Transamination mechanisms

Lee, Y., and Sayre, L. M., 1995, Model studies on the quinone-containing copper amine oxidases unambiguous demonstration of a transamination mechanism, J. Amer. Chem. Soc. 117 11823nll828. 

Figure 23.10. Transamination Mechanism. The external aldimine loses a proton to form a quinonoid intermediate. Reprotonation of this intermediate at the aldehyde carbon atom yields a ketimine. This intermediate is hydrolyzed to generate the a-ketoacid product and pyridoxamine phosphate.

The basic transamination mechanism (pp. 1384-7) can be drawn for both tyrosine and DOPA by writing Ar for either benzene ring. 

Although mechanistic studies have not been carried out, the reaction is likely to follow the standard transamination mechanism (Fig. 28) [126,127]. 

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. 

Copper amine oxidase (CAO) enzymes carry out the aerobic oxidation of primary amines to aldehydes (Scheme 14.8a). While copper is present in the active site, substrate oxidation proceeds by an organocatalytic pathway involving an o-quinone cofactor via a transamination mechanism (Scheme 14.8b). 

In transamination an amine group is transferred from L glutamic acid to pyruvic acid An outline of the mechanism of transamination is presented m Figure 27 4... 

FIGURE 27.4 The mechanism of transamination. All the steps are enzyme-catalyzed. 

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. 

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... 

Figure 29 14 MECHANISM Mechanism of the enzyme-catalyzed, PLP-dependent transamination of an a-amino acid to give an a-keto acid. Individual steps are explained in the text.

Figure 29.15 Mechanism of steps 2-4 of amino acid transamination, the conversion of a PLP-amino acid imine to PMP and an a-keto acid.

Toxicity, chemicals and, 25-26 Trans fatty acid, from hydrogenation of fats, 232-233 from vegetable oils, 1063 Transamination, 1165-1168 mechanism of, 1167... 

Figure 7-4. Ping-pong mechanism for transamination. E—CHO and E—CHjNHj represent the enzyme-pyridoxal phosphate and enzyme-pyridoxamine complexes, respectively. (Ala, alanine Pyr, pyruvate KG, a-ketoglutarate Glu, glutamate).

The deamination of primary amines such as phenylethylamine by Escherichia coli (Cooper et al. 1992) and Klebsiella oxytoca (Flacisalihoglu et al. 1997) is carried out by an oxidase. This contains copper and topaquinone (TPQ), which is produced from tyrosine by dioxygenation. TPQ is reduced to an aminoquinol that in the form of a Cu(l) radical reacts with O2 to form H2O2, Cu(ll), and the imine. The mechanism has been elucidated (Wihnot et al. 1999), and involves formation of a Schiff base followed by hydrolysis in reactions that are formally analogous to those involved in pyridoxal-mediated transamination. 

Glutamate is a commonly occurring amino acid that acts as an excitatory transmitter in CNS. The molecule may be synthesized within the nerve ending either by transamination from 2-oxoglutarate (described in Section 6.3.1.1) or by deamination of glutamine (see Section 8.2.2). However, in common with other synaptic signals, there exists an efficient uptake mechanism in the axon to recycle glutamate that has been released. 

The Schiff base can undergo a variety of reactions in addition to transamination, shown in Fig. 6.4 for example, racemization of the amino acid via the a-deprotonated intermediate. Many of these reactions are catalyzed by metal ions and each has its equivalent nonmetallic enzyme reaction, each enzyme containing pyridoxal phosphate as a coenzyme. Many ideas of the mechanism of the action of these enzymes are based on the behavior of the model metal complexes. 

During the degradation of most amino acids, the a-amino group is initially removed by transamination or deamination. Various mechanisms are available for this, and these are discussed in greater detail in B. The carbon skeletons that are left over after deamination undergo further degradation in various ways. 

In the branched-chain amino acids (Val, Leu, He) and also tyrosine and ornithine, degradation starts with a transamination. For alanine and aspartate, this is actually the only degradation step. The mechanism of transamination is discussed in detail on p. 178. 

---