Amino acids from a-keto acids

An efficient synthesis of (S)-amino acids from a-keto acids via a diastereoselective hydrogenation step with (S)-proline as the chiral inducer was reported (eq 11). Optical yields up to 90% were reached. 

Some general reactions that involve degradation or interconversion of amino acids provide for the synthesis of nonessential amino acids from a-keto acid precursors derived from carbohydrate intermediates. 

Figure 17-4. Synthesis of D-amino acids from a-keto acid, formate, NAD+, D-alanine, and ammonia by coupling of u-alanine dehydrogenase (AlaDH), formate dehydrogenase (FDH), alanine racemase (AlaR), and D-amino acid aminotransferase (d-ATA).

Prolinc has been used133 to synthesize chiral 2-amino acids from a-keto acids. 

Some studies suggest that the principal pathway of glutamate utilization in liver mitochondria is by transamination (81). GDH decreases the distribution coefficient of glutamate-oxaloacetate aminotransferase on Sephadex G-20Q, possibly by forming a complex with that enzyme (82). In addition, in the presence of NADPH and NHC, GDH appears to catalyze the conversion of the pyridoxal phosphate form of the aminotransferase to the pyridoxamine form, which catalyzes the formation of a-amino acids from a-keto acids (83). This reaction is interesting in view of the inhibition of GDH by pyridoxal phosphate (54) (See Section V,A). If the complex exists in mitochondria, it may provide an efficient mode of dehydrogenation of amino acids that are not normally good substrates of GDH (82). 

Bycroft, B. W., and G. R. Lee Efficient Assymmetric Synthesis of a-Amino Acids from a-Keto Acids and Ammonia with Conservation of the Chiral Reagent. J. C. S. Chem. Comm. 1975, 988. 

Efficient Asymmetric Synthesis of a-Amino Acids from a-Keto Acids and... 

Amino acids get used up (making proteins, for example) so, to keep life going, ammonia must be brought in from somewhere. The key amino acid in this link is glutamic acid. A true reductive animation using NADPH and ammonia builds glutamic acid from a-keto-glutaric acid. 

Branched chain fatty acids, such as 2-methylpropanoic, and 2-methylbutanoic and 3-methylbutanoic acids, are not products of the fatty acid synthetic pathway. They are instead derived from oxidation of the aldehydes formed from a-keto acids during amino acid metabolism (Fig 8D.5). The mechanism of regulation is not known. 

Aminotransferases catalyze the transfer of an a-amino group from an a-amino acid to an a-ketoacid. These enzymes, also called transaminases, generally funnel a-amino groups from a variety of amino acids to a-keto-glutarate for conversion into NH4 +. 

The synthesis of a-amino acids has been carried out from a-keto acids by reductive aminadon using a platinum or palladium catalyst, mimicking their biosynthesis. Thus alanine, leucine, phenylalanine, " aspartic acid and glutamic acid - have been synthesized by reductive amination. When an optically active primary amine is used, asymmetric induction can proceed in the course of the reductive amination. 

Studies on the transamination reaction between f-butyl esters of optically active amino acids and methyl pyruvate were carried out, as shown in Scheme 7. The resulting iminodicarboxylic acid (16) was partially hydrolyzed and then oxidized with f-butyl hypochlorite to form alanine. The oxidation is a generally applicable one, and the optical purity of alanine is high (50-70%). Similar asymmetric transamination between an (S)-amino acid and ketones was carried out. Catalytic hydrogenation of the Schiff s bases prepared from a-keto acid esters and amino acid esters was carried out, and a substituent and temperature effect observed (de 40-70%). ... 

Transaminations involve moving a a-amino group from a donor a-amino acid to the keto carbon of an acceptor a-keto acid. These reversible reactions are catalyzed by a group of intracellular enzymes known as transaminases (aminotransferases), which employ covalently bound pyridoxal phosphate as a cofactor. 

The synthesis of chiral a-amino acids starting from a-keto acids by means of a transamination has been reported by NSC Technologies [115]. In this process, L-aspartate serves as the amino donor. The substrate specificity is broad, allowing the conversion of numerous keto acid substrates. 

L-Amino acid transaminases are ubiquitous in nature and are involved, be it directly or indirectly, in the biosynthesis of most natural amino acids. All three common types of the enzyme, aspartate, aromatic, and branched chain transaminases require pyridoxal 5 -phosphate as cofactor, covalently bound to the enzyme through the formation of a Schiff base with the e-amino group of a lysine side chain. The reaction mechanism is well understood, with the enzyme shuttling between pyridoxal and pyridoxamine forms [39]. With broad substrate specificity and no requirement for external cofactor regeneration, transaminases have appropriate characteristics to function as commercial biocatalysts. The overall transformation is comprised of the transfer of an amino group from a donor, usually aspartic or glutamic acids, to an a-keto acid (Scheme 15). In most cases, the equilibrium constant is approximately 1. 

The synthesis of chiral a-amino acids starting from a-keto acids by means of a transamination has been reported by NSC Technologies [26, 27]. In this process, which can be used for the preparation of l- as well as D-amino acids, an amino group is transferred from an inexpensive amino donor, e.g., L-glutamic acid, l-22, or L-aspartic acid, in the presence of a transaminase (= aminotransferase). This reaction requires a cofactor, most commonly pyridoxal phosphate, which is bound to the transaminase. The substrate specificity is broad, allowing the conversion of numerous keto acid substrates under formation of the L-amino acid products with high enantioselectivities [28]. 

In 1961, Hiskey et al.(l) reported the successful asymmetric syntheses of a-amino acids. They demonstrated the synthesis of amino acids in 45-70% enantiomeric purity by catalytic hydrogenation of the Schiff bases prepared from a-keto acids and optically active a-methylbenzylamine followed by hydrogenolysis (Scheme 1). When (S)-amine was used, (S)-a-amino acid resulted. This is a highly stereoselective reaction. However, the authors did not discuss the steric course of the asymmetric hydrogenation process. 

Figure 23.1 Transamination. This process moves the amino group from one keto acid to another. Glutamate and cr-ketoglutarate are always one of the two pairs involved, so that glutamate serves as a clearing house for amino groups. The R in the two compounds on the left is not an unusual atom. It is the chemist s shorthand way of generalising - i.e. this could be any amino acid sidechain.

The transamination of the a-amino group to a keto acid acceptor (reaction 2) has been demonstrated in a number of higher plant studies (Nahler and Ruis, 1973 Streeter, 1977 Lloyd and Joy, 1978). The product of the transamination is 2-oxosuccinamate. This can be deamidated to oxaloacetate by lettuce and spinach leaf preparations (Meister, 1953). A similar reaction was reported by Streeter (1977) in soybean and pea leaf extracts. On the other hand, Joy (1978) reported that the 2-oxosuccinamate is reduced to 2-hydroxysuccinamate in these leaves in vivo. The apparent discrepancy between the results of Streeter (1977) and those of Joy (1978) may be due to the enzyme assay used by the former. It consisted of the oxidation of NADH in the presence of the enzyme extract and 2-oxosuccinamate. The assumption was that deamidation occurred leading to oxaloacetate which then acted as the substrate of endogenous malate dehydrogenase. The work of Davies (1961) showed that plant malate dehydrogenase is not specific for oxaloacetate, and it is possible that the 2-oxosuccinamate may act as a substrate. Meister (1953). actually measured the production of ammonia from 2-oxosuccinamate by his leaf preparations. 

Three nonessential amino acids (glutamate, alanine, and aspartate) are synthesized from a-keto acids via reactions catalyzed by transaminases. For example, alanine is produced from pyruvate and glutamate (which furnishes the amino group) ... 

The amino group of the chromophore (1) is substituted amidically by small dibasic acids which all stem from the glutamic acid metabolism. In addition to the latter (ref. 20) succinic acid and its amide (refs. 18-21), malic acid (ref. 22) and a-keto glutaric acid (refs. 18-20) have been encountered (succinic acid seems to be an artefact stemming either from its amide or from a-keto glutaric acid). The mass of the acid present in the molecule can be deduced from a characteristic retro-Diels-Alder fragment of the quinoline nucleus in the FAB spectra of the free ligands (see m/z 985 in Fig. 1). 

N-Benzyloxy amino acid / -nitrophenyl esters (297) have been obtained from a-keto acids via benzyloxyimino acids (295), their esterifica-... 

Transaminase reactions are freely reversible so that they function in both the synthesis and breakdown of amino acids. All transaminases require pyridoxal phosphate, a derivative of vitamin (page 165), as a cofactor which transfers the a-amino group from an amino add to a keto acid. In general, transaminases have a high value for the appropriate amino add but a much lower for 2-oxoglutarate. 

Cobaltous chloride/potassium cyanide Homogeneous hydrogenation with cobalt complexes a-Amino- from a-keto-carboxylic acids... 

Although natural amino acids are readily available, there is a continuing need for unnatural amino acids. Jon C. AntiUa of the University of South Florida has described J. Am. Chem. Soc. 2007,129, 5830) a promising approach, based on the enantioselective organocatalytic reduction of imines such as 1 derived from a-keto esters. The aryl group is easily removed to give the primary amine. 

Soda et al. [182] described the use of a whole-cell catalyst for the enzymatic synthesis of l- and d- amino acids from a-keto adds with E. coli cells that overexpress heterologous genes. t-Amino adds were produced with a thermostable (S)-amino acid dehydrogenase and formate dehydrogenase (FDH) from a-keto acids and ammonium formate. No exogenous NADH was required as the... 

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