2-Acetylthiamine pyrophosphate

Enolization of cationic ketones is accelerated by electrostatic stabilization of the enolate anion. Rate constants for water-, acetate-, and hydroxide ion-catalysed enolization of 2-acetyl- 1-methylpyridinium ion (94) have been measured13811 and compared with a 2-acetylthiazolium ion (95), a simple analogue of 2-acetylthiamine pyrophosphate.13811 For (94), qh = 1.9 x 102 M-1 s 1, about 1.1 x 106 times that for a typical methyl ketone such as acetone. Thermodynamically, it is >108 times more acidic (pAa values of 11.1 vs 19.3). These increases in kinetic and thermodynamic acidity are derived from through-bond and through-space effects, and the implications for enzymatic catalytic sites with proximal, protonatable nitrogen are discussed. The results for (94) suggest a pAa value of 8.8 for (95), a value that cannot be measured directly due to competing hydrolysis. 

EnoUzation of cationic ketones is accelerated by electrostatic stabilization of the enolate anion. Rate constants for water-, acetate-, and hydroxide ion-catalysed enolization of 2-acetyl-1-methylpyridinium ion (94) have been measured and compared with a 2-acetylthiazolium ion (95), a simple analogue of 2-acetylthiamine pyrophosphate. For (94), = 1-9 x 10 M s , about 1.1 x 10 times that for... 

These observations suggest that 2-acetylthiamine pyrophosphate is an intermediate in the oxidative decarboxylation of pyruvate involving the... 

That 2-acetylthiamine pyrophosphate is an intermediate in the oxidative decarboxylation of pyruvate may also be. inferred from (a) the demonstration by Goedde et al. (1961) that the hydroxyethyl group of 2-hydroxyethyl-thiamine pyrophosphate can be converted to acetyl CoA in the presence of a pyruvate oxidation system from yeast mitochondria and (b) the demonstration by Krampitz et al. (1961) that the hydroxyethyl group of 2-hydrox-yethylthiamine pyrophosphate is oxidized to acetate in the presence of fcrricyanide and the acetoin-forming system from A. aerogenes. 

The mechanism of the clastic cleavages of pyruvate, producing acetyl phosphate and formate or CO2 and Ha, is still obscure. Biotin (Schuster and Lynen, 1960), folic acid (Delavier-Klutchko, 1959), and vitamin B12 derivatives (Rabinowitz, 1960) have recently been implicated in these reactions. It seems possible that these may also be examples of tightly coupled systems in which 2-acetylthiamine pyrophosphate is an intermediate. 

Acetylthiamine pjrrophosphate appears to be yet another form of active acetate. It has been assigned a key role in the lipoic acid-Unked oxidative decarboxylation of pyruvate as the primary product of the oxidation of active acetaldehyde, i.e., 2-hydroxyethylthiamine pyrophosphate. It has been proposed that 2-acetylthiamine pyrophosphate is an intermediate in all oxidative transformations of pyruvate and that 2-succinylthiamine pyrophosphate plays a similar role in oxidation of a-ketoglutarate. Further evaluation of this proposal is anticipated in the near future. 

The decarboxylation of pyruvic acid is an example of a more general type of biochemical reaction the decarboxylation of a-keto acids. The reaction is complex and occurs in several consecutive steps. The intermediates have been identified, but little is known of the enzymes involved. The reaction starts with the complexion of pyruvic acid with one molecule of enzyme-bound thiamine pyrophosphate. This is followed by decarboxylation of pyruvic acid and the formation of an intermediate, 2-acetylthiamine pyrophosphate, in which the aldehyde carbon of the acetyl is bound to the carbon 2 of the thiozole ring of the thiamine pyrophosphate. In the second step, the aldehyde is oxidized, the disulfide bond of enzyme-bound lipoic acid is reduced, and the free enzyme-bound thiamine pyrophosphate is restored. The tWrd step of the reaction involves the transacylation from reduced lipoic acid to CoA. Finally, lipoic acid is reoxidized by the catalytic activity of an NAD-dependent flavoprotein, lipoic dehydrogenase (see Fig. 1-14). 

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