Pyridoxal phosphate, reaction with glutamate

Chen S-S, Engel PC (1975) The equilibrium position of the reaction of bovine fiver glutamate dehydrogenase with pyridoxal 5 -phosphate. Biochem J 147 351-358... 

Cysteine was also an elTective inactivator in vitro] the closely related compound penicillamine (/3,j8-dimethylcysteine) is slightly more reactive - . Penicillamine inhibits the transaminases of rat liver and the glutamic acid decarboxylase of mouse brain . The formation of thiazolidine derivatives by reaction of cysteine or penicillamine with pyridoxal phosphate proceeds... 

The reductant can be NADH, NADPH (in yeast and bacteria), or reduced ferredoxin (in plants). The enzyme that catalyzes this reaction is glutamate synthase it is also known as glutamate oxoglutarate aminotransferase (GOGAT). A GS/GOGAT complex exists in plants and allows them to cope with conditions of limited nitrogen availability. Enzymes that catalyze transamination reactions require pyridoxal phosphate as a coenzyme (Figure 23.8). We discussed this compound in Section 7.8 as a typical example of a coenzyme, and here we can see its mode of action in context. 

The conversion of serine to glycine involves the transfer of a one-carbon unit from serine to an acceptor. This reaction is catalyzed by senne hydroxymethylase, with pyridoxal phosphate as a coenzyme. The acceptor in this reaction is tetra-hydrofolate, a derivative of folic acid and a frequently encountered carrier of one-carbon units in metabolic pathways. Its structure has three parts a substituted pteridine ring, /(-aminobenzoic acid, and glutamic acid (Figure 23.11). Folic acid is a vitamin that has been identified as essential in preventing birth defects consequently, it is now a recommended supplement for all women of... 

The reaction of pyridoxal phosphate (PLP) with glutamate (Glu) in the presence of Cu + has been studied by measuring the initial rate of reaction... 

Most amino acids lose their nitrogen atom by a transamination reaction in which the -NH2 group of the amino acid changes places with the keto group of ct-ketoglutarate. The products are a new a-keto acid plus glutamate. The overall process occurs in two parts, is catalyzed by aminotransferase enzymes, and involves participation of the coenzyme pyridoxal phosphate (PLP), a derivative of pyridoxine (vitamin UJ. Different aminotransferases differ in their specificity for amino acids, but the mechanism remains the same. 

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 2.6.1.39] catalyzes the reversible reaction of 2-aminoadipate with 2-oxoglutarate (or, a-ketoglutarate) to generate 2-oxoadipate and glutamate. The enzyme requires pyridoxal phosphate. 

This enzyme [EC 2.6.1.57] catalyzes the reversible reaction of an aromatic amino acid with a-ketoglutarate to generate an aromatic oxo acid and glutamate. Pyridoxal phosphate is a required cofactor. Methionine can also act as a weak substrate, substituting for the aromatic amino acid. Oxaloacetate substitutes for a-ketoglutarate. 

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... 

This pyridoxal-phosphate-dependent enzyme [EC 2.1.2.5], also known as glutamate formyltransferase, catalyzes the reaction of 5-formiminotetrahydrofolate with L-glutamate to produce tetrahydrofolate and A-formim-ino-L-glutamate. The enzyme will additionally catalyze the transfer of the formyl moiety from 5-formyltetrahy-drofolate to L-glutamate. This protein occurs in eukaryotes as a bifunctional enzyme also having a formiminote-trahydrofolate cyclodeaminase activity [EC 4.3.1.4]. 

This pyridoxal-phosphate-dependent enzyme [EC 2.6.1.4] catalyzes the reaction of glycine with a-ketoglu-tarate (or, 2-oxoglutarate) to produce glyoxylate and l-glutamate. See also GlycineiOxaloacetate Aminotransferase Glyoxylate Aminotransferase A. E. Braunstein (1973) The Enzymes, 3rd ed., 9, 379. 

Many DOHs, such as L-daunosamine, L-epivancosamine or L-ristosamine, contain an amino group at C3, which is introduced by an aminotransferase. The substrate for this reaction is the 3-keto sugar intermediate that arises as a consequence of the action of a 2,3-dehydratase. This transaminahon reaction has been biochemically characterized in the biosynthesis of L-epivancosamine [10]. Using a coupled reaction with EvaB (2,3-dehydratase) and EvaC (aminotransferase), with pyridoxal-5-phosphate (PEP) as a coenzyme and L-glutamate as a cosubstrate, they were able to show conversion of TDP-4-keto-2,6-dideoxyglucose into thymidine-5 -diphospho-3-amino-2,3,6-trideoxy-D-threo-hexopyranos-4-ulose. 

Pyridoxal phosphate is the coenzyme for the enzymic processes of transamination, racemization and decarboxylation of amino-acids, and for several other processes, such as the dehydration of serine and the synthesis of tryptophan that involve amino-acids (Braunstein, 1960). Pyridoxal itself is one of the three active forms of vitamin B6 (Rosenberg, 1945), and its biochemistry was established by 1939, in considerable part by the work of A. E. Braunstein and coworkers in Moscow (Braunstein and Kritzmann, 1947a,b,c Konikova et al 1947). Further, the requirement for the coenzyme by many of the enzymes of amino-acid metabolism had been confirmed by 1945. In addition, at that time, E. E. Snell demonstrated a model reaction (1) for transamination between pyridoxal [1] and glutamic acid, work which certainly carried with it the implication of mechanism (Snell, 1945). 

The phosphate ester of the aldehyde form of vitamin B6, pyridoxal phosphate (pyridoxal-P or PLP), is required by many enzymes catalyzing reactions of amino acids and amines. The reactions are numerous, and pyridoxal phosphate is surely one of nature s most versatile catalysts. The story begins with biochemical transamination, a process of central importance in nitrogen metabolism. In 1937, Alexander Braunstein and Maria Kritzmann, in Moscow, described the transamination reaction by which amino groups can be transferred from one carbon skeleton to another.139 140 For example, the amino group of glutamate can be transferred to the carbon skeleton of oxaloacetate to form aspartate and 2-oxoglutarate (Eq. 14-24). 

Acetamidodeoxyhexoses. A further modification of the 4-keto-inter-mediate has been independently shown by Ashwell and by Strominger and associates (Table I, References 20, 21, 22, 23). Transamination reactions with L-glutamate as the amino donor and pyridoxal phosphate as coenzyme led to formation of 3-amino 3,6-dideoxy- and 4-amino 4,6-dideoxyhexoses, respectively. Acetylation with acetyl coenzyme A yields the naturally-occurring N-acetyl amino sugar derivatives. 

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]. 

Canaline is the product of the hydrolytic cleavage of canavanine with the simultaneous formation of urea. Canaline is an ornithine analogue which also shows neurotoxicity in the adult sexta where it adversely affects central nervous system functions (jj ). It also is a potent inhibitor of vitamin B -containing enzymes (20-22). It forms a stable Schiff base with the pyridoxal phosphate moiety of the enzyme and drastically curtails enzymatic activity. Pyridoxal phosphate-containing enzymes are vital to insects because they function in many essential transamination and decarboxylation reactions. Ornithine is an important metabolic precursor for insect production of glutamic acid and proline (23). 

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). 

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