Mechanism pyruvate decarboxylation

The mechanism of decarboxylation of pyruvic acid is shown in Scheme Xll. The first step is formation of the anion, which then adds to C-2 of pymvate, forming a covalent adduct. This compound has been prepared chemically and its... 

Decarboxylation of p-hydroxyphenylpyruvate begins with nucleophilic addition of TPP ylide to the ketone carbonyl group, followed by loss of CO2 to give an enamine in the usual way. But whereas the enamine formed from pyruvate decarboxylation reacts with lipoamide to give a thioester and regenerated TPP ylide, the enamine from p-hydroxyphenylpyruvate decarboxylation is simply protonated to give an aldehyde plus TPP ylide. The mechanism is shown in Figure 25.8. 

Full details have been published on the oxidative rearrangement of pyruvates to malonates. Whereas the reduction of dimethyl bismethylmalonate with sodium in xylene is reported to yield the keten acetal (9) in the presence of trimethylchlorosilane, when the solvent is ammonia a mixture of products is formed, including the cyclopropanediol di(trimethylsilyl)ether (10). This diol, on treatment with sodium methoxide, gives the products shown in Scheme 39, possibly by Cannizzaro disproportionation of the aldehyde (11). The mechanism of decarboxylation of monoethyl oxaloacetate has been studied as part of a general investigation of ) -keto-acid decarboxylation. 

Step 2 of Figure 29.13 Decarboxylation and Phosphorylation Decarboxylation of oxaloacetate, a jB-keto acid, occurs by the typical retro-aldol mechanism like that in step 3 in the citric acid cycle (Figure 29.12), and phosphorylation of the resultant pyruvate enolate ion by GTP occurs concurrently to give phosphoenol-pyruvate. The reaction is catalyzed by phosphoenolpyruvate carboxykinase. 

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.

The first designed catalyst where there was some understanding of the relationship between structure and function was oxaldie 1, a 14-residue peptide that folds in solution to form helical bundles [11] (Fig. 12). Oxaldie 1 was designed to catalyze the decarboxylation of oxaloacetate, the a-keto acid of aspartic acid, via a mechanism where a primary amine reacts with the ketone carbonyl group to form a carbinolamine that is decarboxylated to form pyruvate. The reaction is piCj dependent and proceeds faster the lower the piC of the primary amine if the reaction is carried out at a pH that is lower than the piCj, of the reactive amine. The sequence contains five lysine residues that in the folded state form... 

Irradiation of pyruvic acid (26) in the vapor phase produces acetaldehyde and carbon dioxide.85 The most reasonable mechanism for the reaction appears to be a concerted decarboxylation via a cyclic four- or five-membered transition state. Although the four-membered transition state has an analogy... 

The mechanism of this reaction is obscure. One suggested mechanism, analogous to the vapor phase reaction, involves concerted decarboxylation of the pyruvic acid to yield a triplet hydroxy carbene which can either dimerize or attack another molecule of pyruvic acid to yield the observed product.91 Dimerization seems to be the less likely process since the carbene can rearrange to acetaldehyde or react with water. Further, this mechanism predicts that acetoin will be formed when pyruvic acid is irradiated in any solvent that does not possess readily abstractable hydrogen atoms, such as benzene, a solvent in which no reaction is observed. One possible explanation of this discrepancy is that the solvation of the pyruvic acid is extremely different in benzene and in water. However, the specific role that the water plays in the reaction has not been determined. 

Problem 16.18 (a) Suggest a mechanism for a ready decarboxylation of malonic acid, HOOCCH,COOH, that proceeds through an activated, intramolecular H-bonded, intermediate complex, (b) Give the decarboxylation products of (i) oxalic acid. HOOC—COOH, and (ii) pyruvic acid, CHjCOCOOH. 

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. 

The literature concerning malo—lactic fermentation—bacterial conversion of L-malic acid to L-lactic acid and carbon dioxide in wine—is reviewed, and the current concept of its mechanism is presented. The previously accepted mechanism of this reaction was proposed from work performed a number of years ago subsequently, several workers have presented data which tend to discount it. Currently, it is believed that during malo-lactic fermentation, the major portion of malic acid is directly decarboxylated to lactic acid while a small amount of pyruvic acid (and reduced coenzyme) is formed as an end product, rather than as an intermediate. It is suspected that this small amount of pyruvic acid has extremely important consequences on the intermediary metabolism of the bacteria. 

When considering the mechanism of the malo-lactic fermentation, the possibility that malic acid may be converted first to oxaloacetic acid (by malic dehydrogenase) must be recognized. This acid could then be decarboxylated to pyruvic acid, and subsequent reaction would yield lactic acid. However, if this were the case, there then should be no situation where malic acid would be decarboxylated faster than oxaloacetic acid. This, however, was shown to occur at pH 6 (14). Similarly, Flesch and Holbach (15) report that malic dehydrogenase has an optimal pH of 10, but that the malo-lactic reaction proceeds at pH 5.6. Therefore, it would not seem likely that the cell would degrade malic acid by this mechanism hence, the oxaloacetic acid intermediate would not be available to the organism. 

One of the first persons to study the oxidation of organic compounds by animal tissues was T. Thunberg, who between 1911 and 1920 discovered about 40 organic compounds that could be oxidized by animal tissues. Salts of succinate, fumarate, malate, and citrate were oxidized the fastest. Well aware of Knoop s (3 oxidation theory, Thunberg proposed a cyclic mechanism for oxidation of acetate. Two molecules of this two-carbon compound were supposed to condense (with reduction) to succinate, which was then oxidized as in the citric acid cycle to oxaloacetate. The latter was decarboxylated to pyruvate, which was oxidatively decarboxylated to acetate to complete the cycle. One of the reactions essential for this cycle could not be verified experimentally. It is left to the reader to recognize which one. 

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... 

Nevertheless, malonyl-CoA is a major metabolite. It is an intermediate in fatty acid synthesis (see Fig. 17-12) and is formed in the peroxisomal P oxidation of odd chain-length dicarboxylic acids.703 Excess malonyl-CoA is decarboxylated in peroxisomes, and lack of the decarboxylase enzyme in mammals causes the lethal malonic aciduria.703 Some propionyl-CoA may also be metabolized by this pathway. The modified P oxidation sequence indicated on the left side of Fig. 17-3 is used in green plants and in many microorganisms. 3-Hydroxypropionyl-CoA is hydrolyzed to free P-hydroxypropionate, which is then oxidized to malonic semialdehyde and converted to acetyl-CoA by reactions that have not been completely described. Another possible pathway of propionate metabolism is the direct conversion to pyruvate via a oxidation into lactate, a mechanism that may be employed by some bacteria. Another route to lactate is through addition of water to acrylyl-CoA, the product of step a of Fig. 17-3. Tire water molecule adds in the "wrong way," the OH ion going to the a carbon instead of the P (Eq. 17-8). An enzyme with an active site similar to that of histidine ammonia-lyase (Eq. 14-48) could... 

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. 

The reductive alkylation of methyl pyruvate with and the t-butyl esters of amino acids using Pd/C catalyst leads to the formation of iminodicarboxylic acids such as 67 in selectivities of 29-75% d.e. depending on the amino acid and solvent used (hexane gave the best results). Hydrolysis of the t-butyl ester to the acid 68 followed by hypochlorite-promoted decarboxylation and imine hydrolysis leads to the formation of ( -alanine 69 in correspondingly high e.e.s278,281. The likely decarboxylation mechanism as far as the imine stage is shown in Scheme 65. 

The process goes through the formation of the intermediate pyruvate CH3—CO— COO—, which further decarboxylates into acetaldehyde, and is reduced finally to ethanol. The complex reaction mechanism is described elsewhere [9], The above reaction takes place only in anaerobic conditions (without oxygen), otherwise the complete oxidation to carbon dioxide and water takes place. 

Another early success in biomimetic chemistry concerns reactions promoted by thiamin. In 1943, more than 35 years ago, Ukai, Tanaka, and Dokowa (12) reported that thiamin will catalyze a benzoin-type condensation of acetaldehyde to yield acetoin. This reaction parallels a similar enzymic reaction where pyruvate is decarboxylated to yield acetoin and acetolactic acid. Although the yields of the nonenzymic process are low, it is clearly a biomimetic process further investigation by Breslow, stimulated by the early discovery of Ugai et al., led to an understanding of the mechanism of action of thiamin as a coenzyme. 

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