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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?

19. Regulation of Pyruvate Carboxylase The carboxy-lation of pyruvate by pyruvate carboxylase occurs at a very low rate unless acetyl-CoA, a positive allosteric modulator, is present. If you have just eaten a meal rich in fatty acids (tri-acylglycerols) but low in carbohydrates (glucose), how does this regulatory property shut down the oxidation of glucose to CO2 and H2O but increase the oxidation of acetyl-CoA derived from fatty acids?

20. Relationship between Respiration and the Citric Acid Cycle Although oxygen does not participate directly in the citric acid cycle, the cycle operates only when O2 is present. Why?

21. Thermodynamics of Citrate Synthase Reaction in Cells Citrate is formed by the condensation of acetyl-CoA with oxaloacetate, catalyzed by citrate synthase:

Oxaloacetate + acetyl-CoA + H2O

In rat heart mitochondria at pH 7.0 and 25 °C, the concentrations of reactants and products are: oxaloacetate, 1 ^m; acetyl-CoA, 1 ^m; citrate, 220 ^m; and CoA, 65 ^m. The standard free-energy change for the citrate synthase reaction is —32.2 kJ/mol. What is the direction of metabolite flow through the citrate synthase reaction in rat heart cells? Explain.

22. Reactions of the Pyruvate Dehydrogenase Complex

Two of the steps in the oxidative decarboxylation of pyruvate (steps © and © in Fig. 16-6) do not involve any of the three carbons of pyruvate yet are essential to the operation of the PDH complex. Explain.

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Atkins Low Carb Diet Recipes

Atkins Low Carb Diet Recipes

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