Pyruvate decarboxylase mechanism

The thiazolium ion then behaves as an electron sink or electrophile and decarboxylation follows. The enolic intermediate, on the other hand, acts as a nucleophile which can be protonated. This intermediate has been isolated. Finally, acetaldelyde is formed and the coenzyme (ylid form) is regenerated at the same time. The liberation of acetaldelyde is the rate-limiting step in the pyruvate decarboxylase mechanism. 

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 ... 

Figure 2. Mechanism of PDH. The three different subunits of the PDH complex in the mitochondrial matrix (E, pyruvate decarboxylase E2, dihydrolipoamide acyltrans-ferase Ej, dihydrolipoamide dehydrogenase) catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA and CO2. E, decarboxylates pyruvate and transfers the acetyl-group to lipoamide. Lipoamide is linked to the group of a lysine residue to E2 to form a flexible chain which rotates between the active sites of E, E2, and E3. E2 then transfers the acetyl-group from lipoamide to CoASH leaving the lipoamide in the reduced form. This in turn is oxidized by E3, which is an NAD-dependent (low potential) flavoprotein, completing the catalytic cycle. PDH activity is controlled in two ways by product inhibition by NADH and acetyl-CoA formed from pyruvate (or by P-oxidation), and by inactivation by phosphorylation of Ej by a specific ATP-de-pendent protein kinase associated with the complex, or activation by dephosphorylation by a specific phosphoprotein phosphatase. The phosphatase is activated by increases in the concentration of Ca in the matrix. The combination of insulin with its cell surface receptor activates PDH by activating the phosphatase by an unknown mechanism.

Benzoylformate decarboxylase (BFD EC 4.1.1.7) belongs to the class of thiamine diphosphate (ThDP)-dependent enzymes. ThDP is the cofactor for a large number of enzymes, including pyruvate decarboxylase (PDC), benzaldehyde lyase (BAL), cyclohexane-1,2-dione hydrolase (CDH), acetohydroxyacid synthase (AHAS), and (lR,6] )-2-succinyl-6-hydroxy-2,4-cyclohexadiene-l-carboxylate synthase (SHCHC), which all catalyze the cleavage and formation of C-C bonds [1]. The underlying catalytic mechanism is summarized elsewhere [2] (see also Chapter 2.2.3). 

Reddy et al. (1983) concluded that NO inactivation of iron-sulfur proteins was the probable mechanism of botulinal inhibition in nitrite-tteated foods. In support of this conclusion, Carpenter et al. (1987) observed decreased activity of clostridial pyruvate-ferredoxin oxidoteductase and lower cytochrome c reducing ability by ferredoxin in extracts of cells treated with nitrite. NO tteatment also inhibits yeast pyruvate decarboxylase (a non-iron-sulfur protein) and py-ruvate-ferredoxin oxidoteductase from C. perfringens (McMindes and Siedler, 1988). They suggested that thiamine-dependent decarboxylation of pyruvate may be an additional site for antimicrobial effects of NO. 

Discuss the mechanism of action of brewer s yeast pyruvate decarboxylase using orbital interaction theory wherever appropriate. For the mechanism, see Zeng, X. Chung, A. Haran, M. Jordan, F., J. Am. Chem. Soc., 1991, 113, 5842. 

Figure B.l. Mechanism for the conversion of pyruvic acid to acetaldehyde and CO2 by pyruvate decarboxylase.

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each halfreaction is an a cleavage which leads to a thiamin- bound enamine (center, Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step b followed by a in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an a4 tetramer and one with an ( P)2 quaternary structure. The isolation of ohydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate52 provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase126 in the biosynthesis of the plant hormone indoIe-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25).1243/127... 

Fluoropyruvate is converted quantitatively by pyruvate decarboxylase from wheat germ into acetate, fluoride (F ), and carbon dioxide. Propose a reaction mechanism. See Gish, G., Smyth, T., and Kluger, R. (1988) /. Am. Chem. Soc. 110, 6230-6234. 

Baker s yeast pyruvate decarboxylase, 301-302 mechanism, 302 Basicities gas phase, 97... 

The identity of the enzyme(s) involved in the latter reaction has been debated (13). However, the formation of the above hydro-xyketone, in analogy with acetoin, has been conceptualized as the consequence of the condensation of the "active" form of acetaldehyde, that is formed by decarboxylative addition of pyruvate to thiamine pyrophospate, with benzaldehyde.The role of pyruvate, in fact has been established. The same mechanism can be invoked for the reaction of cinnamaldehyde.lt is known that the pyruvate decarboxylase (E.C. 4.1.1.1) accepts as substrates a-oxoacids... 

In the early eighties the author s group undertook an effort to detect such an enamine on the enzymes, especially pyruvate decarboxylase (PDC, E.C. 4.1.1.1) (see Scheme 6 for reaction mechanism), as produced from conjugated substrate analogs. 

Despite the apparent simplicity of this highly exergonic reaction (AG° = -33.5 kJ/mol), its mechanism is one of the most complex known. The pyruvate dehydrogenase complex is a large multienzyme structure that contains three enzyme activities pyruvate dehydrogenase (Ej), also known as pyruvate decarboxylase, dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3). Each enzyme activity is present in multiple copies. Table 9.3 summarizes the number... 

Scheme 1 Mechanism of yeast pyruvate decarboxylase YPDC.

The C-2-exchange of azolium salts via an ylide 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 ylide 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 ylide. 

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