Pyruvic acid nonoxidative decarboxylation

The a-keto acid decarboxylases such as pyruvate (E.C. 4.1.1.1) and benzoyl formate (E.C. 4.1.1.7) decarboxylases are a thiamine pyrophosphate (TPP)-dependent group of enzymes, which in addition to nonoxidatively decarboxylating their substrates, catalyze a carboligation reaction forming a C-C bond leading to the formation of a-hydroxy ketones.269-270 The hydroxy ketone (R)-phenylacetylcarbinol (55), a precursor to L-ephedrine (56), has been synthesized with pyruvate decarboxylase (Scheme 19.35). BASF scientists have made mutations in the pyruvate decarboxylase from Zymomonas mobilis to make the enzyme more resistant than the wild-type enzyme to inactivation by acetaldehyde for the preparation of chiral phenylacetylcarbinols.271... 

After biochemical conversion of glucose to pyruvic acid intermediate, the next step in ethanol synthesis is nonoxidative decarboxylation and acetaldehyde formation catalyzed by a native decarboxylase, and then acetaldehyde reduction to ethanol catalyzed by a native dehydrogenase. 

The structure of cocarboxylase (see Fig. 1 for formula) was established by Lohmann and Schuster. A long-established function of cocarboxylase (thiamine pyrophosphate, aneurin) is as the coenzyme of a-ketoaeid carboxylase. Mg++ is also required. The reaction involved is the nonoxidative decarboxylation of an a-keto acid to CO2 and an aldehyde with one less carbon atom. The most important example is the splitting of pyruvic acid to acetaldehyde and CO2 (equation 9). This... 

The coenzyme participates in reactions involving formation and breaking of carbon-carbon bonds immediately adjacent to a carbonyl group. Examples include nonoxidative and oxidative decarboxylations and aldol condensations. For instance it is involved in the nonoxidative decarboxylation of pyruvic acid to acetaldelyde ... 

Pyruvate decarboxylase is able to catalyze two different reactions the nonoxidative decarboxylation of a-keto acids to the corresponding aldehydes [10,15-17] and a car-boxyligase side reaction leading to the formation of hydroxy ketones [18,19]. An understanding of why the last reaction is catalyzed by pyruvate decarboxylase, the physiological role of which is to decarboxylate pyruvate to acetaldehyde, was revealed by the discovery that pyruvate decarboxylase is homologous with acetolactate synthase [20], the enzyme catalyzing an acyloin condensation in the first step of isoleucine-valine biosynthesis. 

The simplest example of such reactions is the decarboxylation of pyruvate. Both model and enzyme studies have shown the intermediacy of covalent complexes formed between the cofactor and the substrate. Kluger and coworkers have studied extensively the chemical and enzymatic behavior of the pyruvate and acetaldehyde complexes of ThDP (2-lactyl or LThDP, and 2-hydroxyethylThDP or HEThDP, respectively) . As Scheme 1 indicates, the coenzyme catalyzes both nonoxidative and oxidative pathways of pyruvate decarboxylation. The latter reactions are of immense consequence in human physiology. While the oxidation is a complex process, requiring an oxidizing agent (lipoic acid in the a-keto acid dehydrogenases , or flavin adenine dinucleotide, FAD or nicotinamide adenine dinucleotide , NAD " in the a-keto acid oxidases and Fe4.S4 in the pyruvate-ferredoxin oxidoreductase ) in addition to ThDP, it is generally accepted that the enamine is the substrate for the oxidation reactions. 

Most of ThDP-dependent enzymes catalyse the processing of 2-keto acids such as pyruvate, branched-chain keto acids and ketoglutarate. Among them, pyruvate is processed by various ThDP-dependent enzymes (Figure 4.2). Its carbonyl group is attacked by the ThDP ylide, yielding a tetrahedral 2-(2-lactyl)-ThDP adduct. The reactions followed by decarboxylation can be roughly classified into oxidative or nonoxidative ones. 

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