Branched aminotransferase

Transaminase enzymes (also called aminotransferases) specifically use 2-oxoglutarate as the amino group acceptor to generate glutamate but some have a wide specificity with respect to the amino donor. For example, the three branched-chain amino acids leucine, isoleucine and valine, all serve as substrates for the same enzyme, branched-chain amino acid transaminase, BCAAT ... 

Xian, M., Alaux, S., Sagot, E. and Gefflaut, T., Chemoenz3fmatic synthesis of glutamic acid analogues substrate specificity and S3mthetic apphcations of branched chain aminotransferase from Escherichia coli. J. Org. Chem., 2007, 72, 7560-7566. 

Figure 8.11 Five near-equilibrium reactions involved in transamination of five different amino adds. Three enzymes are involved in these reactions (1) alanine aminotransferase (2) aspartate aminotransferase (3) branched-chain amino acid aminotransferase, i.e. one enzyme catalyses the three reactions. (The branched-chain amino acids are essential.)...

Branched-Chain Amino Acid Aminotransferase A. E. Braunstein (1973) The Enzymes, 3rd ed., 9, 379. 

BRANCHED-CHAIN AMINO ACID AMINOTRANSFERASE LEUCINE DEHYDROGENASE LEUCINE KINETICS RROTEIN TURNOVER KINETICS SAAM... 

Transamination Removal of the amino groups of all three amino acids is catalyzed by a single enzyme, branched-chain a-amino acid aminotransferase. 

Branched chain aminotransferase Gamma-aminobutyrate aminotransferase CO-Amino acid pyruvate aminotransferase Tyrosine aminotransferase Serine pyruvate aminotransferase... 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

Branched-chain aminotransferases (BCATs) evolved from aspartate aminotransferases (AATs) showed a record 105- to 2 x 106-fold improvement in catalytic efficiency (kcat/KM). Not only were the 13-17 amino acid substitutions concentrated in the most active mutants, but all but one mutated amino acid residues are located far from the active site. With directed evolution, enantioselectivities can be improved on enantiounspecific enzymes (from E = 1.1 to 25.8) and even inverted to yield the opposite enantiomer in comparison to the wild type (40% d- to both 90% d- and 20% L-). 

The aminotransferase class of enzymes (E.C. 2.6.1.x), also known as transaminases, are ubiquitous, PLP-requiring enzymes that have been used extensively to prepare natural L-amino acids and other chiral compounds.30 123 124 The L-aminotransferases catalyze the general reaction shown in Scheme 19.19 where an amino group from one L-amino acid is transferred to an a-keto acid to produce a new L-amino acid and the respective a-keto acid (see also Chapter 3). Those enzymes most commonly used as industrial biocatalysts have been cloned, overexpressed, and generally used as whole-cell or immobilized preparations. These include branched chain aminotransferase (BCAT) (E.C. 2.6.1.42), aspartate aminotransferase (AAT) (E.C. 2.6.1.1), and tyrosine aminotransferase (TAT) (E.C. 2.6.1.5). 

Glucocorticoids also increase the activity of transaminases (aminotransferases), especially in the skeletal muscle. Aminotransferases serve to transfer the amino groups from amino acids to be metabolized to a-keto acids, especially pyruvate. In the latter case, the alanine thus formed is transported from the muscle into the bloodstream and extracted from there by the liver. In the liver, alanine is converted to glucose, and glucose may then return to the muscle as it does in the Cori cycle (Figure 18.4). This is the alanine cycle, and more about this is discussed in Chapter 20. Branched-chain amino acids are the principal donors of nitrogen to pyruvate in the muscle and are thus important actors in the alanine cycle. 

Eden, A., Van Nedervelde, L., Drukker, M., Benvenisty, N., Debourg, A. (2001) Involvement of branched-chain amino acid aminotransferases in the production of fiisel alcohols during fermentation in yeast. Applied Microbiology and Biotechnology, 55, 296-300. 

Wu JY, Kao HJ, Li SC, Stevens R, Hillman S, Millington D, Chen YT. ENU mutagenesis identifies mice with mitochondrial branched-chain aminotransferase deficiency resembling human maple syrup urine disease. J. Clin. Invest. 2004 113 434 440. 

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