Pyruvate dehydrogenase complex overall reaction

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 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-...

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 ... 

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 ... 

Fig. 12-7 The reactions of the pyruvate dehydrogenase complex. The reactants in the overall reaction are shown in boxes. E, = pyruvate decarboxylase (TPP = thiamine pyrophosphate as prosthetic group), E2 = dihydrolipoyl trans-acetylase (oxidized lipoic acid as prosthetic group). E3 = dihydrolipoyl dehydrogenase (FAD as prosthetic group).

Pyruvate must first be transported into mitochondria by a specific carrier that cotransports a proton to maintain electrical neutrality. Inside mitochondria pyruvate undergoes oxidative decarboxylation by three enzymes that function sequentially and are present as a complex known as the pyruvate dehydrogenase complex. The overall reaction is physiologically irreversible, has a high negative AG° (—8.0 kcal/mol, or —33.5 kJ/mol), and commits pyruvate to the formation of acetyl-CoA ... 

The pyruvate dehydrogenase complex catalyzes an irreversible reaction that is the entry point of pyruvate into the TCA cycle (see below) and is under complex regulation by allosteric and covalent modification of the pyruvate dehydrogenase component of the complex. The end products of the overall reaction (NADH and acetyl-CoA) are potent allosteric inhibitors of the pyruvate dehydrogenase... 

The decarboxylation and oxidation of pyruvate to produce acetyl CoA. (a) The overall reaction in which CO, and an Hr are removed from pyruvate and the remaining acetyl group is attached to coenzyme A. This requires the concerted action of three enzymes and five coenzymes, (b) The pyruvate dehydrogenase complex that carries out this reaction is actually a cluster of enzymes and coenzymes. The substrate is passed from one enzyme to the next as the reaction occurs. 

One source of acetyl-CoA molecules for the citric acid cycle is via oxidation of pyruvate in a reaction catalyzed by the pyruvate dehydrogenase complex. The process of converting pyruvate to acetyl-CoA is an oxidative decarboxylation. In the overall reaction (below), the carboxyl group of pyruvate is lost as C02, while the remaining two carbons form the acetyl moiety of acetyl-CoA. 

Pyruvate produced by glycolysis must be oxidized to acetate prior to entry into the citric acid cycle. The enzyme catalyzing this reaction is the pyruvate dehydrogenase complex. The overall reaction ... 

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. 

This conversion requires the three primary enzymes of the pyruvate dehydrogenase complex, as well as the cofactors TPP, FAD, NAD, and lipoic acid. The overall reaction of pyruvate dehydrogenase is the conversion of pyruvate, NAD+, and GoA-SH to acetyl-GoA, NADH + H+, and GOg... 

Reflect and Apply Prepare a sketch showing how the individual reactions of the three enzymes of the pyruvate dehydrogenase complex give rise to the overall reaction. 

List the cofactors that participate in the pyruvate dehydrogenase complex reactions and discuss the roles they play in the overall reaction. 

A metabolically important route for the neration of acyl coenzyme A derivatives is through the oxidation of a-keto adds. The a-keto add dehydrogenase complexes, of which pyruvate dehydrogenase complex is typical, are large multienzyme aggregates. They carry out a comidex reaction sequence to be discussed in section III.D on lipoic acid. The overall reaction given below is an oxidative decarboxylation coupled to thioester formation. 

Formation of acetyl-CoA takes place in the mitochondrial matrix and is catalysed by pyruvate dehydrogenase. This is a multienzyme complex containing three enzymes which together are responsible for the oxidative decarboxylation of the pyruvate. Although the overall reaction may be represented quite simply as follows ... 

Under normal (aerobic) conditions, when oxygen is present, oxygen oxidizes NADH back to NAD" (this happens in the fourth stage of catabolism), and pymvate is converted to acetyl-CoA, which then enters the citric acid cycle. This occurs via a series of reactions catalyzed by a complex of three enzymes and five coenzymes, known collectively as the pyruvate dehydrogenase complex. The overall result of this series of reactions is to transfer the acetyl group of pyruvate to coenzyme A. We looked at the mechanisms for this series of reactions in Section 24.3. 

Oxoglutarate undergoes oxidative decarboxylation to succinyl-CoA, via multi-enzyme reaction similar to the reaction pattern of pyruvate. The multi-enzyme complex (mw about 2 x 10 ) is an octamer of an elementary unit containing each of the three contributing enzyme proteins oxoglutarate decarboxylase, dihydro-lipoyl transacetylase, and dihydrolipoyl dehydrogenase. The overall reaction involves thiamine pyrophosphate, lipoic acid, CoASH and NAD succinyl-CoA is the end product ... 

Pyruvate produced by the glycolytic pathway may be transported into the mitochondria (via an antiport with OH"), where it is converted to acetyl-CoA by the action of the enzyme complex pyruvate dehydrogenase. The pertinent enzyme activities are pyruvate dehydrogenase (PD), lipoic acid acetyltransferase, and dihydrolipoic acid dehydrogenase. In addition, several cofactors are utilized thiamine pyrophosphate (TPP), lipoic acid, NAD+, Co A, and FAD. Only Co A and NAD+ are used in stoichiometric amounts, whereas the others are required in catalytic amounts. Arsenite and Hg2+ are inhibitors of this system. The overall reaction sequence may be represented by Figure 18.5. The NADH generated may enter the oxidative phosphorylation pathway to generate three ATP molecules per NADH molecule reduced. The reaction is practically irreversible its AGq = -9.4 kcal/mol. 

A FIGURE 3-21 Structure and function of pyruvate dehydrogenase, a large multimeric enzyme complex that converts pyruvate into acetyl CoA. (a) The complex consists of 24 copies of pyruvate decarboxylase (Ei), 24 copies of lipoamide transacetylase (E2), and 12 copies of dihydrolipoyl dehydrogenase (E3). The El and E3 subunits are bound to the outside of the core formed by the E2 subunits, (b) The reactions catalyzed by the complex include several enzyme-bound intermediates (not shown). The tight structural integration of the three enzymes increases the rate of the overall reaction and minimizes possible side reactions. 

Decarboxylation and oxidation of alpha-ketoglutarate is carried out by a large multienzyme complex. Both in the overall reaction it catalyzes and in the cofactors used to carry them out—the alpha-ketoglutarate/dehydrogenase complex (alpha-KGDC)—it is similar to the reaction scheme of the pyruvate dehydrogenase (PDC) complex (see Figure 10-6). 

Kinetic data (steady-state and rapid reaction) can be used to identify the sequence of enzyme-containing complexes as substrates are converted to products, and to identify the rates of some or all of the elementary steps in the overall reaction. For instance, in a two-substrate reaction such as that catalyzed by lactate dehydrogenase (lactate+ NAD <- pyruvate+NADH), it can be shown that the... 

Upon entry into the mitochondrial matrix, pyruvate may be converted into acetyl-CoA by the action of a multienzyme complex called the pyruvate dehydrogenase system. The overall non-equilibrium reaction (Figure 12.2a) is exergonic to an... 

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