Thiamine pyrophosphate action, mechanism

Mechanism of thiamine pyrophosphate action. Intermediate (a) is represented as a resonance-stabilized species. It arises from the decarboxylation of the pyruvate-thiamine pyrophosphate addition compound shown at the left of (a) and in equation (2). It can react as a carbanion with acetaldehyde, pyruvate, or H+ to form (b), (c), or (d), depending on the specificity of the enzyme. It can also be oxidized to acetyl-thiamine pyrophosphate (TPP) (e) by other enzymes, such as pyruvate oxidase. The intermediates (b) through (e) are further transformed to the products shown by the actions of specific enzymes. 

Fig. 4. Mechanism of thiamine pyrophosphate action in the fcrricyanide-linked oxidative decarboxylation of pyruvate (Das et al., 1901). The decarbo.xylation process is illustrated in Fig. 2.

Based on the action of thiamine pyrophosphate in catalysis of the pyruvate dehydrogenase reaction, suggest a suitable chemical mechanism for the pyruvate decarboxylase reaction in yeast ... 

The next coenzyme for which a mechanism was established was thiamin pyrophosphate [3]. Ronald Breslow used nmr spectroscopy to show that the hydrogen atom at C-2 of a thiazolium salt rapidly exchanges with deuterium in even slightly alkaline solutions (6), so that the coenzyme offers an anionic centre for catalysis (Breslow, 1957). With this established, Breslow could confidently offer the pathway shown in Scheme 2 for the action of the... 

Mechanism of action of the pyruvate dehydrogenase complex. TPP = thiamine pyrophosphate L = lipoic acid. 

The coenzyme thiamine pyrophosphate (1) plays a central role in many parts of metabolism (Fig. 1). Its mechanism of action involves the formation of a thiazolium zwitterion 2 that was stabilized by a carbene resonance form 3 (10). This discovery opened up studies of the chemistry that such stabilized carbenes could catalyze, as chemists realized that the otherwise impossible chemistry that thiamine pyrophosphate catalyzes in nature could be generalized and adapted for useful synthetic processes. 

The C-2-exchange of azolium salts via an ylide mechanism was discussed in Section 24.1.2.1. Thiamin pyrophosphate acts as a coenzyme in several biochemical processes and in these, its mode of action depends on the intermediacy of a 2-deprotonated species (32.2.4). In the laboratory, thiazolium salts (3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride is commercially available) will act as catalysts for the benzoin condensation, and in contrast to cyanide, the classical catalyst, allow such reactions to proceed with alkanals, as opposed to araldehydes the key steps in thiazolium ion catalysis for the synthesis of 2-hydroxy-ketones are shown below and depend on the formation and nucleophilic reactivity of the C-2-ylide. Such catalysis provides acyl-anion equivalents. 

The enyzyme uses thiamine pyrophosphate as a coenzyme (Figure 14.14) and is very similar in mechanism of action to the pyruvate dehydrogenase complex (Figure 14.10 ... 

In addition to its role in the action of pyruvate dehydrogenase, thiamine pyrophosphate (TPP) serves as a cofactor for other enzymes, such as pyruvate decarboxylase, which catalyzes the nonoxidative decarboxylation of pyruvate. Propose a mechanism for the reaction catalyzed by pyruvate decarboxylase. What product would you expect Why, in contrast to pyruvate dehydrogenase, are lipoamide and FAD not needed as cofactors for pyruvate decarboxylase ... 

The C-2-exchange of azolium salts via an ylid mechanism has already been discussed (section 21.1.2.1). Thiamin pyrophosphate acts as a coenzyme in several biochemical processes and in these its mode of action also depends on the intermediacy of a 2-deprotonated species. For example, in the later stages of alcoholic fermentation, which converts glucose into ethanol and carbon dioxide, the enzyme pyruvate decarboxylase converts pyruvate into ethanal and carbon dioxide, the former then being converted into ethanol by the enzyme alcohol dehydrogenase. It is believed that, in the operation of the former enzyme, the coenzyme, thiamin pyrophosphate, adds as its ylid to the ketonic carbonyl group of pyruvate this is followed by loss of carbon dioxide, then the release of ethanal by expulsion of the original ylid. 

Mechanism of action of transketolase (only the thiazole group of thiamin pyrophosphate is shown). 

In view of the fact that lipoic acid is a cyclic disulfide, it is tempting to speculate on its function as a 2-carbon carrier in pyruvate oxidation through a thioester in a fashion similar to the CoA fimction in this reaction. Similarly the disulfide nature of this factor invites speculation on its possible role as an oxidation-reduction coenzyme by going through a sulfhydryl form. It is a curious fact that three of the factors involved in pyruvate oxidation, thiamine pyrophosphate, CoA, and POF, all contain sulfur. In the first two of these factors, the sulfur appears to play an important role in their mechanism of action. By analogy one would suppose that the disulfide grouping of POF will be prominent in its mechanism of action. 

The mechanism of action of cocarboxylase is still very obscure. The suggestion has been offered that the thiol and disulfide forms of thiamine might constitute an oxidation-reduction system. Subsequently, it has been shown in enzyme experiments with yeast that the isomeric thiazole pyrophosphate exhibited full cocarboxylase activity, whereas the disulfide... 

---