) first showed the presence of a transaminase in cell-free extracts of E. colt that catalyze transamination reactions between glutamate and isoleucine, valine, leucine, norleucine, and norvaline. These monocarboxylic amino acids transaminated with each other as well as with glutamine. Preparations of an E. cdi mutant which did not respond to a-keto- 8-methylvalerate was unable to transaminate isoleucine or valine. The transaminase responsible for activity with the branched-chain amino acids was separated from other transaminases and considerably purified by standard methods of protein purification. It was shown to... 

Branched chain amino acids (BCAAs) are essential amino acids, which together compose approximately a third of the daily amino acid requirement in humans. BCAAs, and especially leucine, play an important role in the regulation of energy and protein metabolism. BCAAs are primarily oxidized in skeletal muscle and not in the liver. BCAAs donate their amino groups to furnish glutamic acid in muscle in transamination reactions yielding... 

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.

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

A minor pathway of valine catabolism is concerned with its conversion to leucine. Because leucine is an essential amino acid, its synthesis from valine is clearly not sufficiently significant to meet the organism s daily demand for leucine. In this reaction, isobutyryl-CoA (see Figure 20.20) is condensed with a molecule of acetyl-CoA to give /3-ketoisocaproate, which is then transaminated to give (3-leucine. A mutase is then used to convert /3-leucine to leucine. This mutase... 

Leucine is a branched chain-amino acid that is essential or required in the diet. Mitochondrial catabolism of excess leucine occurs by the pathway shown in Figure 20-3. The initial transamination step (removal of the amino group) is followed by a decarboxylation reaction to produce isovaleric acid. It is this decarboxylation of the a-keto analogs of the three... 

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). 

Oxo-isovalerate may be formed by the transamination of valine it is also the immediate precursor of valine biosynthesis and an intermediate in the synthesis of leucine (both are essential amino acids in mammals). Oxo-lso-valerate undergoes a hydroxymethyl transfer reaction, in which the donor is... 

The degradation of the hranched-chain amino acids employs reactions that we have encountered previously in the citric acid cycle and fatty acid oxidation. Leucine is transaminated to the corresponding a-ketoacid, a-ketoisocaproate. This a-ketoacid is oxidatively decarboxylated to isovaleryl CoA by the branched-chain a-ketoacid dehydrogenase complex. 

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. 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

Glutamate is derived by transamination of a-keto-glutarate produced in the TCA cycle from citrate via oxaloacetate and acetyl-CoA (Chapter 13). All of the amino acids can produce acetyl-CoA. All except leucine and lysine (which are oxidized solely to acetyl-CoA) can be used in net synthesis of a-ketoglutarate to enhance glutamate synthesis. Ammonia is generated in glutamate dehydrogenase and AMP deaminase reactions (Chapter 21). 

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... 

Leucine. Leucine, one of the branched chain amino acids, is converted to HMG-CoA in a series of reactions that include a transamination, two oxidations, a carboxylation, and a hydration. HMG-CoA is then converted to acetyl-CoA and acetoacetate by HMG-CoA lyase. 

Valine, leucine, and isoleucine - The synthetic pathway from threonine and pyruvate to valine, leucine and isoleucine is outlined in Figure 21.26. The last four reactions in the biosynthesis of valine and isoleucine are catalyzed by the same four enzymes. Threonine dehydratase, which catalyzes the first step in conversion of threonine to isoleucine, is inhibited by isoleucine. Leucine, isoleucine, and valine are all catabolized via transamination followed by oxidative decarboxylation of the respective keto-acids (see here) and oxidation. The oxidation is similar to fatty acid oxidation, except for a debranching reaction for each intermediate. 

While the 2-oxobutyrate needed for isoleucine formation is shown as originating from threonine in Eig. 24-17, bacteria can often make it in other ways, e.g., from glutamate via p-methylaspartate (Eig. 24-8) and transamination to the corresponding 2-oxoacid. It can also be made from pyruvate by chain elongation using acetyl-CoA (Eig. 17-18) citramalate and mesa-conate (Eig. 24-8) are intermediates. This latter pathway is used by some methanogens as are other alternative routes. The first step unique to the biosynthetic pathway to leucine is the reaction of the... 

In leucine biosynthesis, the intermediate 218 on the valine pathway reacts with acetyl-CoA to yield a-isopropylmalate 225 whose configuration has been shown to be S by X-ray crystallography (200). This reaction is analogous to that catalyzed by citrate synthase, and indeed the subsequent reaction, dehydration/rehydration giving ) -isopropylmalate 226, is analogous to the conversion of citrate to isocitrate. The configuration of / -isopropylmalate 226 had been shown to be 2i ,3S (201), and so the stereochemistry of the citrate and isopropylmalate reactions was identical. j -Isopropylmalate 226 was finally converted to leucine 205 by a 4-pro-R NADH specific dehydrogenase (EC 1.1.1.85) (202) and transamination (Scheme 61). 

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