Amino acid deaminase

Because the commercial availability of 2-oxo acids is somewhat limited and their chemical stability is relatively poor, a system was developed to generate these in situ using an amino acid deaminase (amino acid oxidase, E.C. 1.4.3.x) (Scheme 19.3) A This keto acid was then reduced to the chiral 2-hydroxy acid with either L-LdH or D-LdH to produce the respective enantiomer in moderate to high yield with high enantiomeric purity.49... 

An amino acid is deaminated when it loses its nitrogen in the form of NH3 and generates an a-keto acid. It is a redox reaction using NAD+, FAD, or FMN as cofactors, and it may occur in either the mitochondria or the cytosol. The most potent l amino acid deaminase is glutamate dehydrogenase, which catalyzes the reaction... 

The procedure reported in Scheme 13.11 describes deracemization of an amino acid involving oxidation with an L-specific enzyme and transamination with a D-amino transferase using D-aspartate 10, which is generated from L-aspartate 11 by aspartate racemase, as the amino donor. The oxidative enzyme is defined as an L-amino acid deaminase, a flavoprotein from Proteus myxofadens [34]. The transamination reaction is shifted towards the product since the oxalacetate 12 formed decarboxylates spontaneously to give pyruvate and carbon dioxide. 

Recently, L-amino acid deaminase (EC 1.4.3.x) activities have been identified, particularly from the Proteus genus [59]. This enzyme, constituted by 370 residues, is an FAD-containing L-amino acid oxidase flavoprotein that uses molecular oxygen to convert L-amino acids into the corresponding a-keto adds and ammonia but does not produce hydrogen peroxide. L-amino acid deaminase prefers amino acids with aliphatic, aromatic, and sulfur-containing side chains (the best substrates are L-heu, L-Phe, L-Met, and L-Trp) and its kinetic efficiency is quite low because of the low Vnm value (<2 units/mg protein). 

The a-keto acid substrates are available by chemical synthesis. They are also accessible by enzymatic methods, such as amino acid deaminase, that generate these substrates from inexpensive L-amino acids. 

Figure 6.6 Scheme showing the amino acid deaminase test usingpher lalanine... 

Amino acid deaminases, IX, 36-38 biotin and, IX, 38-44 Amino acid deficiency, effect on gonads, X, 96-98 Amino acid oxidase (s), action of, IX, 36 biotin and, IX, 54 effect on biotin, X, 337 flavoprotein nature of, IX, 36 in tissues, steroid hormones and, X, 367 D-Amino acid oxidase, effect of adrenalectomy on, X, 320 of steroid hormones on, X, 319, 320, 352-358... 

A range of (D)-amino acids is also accessible by a fermentation route. The microorganisms are for this purpose modified, so that they no longer express functional (D)-amino acid deaminases. However, they contain instead genes for (L)-amino acid deaminases, (D)-amino acid aminotransferases and a special racemase. Monsanto, for example, has patented such an approach for the preparation of (D)-phenylalanine. 

A racemase brings about inversion of the relatively cheap (L)-isomers of alanine or aspartic acid, but not of (D)-phenylalanine. Only (L)-phenylalanine is deaminated by an (L)-amino acid deaminase, whereas (D)-phenylalanme is not. The latter is generated by ammonia transfer from (D)-alanine or (D)-aspar-tic acid with a (D)-amino acid aminotransferase. The equilibria are moved in favour of the product, either by the metabolism of pyruvic acid or oxosuccinic acid. Since (L)-amino acid deaminases, like (D)-amino add aminotransferases, are non-specific, they also permit the preparation of a variety of other (D)-amino acids. [58]... 

Compared to chemical synthesis, enzymatic transformation has several potential advantages, including its ecofriendly nature, low energy consumption, and reduced production cost. However, to date, there have been few reports on the enzymatic synthesis of a-KG. However, a-KG has been produced from L-glutamic acid by using amino acid deaminase. ... 

Scheme 13.60. A repetition of Scheme 12.25, which was a representation of a pathway from indole-3-glycerolphosphate to tryptophan (Trp, W).The path has been modified with addition of steps (using the amino acid deaminase EC 4.1.1.28) to carry tryptophan (Trp, W) to tryptamine. 

Liu L, Hossain GS, Shin HD, Li J, Du G, Chen J. One-step production of a-ketoglutaric acid from glutamic acid with an engineered L-amino acid deaminase from Proteus mirahilis. J Biotechnol 2013 164 97-104. 

Even though this concept has been proven to be effective, the toxicity of NaCNBHj led to the search for alternative reagents whereby amine-boranes turned out to be competent substitutes [117]. They are stable in water at neutral or basic pH and soluble and unreactive toward a wide range of protic and aprotic solvents. Deracemization of several natural and nonnatural AAs with an L-amino acid deaminase (l-AAD)... 

Multienzyme cascade for the conversion of i-amino acids to a-hydroxy acids employing an L-amino acid deaminase (l-AAD) and isocaproate reductases (Hie). 

---