Enzyme pyruvate decarboxylase

Kluger and Brandi (1986b) also studied the decarboxylation and base-catalysed elimination reactions of lactylthiamin, the adduct of pyruvate and thiamin (Scheme 2). These reactions are nonenzymic models for reactions of the intermediates formed during the reaction catalysed by the enzyme pyruvate decarboxylase. The secondary j3-deuterium KIE for the decarboxylation was found to be 1.09 at pH 3.8 in 0.5 mol dm-3 sodium acetate at 25°C. In the less polar medium, 38% ethanolic aqueous sodium acetate, chosen to mimic the nonpolar reactive site in the enzyme, the reaction is significantly faster but the KIE was, within experimental error, identical to the KIE found in water. This clearly demonstrates that the stabilization of the transition state by hyperconjugation is unaffected by the change in solvent. 

Figure 1.8 Enzyme pyruvate decarboxylase b. Oxidative deamination NH3 released...

R. L. Baxter, and L. Sawyer. Biotin synthesis requires three other enzymes (steps b, c, d). Step b is catalyzed by a PLP-dependent transaminase. At the left is thiamin diphosphate, in the form of its 2-(1 -hydroxyethyl) derivative, an intermediate in the enzyme pyruvate decarboxylase (Dobritzsch et al.,. Biol. Chem. 273,20196-20204,1998). Courtesy of Guoguang Lu. Thiamin diphosphate functions in all living organisms to cleave C-C bonds adjacent to C=O groups. 

The conversion of pyruvate to acetyl-CoA. The reactions are catalyzed by the enzymes of the pyruvate dehydrogenase complex. This complex has three enzymes pyruvate decarboxylase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. In addition, five coenzymes are required thiamine pyrophosphate, lipoic acid, CoASH, FAD, and NAD+. Lipoic acid is covalently attached to... 

Calculate the entropy change for the conversion of pyruvic acid (CH3COCOOH) into acetaldehyde and C02 by the enzyme pyruvate decarboxylase at 25°C and 100 atm. 

Arjunan, P., et al. (1996). Crystal structure of the thiamin diphosphate-dependent enzyme pyruvate decarboxylase from the yeast saccharomyces cerevisiae at 2.3 A Resolution. J. Mol. Biol. 256, 590-600... 

Dyda, F., Fueey, W., Swaminathan, S., Sax, M., Faeeenkope, B., Jordan, F. (1993), Catalytic centers in the thiamin diphosphate dependent enzyme pyruvate decarboxylase at 2.4-A resolution. Biochemistry 32, 6165-6170. 

Under anaerobic conditions, NADH produced in glycolysis builds up. This results in a reduction in the amount of NAD+ available to support continuation of glycolysis. Organisms have two pathways for regenerating NAD+ under anaerobic conditions. Animal cells and lactic acid bacteria use the process of lactic acid fermentation. Yeast convert pyruvate to acetaldehyde in a reaction catalyzed by the enzyme pyruvate decarboxylase. This is followed by reduction of acetaldehyde to ethanol catalyzed by alcohol dehydrogenase. The reaction uses NADH and releases NAD+, which is subsequently used in glycolysis. 

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. 

ANAEROBIC CARBOHYDRATE METABOLISM Yeasts growing in media containing high concentrations of fermentable carbohydrate invariably metabolize it fermentatively to produce ethanol and CO2. If air is present, and when the sugar concentration has been lowered, the ethanol is respired using the metabolic routes described above. Under the anaerobic conditions of a brewery fermentation the hexoses derived from wort fermentable carbohydrates are catabolized by the EMP pathway (Fig. 17.2) to pyruvic acid. The pyruvate produced is decarboxylated by the enzyme pyruvate decarboxylase, with the formation of acetaldehyde and CO2. The enzyme requires the cofactor thiamine pyrophosphate (TPP) for activity and the reaction is shown in Fig. 17.10. The acetaldehyde formed acts (in the absence of the respiratory chain) as an electron acceptor and is used to oxidize NADH with the formation of ethanol ... 

Biological compounds with long chains of carbon atoms are broken down into molecules with shorter chains by the breaking of carbon-carbon bonds. This commonly occurs by the elimination of -COjH groups from carboxylic adds. For example, the enzyme pyruvic decarboxylase acts upon pyruvic add to split off CO2 and produce a compound with one less carbon ... 

Enzymes which degrade a-keto acids to COg and aldehydes contain thiamine pyrophosphate (D 10.4.5) as coenzyme. The keto acid is added in a reversible reaction to carbon atom 2 of the thiazole ring of thiamine pyrophosphate giving an oc-hydroxy acid derivative. This compound is decarboxylated and split to an aldehyde and thiamine pyrophosphate. Figure 25 shows this sequence of reactions for the enzyme pyruvate decarboxylase which splits pyruvate to acetaldehyde and COg. 

Acetaldehyde (AA), a natural aroma component in almost every fruit, accumulates during ripening (Fidler 1968). Acetaldehyde in both climacteric (Dilley 1970) and non-climacteric (Yamashita et al. 1978) fruits is formed from pyruvate by the enzyme, pyruvate decarboxylase (PDC). The two immediate products formed from AA are (1) ethanol, by the enzyme, alcohol dehydrogenase (ADH), and (2) acetyl CoA by the enzyme, aldehyde dehydrogenase (Cossins 1978). 

Figure 4.2 Reactions of pyruvate-processing enzymes pyruvate decarboxylase (PDC), p5Tuvate oxidase (POX), pyruvate ferredoxin oxidoreductase (PFOR) and pyruvate dehydrogenase (PDH). Their reactions diverge after the decarboxylation step. Pyr and PP represent 2,5-dimethyl-4-amino-pyrimidine and the ethyl diphosphate tail, respectively.

F., 1993. Catalytic centers in the thiamin diphosphate dependent enzyme pyruvate decarboxylase at 2.4 A resolution. Biochemistry. 32 6165-6170. 

The conversion of pyruvate (CH3COCO2 ) to ethanol (CH3CH2OH) requires two steps. First, the pyruvate undergoes decarboxylation with participation of the enzyme pyruvate decarboxylase (EC 4.1.1.1). The enzyme uses thiamine diphosphate as a cofactor. As was the case earlier (Scheme 11.7), the nucleophilic ylid adds to the carbon of the carbonyl. In this case, the addition product corresponds to a p,y-unsaturated carboxylic acid, ready for decarboxylation to produce an enol. Protonation followed by elimination of the thiamine diphosphate ylid then yields ethanal (acetaldehyde, CH3CHO).This process is shown in Scheme 11.28. 

In yeast the enzyme pyruvate decarboxylase catalyses the decarboxylation of pyruvate to CO2 and acetaldehyde. P vate decarboxylase requires Mg and thiamine pyrophosphate as a coenzyme. The acetaldehyde is then reduced to ethanol in a reaction catalysed by alcohol dehydrogenase in which Ae NADH produced during glycolysis is oxidized and NAD is regenerated... 

In fermentation, as we have pointed out, pyruvate is decarboxylated to form acetaldehyde. The enzyme pyruvate decarboxylase, or simply carboxylase, requires Mg and thiamine pyrophosphate as cofactors. Acetaldehyde is probably bound in its activated form at first, because it can imdergo several reactions. Under certain conditions, two molecules can combine to form acetoin. Pyruvate decarboxylase is found only in plants. 

The pyruvate dehydrogenase complex comprises three enzymes - pyruvate decarboxylase, lipoamide transacetylase and dihydrolipoyl dehydrogenase. In eukaryotes it is located in the mitochondrial matrix. Pyruvate is decarboxylated to produce NADH, acetyl CoA and CO2. 

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