Transamination reaction mechanism

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. 

Christen, P., and D. E. Metzler, (eds.), Transaminases. New York John Wiley and Sons, 1985. A series of review chapters describing in detail the scope and mechanisms of transamination reactions. 

Pyrazines are formed from transamination reactions, in addition to carbon dioxide and formaldehyde. A requirement is that the carbonyl compound contains a dione and the amino group is alpha to the carboxyl group (16). If the hydrogen on the ct-carbon oI the amino acid is substituted, a ketone is produced. Newell (17) initially proposed a pyrazine formation mechanism between sugar and amino acid precursors. (See Figure 3). The Schiff base cation is formed by addition of the amino acid to the anomeric portion of the aldo-hexose, with subsequent losses of vater and a hydroxyl ion. Decarboxylation forms an imine which can hydrolyze to an aldehyde and a dienamine. Enolization yields a ketoamine, vhich dissociates to amino acetone and glyceraldehyde. 2,5-Dimethylpyrazine is formed by the condensation of the tvo molecules of amino acetone. 

A further method to induce chirality in the pyridoxamine-mediated transamination reactions was developed by Kuzuhara et al. [13]. They synthesized optically resolved pyridinophanes (21, 22) having a nonbranched ansa chain" between the 2 - and 5 -positions of pyridoxamine. With the five-carbon chain in 21 and 22, the two isomers do not interconvert readily. In the presence of zinc(n) in organic solvents such as methanol, tert-butanol, acetonitrile, and nitromethane, they observed stereoselective transamination between pyridinophanes and keto acids. The highest ee%s are 95 % for d-and L-leucine by reaction of the corresponding a-keto acid with (S)- and (R)- 22, respectively. On the basis of kinetic analysis of the transamination reactions, Kuzuhara et al. originally proposed a mechanism for the asymmetric induction through kinetically controlled stereoselective protonation to the carboanion attached to an octahedral Zn(n) chelate intermediate. However, they subsequently raised some questions about this proposal [14]. 

Figure 2.2 Proposed mechanism for enantioselective transamination reaction catalyzed by Murakami s bilayer membrane systems. (Reprinted with permission from Ref 27. Copyright 1995 American Chemical Society.)...

Figure 5.10 Abbreviated mechanism for transamination reactions promoted by pyridoxal-based catalysts.

Different molecular mechanisms have been separately postulated for dibromo-phakellin [74], dibromoagelaspongin [91], agelastatin [92], mauritiamine [88], and palau amine [78]. A1 Mourabit and Potier proposed a universal chemical pathway, starting from the simple precursors 101 and 140 and leading to over 60 pyrrole-imidazole alkaloids [80]. A new biomimetic spontaneous conversion of proline to 2-aminoimidazolinone derivatives using a self-catalyzed intramolecular transamination reaction together with peroxide dismutation as key step has been described [166]. This work has pointed to dispacamide A as the forerunner of oroidin and compounds 101 and 140 as probable hydrolysis products of oroidin and not the precursors. In this... 

The interest in the mechanisms of SchifF base hydrolysis stems largely from the fact that the formation and decomposition of SchifF base linkages play an important role in a variety of enzymatic reactions, for example, carbonyl transfers involving pyridoxal phosphate, aldol condensations, /3-decarboxylations and transaminations. The mechanisms for the formation and hydrolysis of biologically important SchifF bases, and imine intermediates, have been discussed by Bruice and Benkovic (1966) and by Jencks (1969). As the consequence of a number of studies (Jencks, 1959 Cordes and Jencks, 1962, 1963 Reeves, 1962 Koehler et al., 1964), the mechanisms for the hydrolysis of comparatively simple SchifF bases are reasonably well understood. From the results of a comprehensive kinetic investigation, the mechanisms for the hydrolysis of m- and p-substituted benzylidine-l,l-dimethylethylamines in the entire pH range (see, for example, the open circles in Fig. 13) have been discussed in terms of equations (23-26) (Cordes and Jencks, 1963) ... 

These reactions are carried out by pyridoxalphosphate-dependent transaminases. Transamination reactions participate in the synthesis of most amino acids. We shall review the transaminase mechanism (Section 23.3.1) as it applies to amino acid biosynthesis (see Figure 23.10). 

The reaction of naphthols with ammonia and sodium bisulfite is called the Bucherer reaction. Primary amines can be used instead of ammonia, in which case A-substituted naphthylamines are obtained. In addition, primary naphthyla-mines can be converted to secondary (ArNH2 + RNH2 + NaS03 ArNHR), by a transamination reaction. The mechanism of the Bucherer reaction amounts to a kind of overall addition-elimination, via 18 and... 

The normal mechanism for the transamination reaction is shown in Fig. 4.24 (R=H) and involves the condensation of alanine and pyridoxal phosphate to give an imine. A proton is lost from the imine to give a dihydropyridine intermediate. This reaction is catalysed by a basic amino acid provided by the enzyme as well as the electron withdrawing effects of the protonated pyridine ring. The dihydropyridine structure now formed is hydrolysed to give the products. 

Despite the apparent simplicity of the transamination reaction, the mechanism is quite complex. The reaction begins with the formation of a Schiff base between PLP and the a-amino group of an a-amino acid (Figure 14.3). When the a-hydro-gen atom is removed by a general base in the enzyme active site, a resonance-stabilized intermediate forms. With the donation of a proton from a general acid and a subsequent hydrolysis, the newly formed a-keto acid is released from the enzyme. A second a-keto acid then enters the active site and is converted into an a-amino acid in a reversal of the reaction process that has just been described. Transamination reactions are examples of a reaction mechanism referred to as a... 

The postulated catalytic mechanism of amine oxidases starts from the qui-none form of the cofactor (Fig. 17). The distal oxygen atom is replaced by an amino group in a transamination reaction. The amine is then re-oxidized by molecular oxygen to the original quinone. The copper ion is not involved directly in catalysis but is only a cofactor in the synthesis of TOPA quinone (Fig. 18). 

The crucial feature of the transamination mechanism is a tautomerization reaction, or 1,3 prototropic shift, in which a proton is transferred from the substrate carbon, which is directly bound to the amino group to the C4 of the cofactor (Figure 19). In this process, the external aldimine of the first amino acid substrate is isomerized into a ketimine, which is then hydrolyzed, liberating the related oxo acid product and the enzyme with the cofactor in the PMP form. The second half transamination reaction takes place via a reverse mechanism, starting from the enzyme in the PMP form and the second oxo acid substrate. 

Figure 19 Mechanism of a half transamination reaction involving a generic amino acid with side chain R.

The above general mechanism for non-enzymic, pyridoxal-catalysed processes was derived mainly from a study of transamination reactions. Nevertheless, non-enzymic, pyridoxal-catalysed decarboxylations have been reported, for example, that of histidine to histamine. The following pyridoxal-catalysed, non-enzymic decarboxylations of a-aminoisobutyric acid (R = = GHg), a-methylserine (R = GHjOH R = GHg) and... 

Fig. 12. Mechanism of a decarboxylative transamination reaction involving a-aminoisobutyrate and pyruvate as substrates.

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. 

The fact that the isopeptides could be formed as a consequence of thermal treatment of proteins was first demonstrated by Asquith and Otterburn who also showed that the isopeptides had crosslinking functions and that their concentrations increased with increasing severity of heating (40). The mechanism of formation could either be via a condensation or transamination reaction depending on whether the free carboxylic acid or the amide is involved (Figure 1) (20). 

Pong Bi Bi reaction mechanism. As they contain PLP, the mechanism is almost certainly similar to that known for the animal aminotransferases (Fig. 1). Details of this mechanism are discussed by Braunstein (1973) and by Metzler (1977). The apoenzyme moiety determines substrate specificity and confers high catalytic efficiency, as well as suppressing side reactions and eliminating the metal requirement characteristic of nonenzymatic transamination. Initially the amino acid binds to an anchoring site on the enzyme. Condensation then takes place between the amino acid and the enzyme pyridoxal-lysine imine to form an aldimine. Following further rearrangements, a ketimine is produced. Ketimine formation is then followed by a hydrolysis to... 

---