Pyridoxal phosphate, amino acid imines from

Pyridoxal 5 -phosphate is also a coenzyme for the enzyme-catalyzed racemization of amino acids. The key reaction is proton abstraction from the a carbon of the amino acid imine of PLP. This step converts the a carbon, which is a chirality center, from sp to sp. ... 

NMR studies have been carried out on Schiff bases derived from pyridoxal phosphate and amino acids, since they have been proposed as intermediates in many important biological reactions such as transamination, decarboxylation, etc.90 The pK.d values of a series of Schiff bases derived from pyridoxal phosphate and a-amino adds, most of which are fluorinated (Figure 11), have been derived from H and19F titration curves.91 The imine N atom was found to be more basic and more sensitive to the electron-withdrawing effect of fluorine than the pyridine N atom. Pyridoxal and its phosphate derivative are shown in Figure 12a. The Schiff base formation by condensation of both with octopamine (Figure 12b) in water or methanol solution was studied by 13C NMR. The enolimine form is favoured in methanol, while the ketoamine form predominates in water.92... 

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. 

Thiamine pyrophosphate (TPP) is the coenzyme required by enzymes that catalyze the transfer of a two-carbon fragment. Biotin is the coenzyme required by enzymes that catalyze carboxylation of a carbon adjacent to a carbonyl group. Pyridoxal phosphate (PLP) is the coenzyme required by enzymes that catalyze certain transformations of amino acids decarboxylation, transamination, racemiza-tion, C —Cp bond cleavage, and a,j8-elimination. In a transimination reaction, one imine is converted into another imine in a transamination reaction, the amino group is removed from a substrate and transferred to another molecule. 

Recall from Section 20.2 that pyridoxal 5 -phosphate reacts with the a amino group of an a-amino acid to form an imine, or Schiff base. When L-dopa reacts with PLP, the resultant imine undergoes decarboxylation, with the pyridinium ion of PLP acting as the electron acceptor. Hydrolysis then gives dopamine and regenerated PLP. The mechanism is shown in Figure 25.7. 

While it may be surprising that the above diverse reactions require the same cofactor, this will be readily understood when it is realized that these reactions have certain common features. All require imine (Schiff base) formation between the aldehyde carbonyl of the cofactor and the amino group of the substrate. The pyridoxal phosphate becomes an electrophilic catalyst or electron sink, as electrons may be delocalized from the amino acid into the ring structure. It is the direction of this delocalization that dictates the reaction type and in model systems more than one reaction pathway is often observed. Thus the enzyme both enhances the rate of reaction and gives direction to that reaction (see page 428). 

Although it is superficially attractive to write a mechanism that consists of formation of an imine, tautomerization, and hydrolysis, this is not what happens nor does it explain the need for pyridoxal phosphate in the reaction. The pyridoxal phosphate is bonded to a lysine of the enzyme as an imine (14.25). In the transamination reaction, another free amino acid displaces the lysine that is part of the enzyme from pyridoxal, in an imine exchange reaction (Figure 14.33). 

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