Amino transfer reactions

On a genomic level, transaminases are classified into (up to six) subgroups [1717, 1718], among which group 11 comprises the most useful transaminases, which are able to accept non-a-amino acid t5q)e substrates. Since their natural substrates typically encompass co-aminocarboxylic acids, such as ornithine, lysine, (3-alanine, and co-aminobutyrate, they are commonly denoted as m-trans-aminases (co-TA), in contrast a-transaminases, which only act on a-amino acid-type substrates. 

In view to access both stereoisomers of a chiral amine via transamination by choice of an appropriate (R)- or (S)-selective co-TA, screening studies were undertaken which revealed an impressive number of (S)-co-TAs and some more rarely occurring (/ )-selective enzymes [1719-1722]. The most widely used enzymes are obtained from Vibrio fluvialis [1723], Chromobacterium violaceum [1724, 1725], Pseudomonas aeruginosa [1726], Bacillus megaterium [1727], and Alcaligenes denitrificans [1728]. Thermostable mutants were derived from an co-TA from Arthrobacter citreus [1729]. 

Depending on the substrate preference of the employed transaminase, the following couples of sacrificial amine donor/keto acceptors were used, which are often derived from the a-aminoacid pool (such as alanine/pyruvate, phenylalanine/ phenylpyruvate, glutamic acid/a-ketoglutarate, aspartic acid/a-ketosuccinate) or constitute simple amines/ketones, such as 2-propylamine/acetone and 2-butyl-amine/2-butanone. It should be kept in mind that the absolute configuration of a chiral amine-donor has to match the stereospecificity of the co-TA in order to be accepted. 

In transamination, equilibrium constants are close to unity at best and the amino transfer from an a-amino acid to a ketone is strongly disfavored. To even worsen 

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Nicotianamine is converted to a first phytosiderophore, 2 -deoxym ugeneic acid, by amino transfer and subsequent reduction (Figure 4.3). Nicotianamine aminotransferase, which catalyzes the initial amino transfer reaction, the first step... 

The 2-oxoglutarate/L-glutamate couple serves as one amino group acceptor and donor pair in all amino-transfer reactions the specificity of the individual enzymes derives from the particular amino acid that serves as the other donor of an amino group. Thus AST catalyzes the reaction. 

Pyridoxal-5 phosphate (P-5 -P) and its amino analogue, pyridoxamine-5 -phosphate, function as coenzymes in the amino-transfer reactions. The P-5 -P is bound to the apoen-zyme and serves as a true prosthetic group. The P-5 -P bound to the apoenzyme accepts the amino group from the first substrate, aspartate or alanine, to form enzyme-bound pyridoxamine-5 -phosphate and the first reaction product, oxaloacetate or pyruvate, respectively. The coenzyme in amino form then transfers its amino group to the second substrate, 2-oxoglutarate, to form the second product, glutamate. P-5 -P is thus regenerated. 

Alanine appears to be formed primarily by alanine dehydrogenase in A. cylindrica, C. licheniformey P. boryanumy and A. nidulanSy because the inhibition of formation of glutamate and glutamine by methionine sulfoximine is not accompanied by a comparable inhibition in the formation of alanine (Table I). Moreover, alanine formation by these species was not reduced in the presence of aminooxy acetate, an inhibitor of amino transfer reactions (I3,J4). However, in A. variabilis alanine appears to be formed by a transamination reaction, because synthesis of [ N]alanine from was strongly inhibited in the presence of... 

Groups with special conformation were introduced into CD. This could get analogue enzyme model with stereoscopic selectivity. For example, pyridoxamine (PM) was bonded to the first hydroxyl group of p CD. The obtained modified CD could catalyze amino transfer reaction of a-ketone add. The reaction speed was 100 fold that of PM as catalyst and the stereoselectivity was shown during the catalysis [16]. If the PM group was linked to the main surface of -CD through double bonds, the transaminase model would be obtained. It was indicated from studies that when PM group was located on one side of CD, the substrate of meta substituted phenyl pyruvic add would be Unked as priority selection when it was located above the cavity of CD, the substrate of p-substituted phenyl pyruvic acid would be linked as priority selection [17]. 

Transaminases (also termed amino transferases [EC 2.6.l.X]) catalyze the redox-neutral amino-transfer reaction between an amine donor and a carbonyl group serving as acceptor (Scheme 2.225) [94, 1707-1712]. These enzymes require an activated benzaldehyde (pyridoxal-5 -phosphate, PLP, vitamin Bg) as cofactor, which functions as a molecular shuttle for the transfer of the NHa-moiety. In a first step, PLP forms a ketimine Schiff base with the amine-donor. Tautomerization of the C=N bond yields an aldimine, which is hydrolyzed to yield the cofactor in its aminated form (pyridoxamine, PMP). The latter reacts through the same order of events with the carbonyl group of the substrate to form the amine product and... 

III. Aspartate Amino Transfer Reactions and Amide Syntheses. 76... 

III. ASPARTATE AMINO TRANSFER REACTIONS AND AMIDE SYNTHESES... 

Dietary proteins are a source of amino acids which can serve as important precursors for gluconeogenesis. During a fast or starvation, a major contribution is made by alanine which is released along with other amino acids from skeletal muscle. Since labile proteins rich in alanine are not present in muscle, the released alanine appears to result from the activity of alanine aminotransferase (Section 16.1) which produces alanine from pyruvate. This is the basis of the alanine cycle which also operates between skeletal muscle and the liver. The alanine cycle functions only when peripheral tissues reoxidize glycolytic NADH through the oxidative phosphorylation pathway. In the presence of oxygen, pyruvate is not utilized in lactate production and is available for the amino transfer reaction. 

CO2 and the generation of reduced hydrogen carriers. A number of cycle intermediates may be used in biosynthetic pathways. Oxaloacetate and 2-oxoglutarate may be converted into aspartate and glutamate respectively by amino transfer reactions (Section 16.1) and thereby be employed as sources of these amino acids for protein synthesis. 

Oxaloacetate may also be produced by amino transfer reactions involving aspartate or indirectly from pyruvate through the concerted action of two malate dehydrogenase enzymes, EC 1.1.1.40 and EC 1.1.1.37. However, the amino transfer reaction is not anaplerotic since it does not accomplish net synthesis of a tricarboxylate cycle intermediate as it employs 2-oxoglutarate. Some glucogenic amino acids (Table 16.4) may contribute to anaplerosis (Section 16.3). 

In bacteria and plant cells, the 20 amino acids which occur in their proteins may be synthesized from glutamate or glutamine and intermediates of glycolysis, pentose phosphate pathway and tricarboxylate cycle (Table 16.1). In some cases, the synthesis involves a single reaction whilst other pathways may involve as many as 12 reactions. Only the synthesis of tryptophan from phosphoenol-pyruvate and erythrose 4-phosphate does not involve at least one amino transfer reaction... 

Amino transfer reactions are reversible, have a wide occurrence (Table 16.2) and are important in biosynthesis and degradation in animals and humans. The enzymes are frequently specific for a given amino acid and are named accordingly. 

FIGURE 16.4 Double displacement mechanism of amino transfer reactions... 

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