Citrate synthase regulation

Citrate synthase is the first step in this metabolic pathway, and as stated the reaction has a large negative AG°. As might be expected, it is a highly regulated enzyme. NADH, a product of the TCA cycle, is an allosteric inhibitor of citrate synthase, as is succinyl-CoA, the product of the fifth step in the cycle (and an acetyl-CoA analog). 

At least two enzymes compete for acetyl-CoA - the citrate synthase and 3-ke-tothiolase. The affinities of these enzymes differ for acetyl-CoA (Table l),and at low concentrations of it the citrate synthase reaction tends to dominate, provided that the concentration of 2/H/ is not inhibiting. The fine regulation of the citrate synthases of various poly(3HB) accumulating bacteria has been studied [ 14, 47, 48]. They appear to be controlled by cellular energy status indicators (ATP, NADH, NADPH) and/or intermediates of the TCA cycle. The 3-ketothio-lase has also been investigated [10-14,49, 50]. This enzyme is, above all, inhibited by CoASH [10,14,49]. This important feature will be further considered below. 

Citrate synthase, isocitrate dehydrogenase and oxogluta-rate dehydrogenase are key enzymes regulating the flux through the cycle all three catalyse non-equilibrium reactions (Chapter 3). 

The theory of regulation of the cycle is as follows. First, an increase in oxaloacetate concentration increases the activity of citrate synthase and hence the cycle. The concentration of oxaloacetate is regulated by the activity of the enzyme pyruvate carboxylase, which catalyses the reaction ... 

Figure 20.26 Regulation of the activity of aminoacyl-tRNA synthetases and hence the rate of peptide synthesis by the concentration of free tRNAs, and regulation of activity of citrate synthase and hence the rate of the Krebs cycle by the concentration of oxaloacetate. Note that there are two positions in which external regulation occurs in the peptide sythetase pathway and two positions in the Krebs cycle (Chapter 9).

The most important factor in the regulation of the cycle is the NADH/NAD ratio. In addition to pyruvate dehydrogenase (PDH) and oxoglu-tarate dehydrogenase (ODH see p. 134), citrate synthase and isodtrate dehydrogenase are also inhibited by NAD deficiency or an excess of NADH+HT With the exception of isocitrate dehydrogenase, these enzymes are also subject to product inhibition by acetyl-CoA, suc-cinyl-CoA, or citrate. 

Remington, S.J. (1992) Structure and mechanism of citrate synthase. Curr. Tap. Cell. Regul. 33, 209-229. 

Regulation of Citrate Synthase In the presence of saturating amounts of oxaloacetate, the activity of citrate synthase from pig heart tissue shows a sigmoid dependence on the concentration of acetyl-CoA, as shown in the graph. When succinyl-CoA is added, the curve shifts to the right and the sigmoid dependence is more pronounced. 

On the basis of these observations, suggest how succinyl-CoA regulates the activity of citrate synthase. (Hint See Fig. 6-29.) Why is succinyl-CoA an appropriate signal for regulation of the citric acid cycle How does the regulation of citrate synthase control the rate of cellular respiration in pig heart tissue ... 

The condensation of acetyl CoA and oxaloacetate to form citrate is catalyzed by citrate synthase (Figure 9.5). This aldol condensation has an equilibrium far in the direction of citrate synthesis. Citrate synthase is allosterically activated by Ca2+ and ADP, and inhibited by ATP, NADH, succinyl CoA, and fatty acyl CoA derivatives (see Figure 9.9). However, the primary mode of regulation is also deter mined by the availability of its substrates, acetyl CoA and oxaloac etate. [Note Citrate, in addition to being an intermediate in the TCA cycle, provides a source of acetyl CoA for the cytosolic synthesis of... 

Citrate Synthase Is Negatively Regulated by NADH and the Energy Charge... 

Thus far we have discussed the two most important enzymes regulating the supplies of acetyl-CoA and oxaloacetate for the TCA cycle. This leaves us with the three enzymes, all within the cycle, that regulate the activity of the cycle. The first of these, citrate synthase, catalyzes the formation of citrate from acetyl-CoA and oxaloacetate. Regulation, by energy charge and other parameters, of the rate of glycolysis and of the pyruvate dehydrogenase reaction play... 

ADP, acetyl-CoA, succinyl-CoA, and citrate. The major known sites for regulation of the cycle include two enzymes outside the cycle (pyruvate dehydrogenase and pyruvate carboxylase) and three enzymes inside the cycle (citrate synthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase). All of these sites of regulation represent important metabolic branchpoints. 

Citrate synthase catalyzes the step at which acetyl-CoA enters the cycle, and thus regulation of this enzyme controls the activity of the cycle, the rate of production of reduced coenzymes, and thus the rate of cellular respiration. 

Citrate synthase is a highly regulated enzyme. What is one of the ways this citrate synthase reaction is driven forward ... 

TCA cycle substrates oxaloacetate and acetyl-CoA and the product NADH are the critical regulators. The availability of acetyl-CoA is regulated by pyruvate dehydrogenase complex. The TCA cycle enzymes citrate synthase. 

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