4- aminobutyrate transaminase

Among the numerous enzymes that utilize pyridoxal phosphate (PLP) as cofactor, the amino acid racemases, amino acid decarboxylases (e.g., aromatic amino acids, ornithine, glutamic acid), aminotransferases (y-aminobutyrate transaminase), and a-oxamine synthases, have been the main targets in the search for fluorinated mechanism-based inhibitors. Pharmaceutical companies have played a very active role in this promising research (control of the metabolism of amino acids and neuroamines is very important at the physiological level). 

PROBLEM 18.55 y-Aminobutyrate transaminase (GABA-T) is the key enzyme controUing the metabohsm of y-aminobu-tyric acid (GABA), a mammahan inhibitory neurotransmitter. In the following synthesis of the hydrochloride salt of the putative GABA-T inhibitor, 3-amino-1,4-cyclohexadiene-l-... 

The transamination reaction represented by equation (15) involves the reversible transfer of a-amino groups. Although there are probably more enzymes catalysing this type of reaction, some transaminases catalysing w-amino group transfer have also been partially identified. For example, the a-ketoglutarate—y-aminobutyrate transaminase... 

Succinic semialdehyde (SSA) is synthesized in the mitochondria through transamination of y-aminobutyric acid (GABA) by GABA transaminase (GABA-T). Most of the SSA is oxidized by SSA dehydrogenase (SSA-DH) to form succinate, which is used for energy metabolism and results in the end products CO2 + H2O, which are expired. A small portion of SSA (<2%) is converted by SSA reductase (SSA-R) in the cytosol to GHB. GHB may also be oxidized back to SSA by GHB dehydrogenase (GHB-DH). 

Schousboe, A., Wu, J. Y., and Roberts, E. (1973) Purification and characterization of the 4-aminobutyrate-2,ketoglutarate transaminase from mouse brain. Biochemistry 12,2868-2873. 

This enzyme [EC 2.6.1.21], also known as D-aspartate aminotransferase, D-amino acid aminotransferase, and D-amino acid transaminase, catalyzes the reversible pyridoxal-phosphate-dependent reaction of D-alanine with a-ketoglutarate to yield pyruvate and D-glutamate. The enzyme will also utilize as substrates the D-stereoisomers of leucine, aspartate, glutamate, aminobutyrate, norva-hne, and asparagine. See o-Amino Acid Aminotransferase... 

Jaeken J, Casaer P, de Cock P, Corbeel L, Eeckels R, Eggermont E, Schechter PJ, Brucher JM (1984) Gamma-aminobutyric acid-transaminase deficiency a newly recognized inborn error of neurotransmitter metabolism. Neuropediatrics 15 165-169... 

Medina-Kauwe LK, Tobin AJ, De Meirleir L, Jaeken J, Jakobs C, Nyhan WL, Gibson KM (1999) 4-Aminobutyrate aminotransferase (GABA-transaminase) deficiency. J Inherit Metab Dis 22 414-427... 

Schor DS, Struys EA, Hogema BM, Gibson KM, Jakobs C (2001) Development of a stable-isotope dilution assay for gamma-aminobutyric acid (GABA) transaminase in isolated leukocytes and evidence that GABA and beta-alanine transaminases are identical. Clin Chem 47 525-531... 

Although the utility of transaminases has been widely examined, one such limitation is the fact that the equilibrium constant for the reaction is near unity. Therefore, a shift in this equilibrium is necessary for the reaction to be synthetically useful. A number of approaches to shift the equilibrium can be found in the literature.53 124135 Another method to shift the equilibrium is a modification of that previously described. Aspartate, when used as the amino donor, is converted into oxaloacetate (32) (Scheme 19.21). Because 32 is unstable, it decomposes to pyruvate (33) and thus favors product formation. However, because pyruvate is itself an a-keto acid, it must be removed, or it will serve as a substrate and be transaminated into alanine, which could potentially cause downstream processing problems. This is accomplished by including the alsS gene encoding for the enzyme acetolactate synthase (E.C. 4.1.3.18), which condenses two moles of pyruvate to form (S)-aceto-lactate (34). The (S)-acetolactate undergoes decarboxylation either spontaneously or by the enzyme acetolactate decarboxylase (E.C. 4.1.1.5) to the final by-product, UU-acetoin (35), which is meta-bolically inert. This process, for example, can be used for the production of both l- and d-2-aminobutyrate (36 and 37, respectively) (Scheme 19.21).8132 136 137... 

Silverman and associates explored a variety of potential inactivators for GABA [y-aminobutyric acid, H3T (CH2)3COOH] transaminase, another pyridoxal-dependent enzyme. In the reaction of the enzyme with 4-amino-5-fluoropentanoic acid, Silverman and Invergo wrote the mechanism in equation 25 for the covalent interaction of the enzyme with the inactivator161. The mechanism, dubbed the enamine mechanism, was earlier suggested by Metzler s group162, who had also proposed, as a test, alkaline treatment of the inactivated enzyme that would result in the release of the coenzyme-bound modified inactivator. 

Figure 5. Inhibition of y-aminobutyric acid transaminase by 4-aminohex-5-ynoic acid (y-acetylenic GABA)...

Cassel, G., Karlsson, L., Sellstrom, A. (1991). On the inhibition of glutamic acid decarboxylase and gamma-aminobutyric acid transaminase by sodium cyanide. P/iarmaco/. Toxicol. 69 238-41. 

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