Pyridoxal phosphate enzymes transaminase reactions

Transamination, often also referred to as aminotransfer, is applied to those enzymatic reactions in which an amino group is exchanged between an amino acid and an a-keto acid. This type of reaction is catalyzed by a group of transferases called transaminases or aminotransferases. They are active in both the cytosol and the mitochondria of most cells. An essential prosthetic group of such enzymes is pyridoxal phosphate, and the reaction is generally of the ping-pong type. 

In the transaminase reaction, the L-amino acid is the most common donor, but D-amino acids have also been described in that role. Studies on the specificity of transaminase suggest that the enzyme is specific for both the donor of the amino group and the acceptor keto acid. Transaminase requires pyridoxal phosphate as coenzyme for activity (the role of the pyridoxal phosphate in the reaction is discussed further in the section on vitamins). 

A few years later, in 1953, the versatility of pyridoxal phosphate was illustrated by Snell and his collaborators who found many of the enzyme reactions in which pyridoxal phosphate is a coenzyme could be catalyzed non-enzymically if the substrates were gently heated with pyridoxal phosphate (or free pydridoxal) in the presence of di- or tri-valent metal ions, including Cu2+, Fe3+, and Al3+. Most transaminases however are not metal proteins and a rather different complex is formed in the presence of the apoprotein. 

Glutamate can then participate in the formation of other amino acids via the process called transamination. Transamination is the exchange of the amino group from an amino acid to a keto acid, and provides the most common process for the introduction of nitrogen into amino acids, and for the removal of nitrogen from them. The reaction is catalysed by a transaminase enzyme, and the coenzyme pyridoxal phosphate (PLP) is required. 

Among the NH2 transfer reactions, transaminations (1) are particularly important. They are catalyzed by transaminases, and occur in both catabolic and anabolic amino acid metabolism. During transamination, the amino group of an amino acid (amino acid 1) is transferred to a 2-oxoacid (oxoacid 2). From the amino acid, this produces a 2-oxo-acid (a), while from the original oxoacid, an amino acid is formed (b). The NH2 group is temporarily taken over by enzyme-bound pyridoxal phosphate (PLP see p. 106), which thus becomes pyridoxamine phosphate. 

This enzyme [EC 2.6.1.2], also known as glutamic-pyruvic transaminase and glutamic-alanine transaminase, catalyzes the pyridoxal-phosphate-dependent reaction of alanine with 2-ketoglutarate, resulting on the production of pyruvate and glutamate. 2-Aminobutanoate will also react, albeit slowly. There is another alanine aminotransferase [EC 2.6.1.12], better known as alanine-oxo-acid aminotransferase, which catalyzes the pyridoxal-phosphate-dependent reaction of alanine and a 2-keto acid to generate pyruvate and an amino acid. See also Alanine Glyoxylate Aminotransferase... 

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... 

This enzyme [EC 2.6.1.1] (also known as transaminase A, glutamicioxaloacetic transaminase, and glutamic aspartic transaminase) catalyzes the reversible reaction of aspartate with a-ketoglutarate to produce oxaloace-tate and glutamate. Pyridoxal phosphate is a required cofactor. The enzyme has a relatively broad specificity, and tyrosine, phenylalanine, and tryptophan can all serve as substrates. 

This enzyme [EC 2.6.1.42], also referred to as transaminase B, catalyzes the reversible reaction of leucine with a-ketoglutarate (or, 2-oxoglutarate) to produce 4-methyl-2-oxopentanoate and glutamate. The pyridoxal-phosphate-dependent enzyme will also utilize isoleucine and valine as substrates. However, this enzyme is distinct from that of valine pyruvate aminotransferase [EC 2.6.1.66]. See also Leucine Aminotransferase... 

The covalent intermediates can be attacked by a second nucleophile to cause the release of the product. When the second nucleophile is water, the overall reaction is called hydrolysis. Also, in many cases the nucleophile is not simply an amino acid side chain of the enzyme but a prosthetic group an example is pyridoxal phosphate in the transaminases (Chap. 15). 

Vitamin Be has a central role in the metabolism of amino acids in transaminase reactions (and hence the interconversion and catabolism of amino acids and the synthesis of nonessential amino acids), in decarboxylation to yield biologically active amines, and in a variety of elimination and replacement reactions. It is also the cofactor for glycogen phosphorylase and a variety of other enzymes. In addition, pyridoxal phosphate, the metabolically active vitamer, has a role in the modulation of steroid hormone action and the regulation of gene expression. 

The result of this half-transaminase reaction is formation of pyridoxamine phosphate at the active site of the enzyme, and hence loss of activity. Pyridoxamine phosphate dissociates from the active site, so that if adequate pyridoxal phosphate is available the resultant apoenzyme can be reactivated. 

The coenzyme form of pyridoxine is known as pyridoxal phosphate (PP) The most common type of reaction requiring PP as a coenz5mie is transamination. Enzymes catalysing such reactions are known as transaminases or aminotransferases. The coenzyme binds to its apoenzyme via Schiff s base between its aldehyde group and the epsilon amino group of a lysine in the... 

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 first step in the catabolism of most amino acids is the transfer of the o-amino group from the amino acid to a-ketoglutarate (tx-KG). This process is catalyzed by transaminase (aminotransferase) enzymes that require pyridoxal phosphate as a cofactor. The products of this reaction are glutamate (Glu) and the a-ketoacid analog of the amino acid destined for catabolic breakdown. For example, aspartate is converted to its a-keto analog, oxalo-acetate, by the action of aspartate transaminase (AST), which also produces Glu from a-KG. The transamination process is freely reversible, and the direction in which the reaction proceeds is dependent on the concentrations of the reactants and products. These reactions do not effect a net removal of amino nitrogen the amino group is only transferred from one amino acid to another. 

Pyridoxal phosphate The coenzyme that is required for transaminase (aminotransferase) reactions, as weU as other enzymes. It is the active form of pyridoxine (vitamin B ). 

The glycolytic pathway includes three such reactions glucose 6-phosphate isomer-ase (1,2-proton transfer), triose phosphate isomerase (1,2-proton transfer), and eno-lase (yS-elimination/dehydration). The tricarboxylic acid cycle includes four citrate synthase (Claisen condensation), aconitase (j5-elimination/dehydration followed by yS-addition/hydration), succinate dehydrogenase (hydride transfer initiated by a-proton abstraction), and fumarase (j5-elimination/dehydration). Many more reactions are found in diverse catabolic and anabolic pathways. Some enzyme-catalyzed proton abstraction reactions are facilitated by organic cofactors, e.g., pyridoxal phosphate-dependent enzymes such as amino acid racemases and transaminases and flavin cofactor-dependent enzymes such as acyl-C-A dehydrogenases others. 

Pyridoxal phosphate is a co-enzyme for numerous enzymes, notably amino acid decarboxylases, amino acid transaminases, histaminase and probably diamine oxidase Ais.iw. As most of the evidence on which the mechanism of action of pyridoxal-dependent enzymes is based has been obtained from studies of the non-enzymic interaction of pyridoxal with amino acids, these non-enzymic reactions will be considered first in some detail. 

Another subset of group transfer reactions consists of transaminations (Fig. 8.20). In this type of reaction, the nitrogen group from an amino acid is donated to an a-keto acid, forming a new amino acid and the a-keto acid corresponding to the donor amino acid. Enzymes catalyzing this last type of reaction are called transaminases or aminotransferases. The coenzyme pyridoxal phosphate is reqnired for all transaminases (see Fig. 8.13). 

All amino acids except lysine and threonine undergo transamination reactions. The enzymes catalyzing these reactions are known as transaminases or aminotransferases. For most of these reactions, a-ketoglutarate and glutamate serve as one of the a-keto acid-amino acid pairs. Pyridoxal phosphate is the cofactor, and the mechanism of the reaction is indicated in Figure 38.4. 

Fig. 38.3. Transamination. The amino group from one amino acid is transferred to another. Pairs of amino acids and their corresponding a-keto acids are involved in these reactions, a-ketoglutarate and glutamate are usually one of the pairs. The reactions, which are readily reversible, use pyridoxal phosphate (PLP) as a cofactor. The enzymes are called transaminases or aminotransferases. A. A generalized reaction. B. The aspartate transaminase reaction.

This enzyme is a pyridoxal phosphate protein, which catalyses the formation of 7-aminobutyric acid, a possible chemical transmitter in the CNS. It can be used in an indicator reaction for the radiochemical assay of transaminases. It is assayed radiochemically using Clabelled glutamate and COj evolution [391]. 

In the presence of a-ketoglutarate and pyridoxal phosphate, tyrosine loses its amino group to yield hy-droxyphenylpyruvate. The enzyme involved in that reaction—tyrosine transaminase—has been extensively purified from dog liver, and some aspects of its... 

Kynurenic acid and xanthurenic acid, side products of the reaction, are the products of the transamination of the a-amino group of kynurenine and 3-hydroxy-kynurenine to a-ketoglutaric acid in the presence of pyridoxal phosphate and an enzyme found in mammalian liver and kidney, kynurenine transaminase. The keto acid resulting from the transamination reaction condenses spontaneously. Liver homogenate also decarboxylates 3-hydroxykynurenine to yield 4,8-de-hydroxyquinoline. Kynurenase may catalyze the cleavage of the side chain of kynurenine or 8-hydroxy-kynurenine and lead to the formation of alanine and... 

These reactions involve the activities of transaminases and decarboxylases (see p. 210), and over 50 pyridoxal phosphate-dependent enzymes have been identified. In transamination, pyridoxal phosphate accepts the a-amino group of the amino acid to form pyridoxamine phosphate and a keto acid. The amino group of pyri-doxamine phosphate can be transferred to another keto acid, regenerating pyridoxal phosphate. The vitamin is believed to play a role in the absorption of amino acids from the intestine. 

Vitamin Bg (pyridoxine also known as pyridoxol, 118) is an essential growth factor in the diet of many organisms and animals. It forms part of a coenzyme (pyridoxylphosphate) and it is a cofactor for a class of enzymes known as transaminases. A transaminase or an aminotransferase is an enzyme that catalyzes a type of reaction between an amino acid and an a-keto acid. The presence of elevated transaminase levels can be an indicator of liver damage. Vitamin Bg has both an aldehyde form (pyridoxal, 119) and an amine form (pyridoxamine, 120), and it is known that pyridoxal phosphate is a carrier of amino groups and sometimes amino acids. ... 

Kynurenine Transaminase. Both kynurenine and 3-hydroxykynu-renine participate in reactions with two enzymes that use pyridoxal phosphate. One is a transaminase that presumably forms the a-keto derivative of kynurenine. This compound does not accumulate, however ... 

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