Pyruvate dehydrogenase complex reaction catalyzed

The formation of acetyl-CoA from pyruvate in animals is via the pyruvate dehydrogenase complex, which catalyzes the irreversible decarboxylation reaction. Carbohydrate is synthesized from oxaloacetate, which in turn is synthesized from pyruvate via pyruvate carboxylase. Since the pyruvate dehydrogenase reaction is irreversible, acetyl-CoA cannot be converted to pyruvate, and hence animals cannot realize a net gain of carbohydrate from acetyl-CoA. Because plants have a glyoxylate cycle and animals do not, plants synthesize one molecule of succinate and one molecule of malate from two molecules of acetyl-CoA and one of oxaloacetate. The malate is converted to oxaloacetate, which reacts with another molecule of acetyl-CoA and thereby continues the reactions of the glyoxylate cycle. The succinate is also converted to oxaloacetate via the enzymes of the citric acid cycle. Thus, one molecule of oxaloacetate is diverted to carbohydrate synthesis and, therefore, plants are able to achieve net synthesis of carbohydrate from acetyl-CoA. 

A fourth fate of pyruvate is its oxidative decarboxylation to acetyl CoA. This irreversible reaction inside mitochondria is a decisive reaction in metabolism it commits the carbon atoms of carbohydrates and amino acids to oxidation by the citric acid cycle or to the synthesis of lipids. The pyruvate dehydrogenase complex, which catalyzes this irreversible funneling, is stringently regulated by multiple allosteric interactions and covalent modifications. Pyruvate is rapidly converted into acetyl CoA only if ATP is needed or if two-carbon fragments are required for the synthesis of lipids. 

T. vaginalis (16), T. foetus (17) and E. histolytica (18), and detected in G. lamblia (19). It is similar in properties and has sequence homology to isofunctional enzymes in anaerobic eubacteria. The enzyme is, however, fundamentally different from the mitochondrial pyruvate dehydrogenase complex, which catalyzes the same overall reaction but involves a different electron acceptor. The electron acceptor used by the enzyme of these anaerobic parasites is known to be ferredoxin, also an iron-sulfur protein (1). Interestingly, however, the trichomonad protein, a [2Fe-2S] ferredoxin with some similarities to mitochondrial proteins, belongs to a different subfamily to the protein in E. histolytica, which is a 2[4Fe-4S] ferredoxin like those in anaerobic eubacteria. The G. lamblia ferredoxin is probably similar to the latter, although it is yet to be fully characterized. 

The pyruvate dehydrogenase complex (PDC) is a noncovalent assembly of three different enzymes operating in concert to catalyze successive steps in the conversion of pyruvate to acetyl-CoA. The active sites of ail three enzymes are not far removed from one another, and the product of the first enzyme is passed directly to the second enzyme and so on, without diffusion of substrates and products through the solution. The overall reaction (see A Deeper Look Reaction Mechanism of the Pyruvate Dehydrogenase Complex ) involves a total of five coenzymes thiamine pyrophosphate, coenzyme A, lipoic acid, NAD+, and FAD. 

The conversion occurs through a multistep sequence of reactions catalyzed by a complex of enzymes and cofactors called the pyruvate dehydrogenase complex. The process occurs in three stages, each catalyzed by one of the enzymes in the complex, as outlined in Figure 29.11 on page 1152. Acetyl CoA, the ultimate product, then acts as fuel for the final stage of catabolism, the citric acid cycle. All the steps have laboratory analogies. 

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

We know that anaerobic glycolysis of glncose yields pyruvate and/or lactate, interconvertable metabolites. Pyruvate is converted into acetyl-SCoAin the following reaction, catalyzed by the pyruvate dehydrogenase complex ... 

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

Pyruvate dehydrogenase (lipoamide) [EC 1.2.4.1], which requires thiamin pyrophosphate, catalyzes the reaction of pyruvate with lipoamide to produce 5-acetyldihydroli-poamide and carbon dioxide. It is a component of the pyruvate dehydrogenase complex (which also includes dihydrolipoamide dehydrogenase [EC 1.8.1.4] and dihy-drolipoamide acetyltransferase [EC 2.3.1.12]). Pyruvate dehydrogenase (cytochrome) [EC 1.2.2.2] catalyzes the... 

The pyruvate dehydrogenase complex from Escherichia coli is considerably more complex than tryptophan synthetase. It has a molecular weight of approximately 4.6 millon and contains three enzymes pyruvate dehydrogenase (Et), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).82 The overall reaction catalyzed by the complex is... 

Fig. 9. A schematic drawing of a possible mechanism for the reaction catalyzed by the pyruvate dehydrogenase complex. The three enzymes Elf E2, and E3 are located so that lipoic acid covalently linked to E2 can rotate between the active sites containing thiamine pyrophosphate (TPP) and pyruvate (Pyr) on Elt CoA on E2, and FAD on E3. Acetyl-CoA and GTP are allosteric effectors of E, and NAD+ is an inhibitor of the overall reaction.

The overall reaction catalyzed by the pyruvate dehydrogenase complex is an oxidative decarboxylation,... 

FIGURE 16-2 Overall reaction catalyzed by the pyruvate dehydrogenase complex. The five coenzymes participating in this reaction, and the three enzymes that make lip the enzyme complex, are discussed in the text. 

Figure 16-6 shows schematically how the pyruvate dehydrogenase complex carries out the five consecutive reactions in the decarboxylation and dehydrogenation of pyruvate. Step CD is essentially identical to the reaction catalyzed by pyruvate decarboxylase (see Fig. 14-13c) C-l of pyruvate is released as C02, and C-2, which in pyruvate has the oxidation state of an aldehyde, is attached to TPP as a hydroxyethyl group. This first step is the slowest and therefore limits the rate of the overall reaction. It is also the point at which the PDH complex exercises its substrate specificity. In step (2) the hydroxyethyl group is oxidized to the level of a car-...

Reactions of the TCA cycle Enzyme that oxidatively decarboxylates pyruvate, its coenzymes, activators, and inhibitors REACTIONS OF THE TRICARBOXYLIC ACID CYCLE (p. 107) Pyruvate is oxidatively decarboxylated by pyruvate dehydrogenase complex producing acetyl CoA, which is the major fuel for the tricarboxylic acid cycle (TCA cycle). The irreversible set of reactions catalyzed by this enzyme complex requires five coenzymes thiamine pyrophosphate, lipoic acid, coenzyme A (which contains the vitamin pantothenic acid), FAD, and NAD. The reaction is activated by NAD, coenzyme A, and pyruvate, and inhibited by ATP, acetyl CoA, and NADH. 

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... 

In the reaction catalyzed by dihydrolipoyl dehydrogenase, one of three enzymes in the pyruvate dehydrogenase complex (fig. 13.5), electrons flow from oxidized lipoic acid to enzyme-bound FAD to NAD+. Compare the flow of electrons in the latter part of this scheme (FAD to NAD + ) to the flow of electrons in the electron transport scheme (Complex I). Is there a distinct difference in the flow of electrons in the two schemes If so, can you provide a possible explanation for this difference ... 

Problem 6.2 illustrates the use of equation 6.2-1 by applying it to four net reactions that represent the oxidation of glucose to carbon dioxide and water (1) the net reaction for glycolysis, (2) the net reaction catalyzed by the pyruvate dehydrogenase complex, (3) the net reaction for the citric acid cycle, and (4) the net reaction for oxidative phosphorylation. The v in equation 6.2-1 is the apparent stoichiometric number matrix for these four reactions. The net reaction is... 

There are multienzyme complexes that efficiently catalyze sequential reactions in some metabolic pathways. The overall rate of a sequential reaction is greatly enhanced by the assembly of subunits possessing sequential metabolic activities, because the metabolites are transferred directly from one active site to another without diffusing in the solution.9 Typical examples are the mammalian pyruvate dehydrogenase complex (three catalytic subunits)10 and the bacterial trytophan synthase (two catalytic subunits).11 ... 

Answer The pyruvate dehydrogenase complex can be thought of as performing five enzymatic reactions. The first three (see Fig. 16-6) catalyze the oxidation of pyruvate to acetyl-CoA and reduction of the enzyme. The last two reactions are essential to reoxidize the reduced enzyme, reducing NAD+ to NADH + H+. The moiety on the enzyme that is oxidized/reduced is the lipoamide cofactor. 

Figure 7-1. Pathways of fuel metabolism and oxidative phosphorylation. Pyruvate may be reduced to lactate in the cytoplasm or may be transported into the mitochondria for anabolic reactions, such as gluconeogenesis, or for oxidation to acetyl-CoA by the pyruvate dehydrogenase complex (PDC). Long-chain fatty acids are transported into mitochondria, where they undergo [ -oxidation to ketone bodies (liver) or to acetyl-CoA (liver and other tissues). Reducing equivalents (NADH, FADII2) are generated by reactions catalyzed by the PDC and the tricarboxylic acid (TCA) cycle and donate electrons (e ) that enter the respiratory chain at NADH ubiquinone oxidoreductase (Complex 0 or at succinate ubiquinone oxidoreductase (Complex ID- Cytochrome c oxidase (Complex IV) catalyzes the reduction of molecular oxygen to water, and ATP synthase (Complex V) generates ATP fromADP Reprinted with permission from Stacpoole et al. (1997).

The pyruvate dehydrogenase complex catalyzes the conversion of pyruvate to acetyl-CoA. This conversion links the breakdown of carbohydrates to the processes of respiration and oxidative phosphorylation (Chap. 12). The overall reaction is ... 

This reaction, which is catalyzed hy the pyruvate dehydrogenase complex, will he discussed in detail in Chapter 18. The NAD+ required for this reaction and for the oxidation of glyceraldehyde 3-phosphate is regenerated when NADH ultimately transfers its electrons to O2 through the electron-transport chain in mitochondria. 

Figure 17.9. Reactions of the Pyruvate Dehydrogenase Complex. At the top (center), the enzyme (represented by a yellow, a blue, and two red spheres) is unmodified and ready for a catalytic cycle. (1) Pyruvate is decarboxylated to form the hydroxy ethyl TPP. (2) The dihydrolipoyl arm of E2 moves into the active site of E. (3) Ej catalyzes the transfer of the two-carbon group to the dihydrolipoyl group to form the acetyl-lipoyl complex. (4) E2 catalyzes the transfer of the acetyl moiety to CoA to form the product acetyl CoA. The disulfhydryl lipoyl arm then swings to the active site of E3. E3...

Active form of thiamine, that is thiamine pyrophosphate (TPP) is required for the key reactions catalyzed by Pyruvate dehydrogenase complex and alpha-ketoglutarate dehydrogenase complex (TCA cycle). 

---