Control pyruvate dehydrogenase

Figure 5.3 Major control points of glycolysis and the TCA cycle. Enzymes I, hexokinase II, phosphofructokinase III, pyruvate kinase IV, pyruvate dehydrogenase V, citrate synthase VI, aconitase VII, isocitrate dehydrogenase VIII, a-oxoglutarate dehydrogenase.

Ravindran, S., Radke, G.A., Guest, J.R. and Roche, T.E. (1996) Lipoyl domain-based mechanism for the integrated feedback control of the pyruvate dehydrogenase complex by enhancement of pyruvate dehydrogenase kinase activity. Journal of Biological Chemistry 271,653-562. 

Substrate availability for certain reactions can be optimized by anaplerotic ( topping-up ) reactions. For example, citrate synthase is a key control point of the TCA cycle. The co-substrates of citrate synthase are acetyl-CoA and oxaloacetate (OAA) and clearly, restriction in the availability of either substrate will decrease the rate of the citrate synthase reaction. Suppose, for example, a situation arises when acetyl-CoA concentration is significantly higher than that of OAA, the concentration of the latter can be topped-up and the concentration of acetyl-CoA simultaneously reduced by diverting some of the pyruvate away from acetyl-CoA synthesis (via pyruvate dehydrogenase) to OAA synthesis (via pyruvate carboxylase) as shown in Figure 3.1. The net effect is to balance the relative concentrations of the two co-substrates and thus to promote citrate synthase activity. 

Where two enzymes compete for the same substrate, we expect to see some form of metabolic control and in this case the concentrations of NADH and acetyl-CoA are the key controlling factors (Figure 6.44). When glucose is not available as a fuel, metabolism switches to 3- oxidation of fatty acids, which generates more than sufficient quantities of both NADH and acetyl-CoA to drive the TCA cycle and to maintain oxidative phosphorylation. Pyruvate dehydrogenase activity is suppressed and pyruvate carboxylase is stimulated by ATP, NADH and acetyl-CoA (strictly speaking by low mitochondrial ratios of ADP/ATP, NAD+/NADH and coenzyme A/acetyl-CoA), so... 

Pyruvate dehydrogenase is inhibited by its product acetyl CoA. This control is important in several contexts and shoidd be considered along with pyruvate carboxylase, the other mitochondrial enzyme that uses pyruvate (introduced in gluconeogenesis, Chapter 14, Figure 1-14-5). 

Elaborate cascades initiate the clotting of blood (Chapter 12) and the action of the protective complement system (Chapter 31). Cascades considered later in the book are involved in controlling transcription (Fig. 11-13) and in the regulation of mammalian pyruvate dehydrogenase (Eq. 17-9), 3-hydroxy-3-methyl-glutaryl-CoA reductase and eicosanoids (Chapter 21), and glutamine synthetase (Chapter 24). 

The committed step in a metabolic pathway is usually under metabolic control. Inhibition of the committed step in a metabolic sequence or pathway prevents the accumulation of unneeded intermediates and effectively precludes activity of the enzymes using those intermediates as substrates. The decarboxylation of pyruvate and the oxidative transfer of the hydroxyethyl group by pyruvate dehydrogenase constitutes the committed step in the pyruvate dehydrogenase catalytic sequence and is a logical control point. 

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 not directly a part of the reactions that constitute the citric acid cycle. It is the link between glycolysis and the citric acid cycle, and its activity is controlled by the energy status of the mitochondria. 

The product of this reaction, oxaloacetate, can either enter the gluconeogenic pathway (Chap. 11) by way of malate or condense with acetyl-CoA to yield citrate. Pyruvate carboxylase is an allosteric enzyme, and it is activated by the heterotropic effector, acetyl-CoA. Thus, pyruvate in the mitochondria is the substrate for either pyruvate dehydrogenase or pyruvate carboxylase, the activities of which, in turn, are controlled by reactants associated with the citric acid cycle. The interplay among pyruvate dehydrogenase, pyruvate carboxylase, pyruvate, and the citric acid cycle is shown in Fig. 12-9. 

Fig. 12-8 The control of the pyruvate dehydrogenase complex via phosphory-lation/dephosphorylation of the pyruvate decarboxylase part of the complex.

To understand how the TCA cycle responds kinetically to changes in demand, we can examine the predictions in time-dependent reaction fluxes in response to changes in the primary controlling variable NAD. Figure 6.4 plots predicted reaction fluxes for pyruvate dehydrogenase, aconitase, fumarase, and malate dehydrogenase in response to an instantaneous change in NAD. The initial steady state is obtained... 

The aconitase flux (reaction 3) and the fumarase flux (reaction 8) display overshoots that are small compared to pyruvate dehydrogenase. The aconitase reaction flux reaches a peak value that is only a few percent greater than the final steady state value. The fumarase flux does overshoot the final steady state, but the overshoot is too small to be observed on the scale plotted. This behavior occurs because these reactions are downstream of any reactions directly using NAD as a substrate. Therefore their response is muted compared to reactions that are directly controlled byNAD/NADH. 

After comparing the protein profiles of myocardial mitochondria between a chronic restraint stress group and a control group, 11 protein spots were found to change, of which seven were identified. Five of these proteins, carnitine palmitoyltransferase 2, mitochondrial acyl-CoA thioesterase 1, isocitrate dehydrogenase 3 (NAD ) alpha, fumarate hydratase 1, and pyruvate dehydrogenase beta, were foimd to decrease in abimdance following chronic restraint stress with fimctional roles in the Krebs cycle and lipid metabolism in mitochondria. The other two proteins, creatine kinase and prohibitin, increased after chronic restraint stress (liu et ak, 2004). 

The importance of this covalent control is illustrated in people with a phosphatase deficiency. Because pyruvate dehydrogenase is always phosphorylated and thus inactive, glucose is processed to lactic acid. This condition results in unremitting lactic acidosis (high Wood levels of lactic acid), which leads to the malfunctioning of many tissues, most notably the central nervous system (Section 17.3.2). 

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