BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 13. THE CITRIC ACID CYCLE

QUESTIONS AND PROBLEMS

1. What is The Fate of the radioactive label if each of the following compounds is added to a Cell extract containing the Enzymes and Cofactors of The Glycolytic Pathway and The Citric Acid Cycle, as well as the Pyruvate dehydrogenase complex? (The ¹⁴C label is indicated by an asterisk.)

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2. Will oxaloacetate accumulate if acetyl-CoA is added to an extract containing the enzymes and cofactors of The Citric Acid cycle?

3. What are the relative concentrations of citrate, isocitrate, and cis-aconitate at equilibrium? (Use the data in Table 13.1.)

4. What is the value of ∆G⁰' for The oxidation of the acetyl moiety of acetyl-CoA in the citric acid cycle?

5. Incubation of H₃C—CD₂—OH and NAD⁺ with Alcohol dehydrogenase yielded a preparation of deuterated reduced NAD. This reduced coenzyme was added to a solution of 1,3-BPG and glyceraldehyde-3-phosphate dehydrogenase. The NAD⁺ formed in this second reaction contains one deuterium atom, whereas the glyceraldehyde-3-phosphate molecule, the other reaction product, contains no deuterium. What Conclusion regarding the stereospecificity of glyceraldehyde-3-phosphate dehydrogenase can be drawn from this experiment?

6. Thiamin thiazolone pyrophosphate binds to pyruvate dehydrogenase approximately 20,000 times more strongly than thiamin pyrophosphate and competitively inhibits the enzyme. Why?

7. The NAD⁺-linked oxidation of malate to form oxaloacetate is a highly endergonic reaction under standard conditions (∆G⁰' = + 7 kcal/mol). Under physiological conditions, the reaction proceeds readily in the direction from malate to oxaloacetate because the equilibrium concentrations of the products are low compared to those of the substrates. Assuming that the [NAD⁺]/[NADH] ratio is 8 and the pH is 7, calculate the minimum [Malate]/[Oxaloacetate] ratio at which The formation of oxaloacetate from malate can occur.