Pyridoxal phosphate imines

An example of a biologically important aide hyde is pyridoxal phosphate which is the active form of vitamin Bg and a coenzyme for many of the reac tions of a ammo acids In these reactions the ammo acid binds to the coenzyme by reacting with it to form an imine of the kind shown in the equation Re actions then take place at the ammo acid portion of the imine modifying the ammo acid In the last step enzyme catalyzed hydrolysis cleaves the imme to pyridoxal and the modified ammo acid... 

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... 

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. 

The following are topics that may be especially valuable to the student and which might be read initially in Chapter 12, lysozyme (Section B,5), chymo-trypsin (Section C,l), kinases (Section D,9), multiple displacement, reactions (Section G) in Chapter 13, imines (Section A,2), addition to C=C bonds (Section A, 4,5), beta cleavage and condensation (Section C) in Chapter 14, thiamin diphosphate (Section D), pyridoxal phosphate (Section E) in Chapter 15, NAD (Section A). 

Tissues of the mammalian central nervous system contain a pyridoxal phosphate-dependent glutamate decarboxylase that catalyzes conversion of Glu to y-aminobutyrate (GABA), an inhibitory synaptic transmitter. GABA is degraded by trans-imination with a-oxoglutarate as the acceptor to yield succinic semialdehyde, which then is oxidized to succinate by an NAD-linked dehydrogenase. 

There is an important biochemical counterpart of the deamination reaction that utilizes pyridoxal phosphate, 7, as the aldehyde. Each step in the sequence is catalyzed by a specific enzyme. The a-amino group of the amino acid combines with 7 and is converted to a keto acid. The resulting pyridoxamine then reacts to form an imine with a different a-keto acid, resulting in formation of a new a-amino acid and regenerating 7. The overall process is shown in Equation 25-6 and is called transamination. It is a key part of the process whereby amino acids are metabolized. 

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... 

Pyridoxal phosphate forms a Schiff base (imine) with the glycine. A carbon-bound hydrogen is labile, and the resulting carbanion stabilized by resonance back into the pyridoxal phosphate. The carbanion approaches the carbonyl carbon of the succinyl-CoA. Following the elimination of the CoASH, the intermediate shown in figure 22.13 is formed. The intermediate then loses a C02, forming a carbanion that is resonance stabilized back into the pyridoxal phosphate. 

The glycine-dependent aldolases contain a cofactor pyridoxal phosphate (PLP). Binding of glycine to it as an imine enables the deprotonation necessary for the carbon-carbon bond forming reaction, with pyridine acting as an electron sink. The subsequent 100% atom efficient reaction with an aldehyde establishes the new bond and two new stereocenters (Scheme 5.30). Of all the glycine-dependent aldolases only L-threonine aldolase (LTA) is commonly used [40, 43, 52]. 

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) ... 

Imines are particularly common as intermediates in many biological pathways, where they arc often called Schiff bases. The amino acid alanine, for instance, is metabolized in the body by reaction with the aldehyde pyridoxal phosphate (PLP), a derivative of vitamin Bg, to yield a Schiff base that is further degraded. 

Hydrolysis of the imine then yields an a-keto acid along with a nitrogen-containing pyridoxal phosphate derivative. 

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