Regulation of Pyruvate Dehydrogenase Activity

1 Regulation of Pyruvate Dehydrogenase Activity Pyruvate dehydrogenase is the key enzyme that commits pyruvate (and hence the products of carbohydrate metabolism) to complete oxidation (via the tricarboxyUc acid cycle) or lipogenesis. It is subject to regulation by both product inhibition and a phosphorylation/dephosphorylation mechanism. Acetyl CoA and NADH are both inhibitors, competing with coenzyme A and NAD+. 

The kinases are inhibited by pyruvate and adenosine disphosphate (ADP), and the phosphatases are activated by calcium ions. There is normally a constant process of phosphorylation and dephosphorylation of the enzyme, so that it is very sensitive to changes in intraceUular free calcium and the adenosine triphosphate (ATP) ADP ratio. 

Pyruvate dehydrogenase kinases are induced by glucocorticoid hormones and long-chain fatty acids (acting via the peroxisome proliferation-activated 

In addition to its cofactor role, thiamin diphosphate, together with calcium or other divalent cations, activates pyruvate dehydrogenase by binding to a regulatory site and reducing the for pyruvate (Czerniecki and Czygier, 2001). 

2 Thiamin-Responsive Pyruvate Dehydrogenase Deficiency Genetic deficiency of pyruvate dehydrogenase Ela (which is on the X chromosome) leads to potentially fatal lactic acidosis, with psychomotor retardation, central nervous system damage, atrophy of muscle fibers and ataxia, and developmental delay. At least some cases respond to the administration of high doses (20 to 3,000 mg per day) of thiamin. In those cases where the enzyme has been studied, there is a considerable increase in the of the enzyme for thiamin diphosphate. Female carriers of this X-linked disease are affected to a variable extent, depending on the X-chromosome inactivation pattern in different tissues (Robinson et al., 1996). 

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 

Schematic representation of the relationship between the three enzymes of the pyruvate dehydrogenase complex. The lipoyl-lysyl moiety of the transacetylase delivers the acetyl group to CoA and the reducing equivalents to FAD. TPP = Thiamine pyrophosphate. 

An analogue of pyruvate, dichloroacetate (CHCl2COO ), inhibits pyruvate dehydrogenase kinase and maintains the pyruvate dehydrogenase complex 

Regulation of pyruvate dehydrogenase (PD) by inactivation and reactivation by a non-cAMP-dependent phosphorylation-dephosphorylation cycle. Although PD kinase phosphorylates three specific seryl residues in the a-subunit of PD, phosphorylation at any of these sites inactivates PD. The kinase and the phosphatase are under the influence of several regulators, and the dephospho-active PD is also regulated by end products. 0 = Activation 0 = inhibition E2 = dihydrolipoyl transacetylase E3 = dihydrolipoyl dehydrogenase. 

Thiamine deficiency causes decreased pyruvate oxidation, leading to accumulation of pyruvate and lactate, particularly in the blood and brain, and is accompanied by impairment of the cardiovascular, nervous, and gastrointestinal systems (Chapter 38). Inherited deficiency of pyruvate dehydrogenase complex is accompanied by lactic acidemia and abnormalities of the nervous system (e.g., ataxia and psychomotor retardation). Pyruvate carboxylase deficiency causes similar abnormalities (Chapter 15). Both inherited disorders of pyruvate utilization are autosomal recessive. 

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Kolobova E,Tuganova A, BoulatnikovI, and Popov KM (2001) Regulation of pyruvate dehydrogenase activity through phosphorylation at multiple sites. Biochemical Journal 358, 69-77. 

Rutter GA, Burnett P, Rizzuto R et al (1996) SubceUular imaging of intramitochondrial Ca with recombinant targeted aequorin significance for the regulation of pyruvate dehydrogenase activity. Proceedings of the National Academy of Science, USA 93 5489-5494. 

Severson. D. L., Denton, R. M., Bridges, B. J., and Randle, P. J., 1976, Exchangeable and total calcium pools in mitochondria of rat epidid3nnal fat pads and isolated fat cells. Role in the regulation of pyruvate dehydrogenase activity. Biochem. ]. 154 209. 

Figure 17-6. Regulation of pyruvate dehydrogenase (PDH). Arrows with wavy shafts indicate allosteric effects. A Regulation by end-product inhibition. B Regulation by interconversion of active and inactive forms.

Holness MJ, Sugden MC Regulation of pyruvate dehydrogenase complex activity by reversible... 

Huang B, Wu P, Bowker-Krnley MM, and Harris RA (2002) Regulation of pyruvate dehydrogenase kinase expression by peroxisome proUferator-activated receptor-alpha ligands, glucocorticoids, and insulin. Diabetes 51,276-83. 

Fig. 20.16. Regulation of pyruvate dehydrogenase complex (PDC). PDC kinase, a subunit of the enzyme, phosphorylates PDC at a specific serine residue, thereby converting PDC to an inactive form. The kinase is inhibited by ADP and pyruvate. PDC phosphatase, another subunit of the enzyme, removes the phosphate, thereby activating PDC. The phosphatase is activated by Ca ". When the substrates, pyruvate and CoASH, are bound to PDC, the kinase activity is inhibited and PDC is active. When the products acetyl CoA and NADH bind to PDC, the kinase activity is stimulated, and the enzyme is phosphorylated to the inactive form. E] and the kinase exist as tissue-specific isozymes with overlapping tissue spiecificity, and somewhat different regulatory properties.

Pawelczyk, T. Olson, M.S. Regulation of pyruvate dehydrogenase kinase activity from pig kidney cortex. Biochem. J., 288, 369-373 (1992)... 

Constantin-Teodosiu, D., Regulation of Pyruvate Dehydrogenase Complex Activity and Acetyl Group Formation in Skeletal Muscle during Exercise. Ph.D. dissertation, Huddinge University, Sweden, 1992. 

Control of pymvate dehydrogenase activity is via covalent modification a specific kinase causes inactivation of the PDH by phosphorylation of three serine residues located in the pyruvate decarboxylase/dehydrogenase component whilst a phosphatase activates PDH by removing the phosphates. The kinase and phosphatase enzymes are non-covalently associated with the transacetylase unit of the complex. Here again we have an example of simultaneous but opposite control of enzyme activity, that is, reciprocal regulation. 

Hoshi, M. Takashima, A. Noguchi, K. Murayama, M. Sato, M. Kondo, S. Saitoh, Y. Ishiguro, K. Hoshino, T. Imahori, K. Regulation of mitochondrial pyruvate dehydrogenase activity by r protein kinase I/glycogen synthase kinase 3/1 in brain. Proc. Natl. Acad. Sci. USA, 93, 2719-2723 (1996)... 

DCA is an inhibitor of pyruvate dehydrogenase kinase, the enzyme that regulates pyruvate dehydrogenase (PDH) activity by phosphorylation of the Ex subunit, leading to a decrease in PDH activity (see Chapter 7). DCA has been used to lower lactate levels in some patients with MELAS. Stimulation of PDH activity reduces the release of lactate from peripheral tissues and enhances its metabolism by the liver. Since prolonged use of DCA may lead to sensory neuropathy, treatment must be closely monitored. 

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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