Pyridoxal phosphate imine formation from

The synthesis pathway of quinolizidine alkaloids is based on lysine conversion by enzymatic activity to cadaverine in exactly the same way as in the case of piperidine alkaloids. Certainly, in the relatively rich literature which attempts to explain quinolizidine alkaloid synthesis °, there are different experimental variants of this conversion. According to new experimental data, the conversion is achieved by coenzyme PLP (pyridoxal phosphate) activity, when the lysine is CO2 reduced. From cadeverine, via the activity of the diamine oxidase, Schiff base formation and four minor reactions (Aldol-type reaction, hydrolysis of imine to aldehyde/amine, oxidative reaction and again Schiff base formation), the pathway is divided into two directions. The subway synthesizes (—)-lupinine by two reductive steps, and the main synthesis stream goes via the Schiff base formation and coupling to the compound substrate, from which again the synthetic pathway divides to form (+)-lupanine synthesis and (—)-sparteine synthesis. From (—)-sparteine, the route by conversion to (+)-cytisine synthesis is open (Figure 51). Cytisine is an alkaloid with the pyridone nucleus. 

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 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) ... 

The reversible formation of imines from amino groups and carbonyls is the key mechanistic step in transamination reactions. To create imines, all transaminases require pyridoxal phosphate (FTP), a coenzyme derived from pyridoxine (vitamin Bg). This coenzyme... 

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). 

There are several enzymes that form a Schiff base between their substrates and either a lysine -amino group from the protein, the aldehyde electrophile of pyridoxal-5-phosphate or the keto group of a covalently bound pyruvyl moiety. The electron sink under all these conditions is either the imine or its protonated iminium form, that in many cases leads to the formation of an enamine by the variety of pathways outlined below. 

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