Pyruvate dehydrogenase complex phosphorylation

This enzyme [EC 2.7.1.99] catalyzes the reaction of ATP with [pyruvate dehydrogenase (lipoamide)] to produce ADP and [pyruvate dehydrogenase (lipoamide)] phosphate. This is an enzyme that is associated with the pyruvate dehydrogenase complex. Phosphorylation of pyruvate dehydrogenase (lipoamide) [EC 1.2.4.1] inactivates that enzyme. 

TABLE 16-1 Stoichiometry of Coenzyme Reduction and ATP Formation in the Aerobic Oxidation of Glucose via Glycolysis, the Pyruvate Dehydrogenase Complex Reaction, the Citric Acid Cycle, and Oxidative Phosphorylation... 

Experiments with rats have shown that the branched-chain a-keto acid dehydrogenase complex is regulated by covalent modification in response to the content of branched-chain amino acids in the diet. With little or no excess dietary intake of branched-chain amino acids, the enzyme complex is phosphorylated and thereby inactivated by a protein kinase. Addition of excess branched-chain amino acids to the diet results in dephosphoiylation and consequent activation of the enzyme. Recall that the pyruvate dehydrogenase complex is subject to similar regulation by phosphorylation and dephosphorylation (p. 621). 

The conversion of pyruvate to acetyl CoA and C02 A. is reversible. B. involves the participation of lipoic acid. C. is activated when pyruvate dehydrogenase complex is phosphorylated by a protein kinase in the pres ence of ATP. D. occurs in the cytosol. E. depends on the coenzyme biotin. Correct answer = B. Lipoic acid is an intermedi ate acceptor of the acetyl group formed in the reaction. Pyruvate dehydrogenase complex cat alyzes an irreversible reaction that is inhibited when the enzyme is phosphorylated. The enzyme is located in the mitochondrial matrix. 

The pyruvate dehydrogenase complex may have been regulated by phosphorylation of any one of the three different enzymes in the complex, yet regulation occurs on the first enzyme of the complex. How is regulation of the complex consistent with the regulation observed in metabolic pathways whose enzymes are not physically associated ... 

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... 

Regulation of the Pyruvate Dehydrogenase Complex In animal tissues, the rate of conversion of pyruvate to acetyl-CoA is regulated by the ratio of active, phosphorylated to inactive, unphosphory-lated PDH complex. Determine what happens to the rate of this reaction when a preparation of rabbit muscle mitochondria containing the PDH complex is treated with (a) pyruvate dehydrogenase kinase, ATP, and NADH (b) pyruvate dehydrogenase phosphatase and Ca2 + (c) malonate. 

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 ... 

The activity of the pyruvate dehydrogenase complex (PDHC) in mammals is altered by an enzyme-catalyzed phosphorylation (Chap. 12) of the complex... 

Control of the activity of the pyruvate dehydrogenase complex is exerted by the phosphorylation of pyruvate decarboxylase (E[), which renders it inactive. This process is catalyzed by pyruvate dehydrogenase kinase, which is always tightly bound to E[. The kinase is activated by high-energy conditions, and it requires ATP to accomplish the phosphorylation step. Another enzyme, phosphoprotein phosphatase, is weakly bound to E, and reactivates the system by removing the inhibitory phosphate group (Fig. 12-8). 

The Pyruvate Dehydrogenase Complex Is Regulated Allosterically and by Reversible Phosphorylation... 

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.

The activity of pyruvate dehydrogenase is regulated by two mechanisms product inhibition and covalent modification (Section 6.5). The enzyme complex is allosterically activated by NAD+, CoASH, and AMP. It is inhibited by high concentrations of ATP and the reaction products acetyl-CoA and NADH. In vertebrates these molecules also activate a kinase, which converts the active pyruvate dehydrogenase complex to an inactive phosphorylated form. High concentrations of the substrates pyruvate, CoASH, and NAD+ inhibit the activity of the kinase. The pyruvate dehydrogenase complex is reactivated by a dephosphorylation reaction catalyzed by a phosphoprotein phosphatase. The phosphoprotein phosphatase is activated when the mitochondrial ATP concentration is low. 

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