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Chapter 19: Problem 15

In most cells, fatty acids are synthesized from acetate units in the cytosol. However, the primary source of acetate units is the TCA cycle in mitochondria, and acetate cannot be transported directly from the mitochondria to the cytosol. Cells solve this problem by exporting citrate from the mitochondria and then converting citrate to acetate and oxaloacetate. Then, because cells cannot transport oxaloacetate into mitochondria directly, they must convert it to malate or pyruvate, both of which can be taken up by mitochondria. Draw a complete pathway for citrate export, conversion of citrate to malate and pyruvate, and import of malate and pyruvate by mitochondria. a. Which of the reactions in this cycle might require energy input? b. What would be the most likely source of this energy? c. Do you recognize any of the enzyme reactions in this cycle? d. What coenzymes might be required to run this cycle?

Short Answer

Expert verified
Energy is likely required for the conversion of oxaloacetate to malate or pyruvate. The most likely source of this energy is ATP. Some enzyme reactions that might be recognizable include those involving aconitase, malate dehydrogenase, and pyruvate kinase. Coenzymes such as NAD+, NADP+, and coenzyme A could be required for these reactions.

Step by step solution

01

Draw the Pathway

Start by drawing the pathway as directed by the exercise. Beginning with citrate exported from mitochondria, it is converted into acetate and oxaloacetate in the cytosol. Then, the oxaloacetate is converted into malate or pyruvate, both of which can be imported back into the mitochondria.
02

Identify Energy-Dependent Reactions

Next, examine the reactions in the cycle carefully. Reactions that convert one form of molecule to another, such as the conversion of oxaloacetate to malate or pyruvate, often require energy input.
03

Determine the Energy Source

The most likely source of energy for these reactions would be from ATP (adenosine triphosphate), which is the principal molecule for storing and transferring energy in cells.
04

Recognize the Enzyme Reactions

Recognition of enzyme reactions within this cycle will be based on prior knowledge. Some known enzymes that might be involved in this cycle include aconitase (converts citrate to isocitrate in the TCA cycle), malate dehydrogenase (converts malate to oxaloacetate in the TCA cycle), and pyruvate kinase (converts phosphoenolpyruvate to pyruvate in glycolysis).
05

Predict Necessary Coenzymes

Necessary coenzymes for these types of reactions often include NAD+ and NADP+ for the dehydrogenase enzymes and coenzyme A, needed for acetyl-CoA production and thus would be required for the incorporation of acetate into the cycle.

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Most popular questions from this chapter

(Integrates with Chapter \(15 .\) ) The serine residue of isocitrate dehydrogenase that is phosphorylated by protein kinase lies within the active site of the enzyme. This situation contrasts with most other examples of covalent modification by protein phosphorylation, where the phosphorylation occurs at a site remote from the active site. What direct effect do you think such active-site phosphorylation might have on the catalytic activity of isocitrate dehydrogenase? (See Barford, D., 1991. Molecular mechanisms for the control of enzymic activity by protein phosphorylation. Biochimica et Biophysica Acta \(1133: 55-62 .\)

Describe the labeling pattern that would result from the introduction into the TCA cycle of glutamate labeled at \(\mathrm{C}_{\gamma}\) with \(^{14} \mathrm{C}\)Describe the labeling pattern that would result from the introduction into the TCA cycle of glutamate labeled at \(\mathrm{C}_{\gamma}\) with \(^{14} \mathrm{C}\)

Aconitase is rapidly inactivated by \(2 R, 3 R\) -fluorocitrate, which is produced from fluoroacetate in the citrate synthase reaction. Interestingly, inactivation by fluorocitrate is accompanied by stoichiometric release of fluoride ion (i.e., one F-ion is lost per aconitase active site \() .\) This observation is consistent with "mechanism-based inactivation" of aconitase by fluorocitrate. Suggest a mechanism for this inactivation, based on formation of 4 -hydroxy-trans-aconitate, which remains tightly bound at the active site. To assess your answer, consult this reference: Lauble, H., Kennedy, M., et al., 1996. The reaction of fluorocitrate with aconitase and the crystal structure of the enzyme-inhibitor complex. Proceedings of the National Academy of Sciences \(93: 13699-13703\)

(Integrates with Chapter 3 .) Aconitase catalyzes the citric acid cycle reaction: Citrate \(\rightleftharpoons\) isocitrate The standard free energy change, \(\Delta G^{\circ \prime}\), for this reaction is +6.7 \(\mathrm{kJ} / \mathrm{mol}\). However, the observed free energy change \((\Delta G)\) for this reaction in pig heart mitochondria is \(+0.8 \mathrm{kJ} / \mathrm{mol}\). What is the ratio of [isocitrate]/[citrate] in these mitochondria? If [isocitrate] = \(0.03 \mathrm{m} M,\) what is [citrate]?

Describe the labeling pattern that would result in the glyoxylate cycle if a plant were fed acetyl-CoA labeled at the \(-\mathrm{CH}_{3}\) carbon.
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