Pyridoxal, reactions with amino acids

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. 

Another interesting example of metal-directed chemistry involving the stabilisation and reactivity of imines is seen in the reaction of pyridoxal with amino acids. This reaction is at the basis of the biological transamination of amino acids to a-ketoacids, although the involvement of metal ions in the biological systems is not established. The reaction of pyridoxal (5.27) with an amino acid generates an imine (5.28), which is stabilised by co-ordination to a metal ion (Fig. 5-55). 

Vanadyl and copper(n) ions catalyse the /J-elimination reaction of O-phospho-threonine in the presence of pyridoxal.429 Equilibrium spectroscopic studies of the threonine-metal ion-pyridoxal system have identified a metal-ion complex of the amino-acid-pyridoxal Schiff base. The catalytic effect of the metal is ascribed to its electron-with drawing effecCIt was suggested that the specific catalytic effect of Cu2 + and V02+ arises from their reluctance to co-ordinate the phosphate in an axial position. Other metal ions such as nickel can also form the Schiff base complex but probably stabilize the phosphothreonine system by chelate formation. 

Pyridoxal phosphate is the coenzyme in a large number of amino acid reactions. At this point it is convenient to consider together 1,he mechanism of those pyridoxal-dependent reactions concerned with aromatic amino acids. The reactions concerned are (1) keto acid formation (e.g., from kynurenine, above), 2) decarboxylation (e.g., of 5-hydroxytrypto-phan to 5-hydroxytryptamine, p. 106), (3) scission of the side claain (e.g., 3-tyrosinase, p. 78 tryptophanase, p. 110 and kynureninase, above), and 4) synthesis (e.g., of tryptophan from indole and serine, p. 40). Many workers have considered the mechanism of one or more of these reactions (e.g., 24, 216, 361, 595), but a unified theory is primarily due to Snell and his colleagues (summarized in 593). Snell s experiments have been carried out largely in vitro, and it should be emphasized that in vivo it is the enzyme protein which probably directs the electromeric changes. 

The dehydro amino acid esters react with various aldehydes, including pyri-doxal, to form imines. These imines (4) are believed to be involved in several reactions of pyridoxal-containing enzymes. ... 

Scheme 12.92. A representation of the conversion of 1,7-heptanedioic acid ([H02C(CH2)5C02H], pimeUc acid) (pimeloyl-SCoA) to 7-keto-8-aminonanoate (7-keto-8-aminopellargonic acid) on reaction with alanine (Ala, A) in a pyridoxal phosphate-catalyzed process (EC 2.3.1.4). Transamination (pyridoxal) with S-adenosyhnethionine deUvering the amino group to form 7,8-diaminonanoate (7,8-diaminoplargonic acid, EC 2.6.1.62) and S-adenosyl-4-methylthio-2-oxobutanoate.

P-Hydroxy-a-amino Acids. Another reaction of glycine is catalyzed by pyridoxal. This is the reversible condensation with aldehydes. Glycine and formaldehyde form serine in a model of the reaction observed in biological systems to proceed with tetrahydrofolic acid as a formaldehyde acceptor and donor. When glyoxylate reacts with pyridoxamine, the gly-... 

Non-enzymic Decarboxylation of Amino Acids Pyridoxal phosphate is a co-enzyme for numerous enzymes, notably amino acid decarboxylases, amino acid transaminases, histaminase and probably diamine oxidase - -. 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. 

An example of a biologically important aldehyde is pyridoxal phosphate, which is the active form of vitamin Bg and a coenzyme for many of the reactions of a-amino acids. In these reactions the amino acid binds to the coenzyme by reacting with it to form an imine of the kind shown in the equation. Reactions then take place at the amino acid portion of the imine, modifying the amino acid. In the last step, enzyme-catalyzed hydrolysis cleaves the imine to pyridoxal and the modified amino acid. 

FIGURE 18.27 Pyridoxal-5-phosphate forms stable Schiff base adducts with amino acids and acts as an effective electron sink to stabilize a variety of reaction intermediates. 

The amino acid methionine is biosynthesized by a multistep roule that includes reaction of an inline of pyridoxal phosphate (PLP) to give an unsaturated imine. which then reacts with cysteine. What kinds of reactions are occurring in the two steps ... 

The mechanism of the first part of transamination is shown in Figure 29.14. The process begins with reaction between the a-amino acid and pyridoxal phosphate, which is covalently bonded to the aminotransferase by an iminc linkage between the side-chain -NTI2 group of a lysine residue and the PLP aldehyde group. Deprotonation/reprotonation of the PLP-amino acid imine in steps 2 and 3 effects tautomerization of the imine C=N bond, and hydrolysis of the tautomerized imine in step 4 gives an -keto acid plus pyridoxamine... 

Another interesting example is SHMT. This enzyme catalyzes decarboxylation of a-amino-a-methylmalonate with the aid of pyridoxal-5 -phosphate (PLP). This is an unique enzyme in that it promotes various types of reactions of a-amino acids. It promotes aldol/retro-aldol type reactions and transamination reaction in addition to decarboxylation reaction. Although the types of apparent reactions are different, the common point of these reactions is the formation of a complex with PLP. In addition, the initial step of each reaction is the decomposition of the Schiff base formed between the substrate and pyridoxal coenzyme (Fig. 7-3). 

Identification of pyridoxal phosphate as coenzyme suggested the aldehyde group on pyridoxine might form an intermediate Schiff s base with the donor amino acid. Pyridoxamine phosphate thus formed would in turn donate its NH2 group to the accepting a-ketonic acid, a scheme proposed by Schlenk and Fisher. 15N-labeling experiments and, later, the detection of the Schiff s base by its absorption in UV, confirmed the overall mechanism. Free pyridoxamine phosphate however does not participate in the reaction as originally proposed. Pyridoxal phosphate is invariably the coenzyme form of pyridoxine. 

Pyridoxal phosphate is a required coenzyme for many enzyme-catalyzed reactions. Most of these reactions are associated with the metabolism of amino acids, including the decarboxylation reactions involved in the synthesis of the neurotransmitters dopamine and serotonin. In addition, pyridoxal phosphate is required for a key step in the synthesis of porphyrins, including the heme group that is an essential player in the transport of molecular oxygen by hemoglobin. Finally, pyridoxal phosphate-dependent reactions link amino acid metabolism to the citric acid cycle (chapter 16). 

Pyridoxal phosphate (4) is the most important coenzyme in amino acid metabolism. Its role in transamination reactions is discussed in detail on p. 178. Pyridoxal phosphate is also involved in other reactions involving amino acids, such as decarboxylations and dehydrations. The aldehyde form of pyridoxal phosphate shown here (left) is not generally found in free form. In the absence of substrates, the aldehyde group is covalently bound to the e-amino group of a lysine residue as aldimine ( Schiffs base ). Pyridoxamine phosphate (right) is an intermediate of transamination reactions. It reverts to the aldehyde form by reacting with 2-oxoacids (see p. 178). 

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 2.6.1.57] 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. 

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