Pyruvate dehydrogenase complex components

The pyruvate dehydrogenase complex consists of a number of polypeptide chains of each of the three component enzymes, all organized in a regular spatial configuration. Movement of the individual enzymes appears to be restricted, and the metabofic intermediates do not dissociate freely but remain bound to the enzymes. Such a complex of enzymes, in which the sub-... 

Figure 17-5. Oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex. Lipoic acid is joined by an amide link to a lysine residue of the transacetylase component of the enzyme complex. It forms a long flexible arm, allowing the lipoic acid prosthetic group to rotate sequentially between the active sites of each of the enzymes of the complex. (NAD nicotinamide adenine dinucleotide FAD, flavin adenine dinucleotide TDP, thiamin diphosphate.)...

Lipoic acid (the other names are a-lipoic acid or thioctic acid) (Figure 29.9) is a natural compound, which presents in most kinds of cells. Lipoic acid (LA) is contained in many food products, in particular in meat, but it is also synthesized in human organism from fatty acids. Earlier, it has been shown that in humans lipoic acid functions as a component of the pyruvate dehydrogenase complex. However, later on, attention has been drawn to the possible antioxidant activity of the reduced form of lipoic acid, dihydrolipoic acid (DHLA) (Figure 29.9). 

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... 

Lewisite is the most important of the organo-arseni-cal CW agents. Exposure to lewisite is quite painful, and onset of symptoms occurs rapidly (seconds to minutes) (31) in contrast to sulfur mustard for which a latency period occurs of several hours between exposure and symptoms (32). Although it is not known to have been used as a CW agent, lewisite is still considered a potential threat due to the relative ease of production and its rapid onset of action. Moreover, substantial stockpiles of lewisite are present in the United States, Russia, and in China abandoned by the Japanese Imperial Army. This may constitute a potential hazard for public health (33). The toxicity of lewisite is inter alia caused by the high affinity for the vicinal di-thiol system present in dihydrolipoic acid, a component of the pyruvate dehydrogenase complex, as is the case for other arsenicals (34). This prevents the formation of acetyl coenzyme A from pyruvate. 

TDP-dependent enzymes include transketolase, an enzyme component of the pentose shunt pathway, pyruvate dehydrogenase complex, and aKGDH a tricarboxylic acid cycle enzyme (Fig. 3). Branched-chain ketoacid dehydrogenases are also TDP-dependent. 

Although the structure of an intact member of the pyruvate dehydrogenase complex family has not yet been determined in atomic detail, the structures of all of the component enzymes are now known, albeit from different complexes and species. Thus, it is now possible to construct an atomic model of the complex to understand its activity (Figure 17.7). 

Figure 17.17. Regulation of the Pyruvate Dehydrogenase Complex. The complex is inhibited by its immediate products, NADH and acetyl CoA. The pyruvate dehydrogenase component is also regulated by covalent modification. A specific kinase phosphorylates and inactivates pyruvate dehydrogenase, and a phosphatase actives the dehydrogenase by removing the phosphoryl. The kinase and the phosphatase also are highly regulated enzymes.

Figure 17.20. Arsenite Poisoning. Arsenite inhibits the pyruvate dehydrogenase complex by inactivating the dihydrolipoamide component of the transacetylase. Some sulfhydryl reagents, such as 2,3-dimercaptoethanol, relieve the inhibition by forming a complex with the arsenite that can be excreted.

The oxidation of pyruvate to acetyl CoA is accomplished by the Pyruvate Dehydrogenase complex, a large, multi-component enzyme with three main enzyme subunits. 

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 conversion of pyruvate into acetyl CoA by the pyruvate dehydrogenase complex is the link between glycolysis and cellular respiration because acetyl CoA is the fuel for the citric acid cycle. Indeed, all fuels are ultimately metabolized to acetyl CoA or components of the citric acid cycle. 

Hiromasa, Y., Fujisawa, T., Aso. Y, and Roche, T. E. 2004. Organization of the cores of the maminalian pyruvate dehydrogenase complex formed by E2 and E2 plus the E3-binding proteins and their capacities to bind the El and E3 components. J. Biol Chem. 279 6921-6933. 

Describe the reaction that results in the conversion of pyruvate to acetyl CoA, describing the location of the reaction and the components of the pyruvate dehydrogenase complex. 

Figure 20 Putative Information transfer pathway between ThDPs on two monomers of the E. coli pyruvate dehydrogenase complex El component.

Komuniecki, R., Rhee, R., Bhat, D., Duran, E., Sidawy. E. and Song, H. (1992) The pyruvate dehydrogenase complex from the parasitic nematode, Ascaris suum novel subunit composition and domain structure of the dihydrolipoyl transacetylase component. Arch. Biochem. Biophys. 296 115 121. 

FIGURE 17.7 Schematic representation of the pyruvate dehydrogenase complex. The transacetylase core (E2) is shown in red, the pyruvate dehydrogenase component (E,) in yellow, and the dihydrolipoyl dehydrogenase (E3) in green. 

Linn, T.C. Pelley, J.W. Pettit, RH. Hucho, R Randall, D.D. Reed, L.J. a-Keto acid dehydrogenase complexes. XV. Purification and properties of the component enzymes of the pyruvate dehydrogenase complexes from bovine kidney and heart. Arch. Biochem. Biophys., 148, 327-342 (1972)... 

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