Chapter 18

1. The pyruvate dehydrogenase complex catalyzes the following reaction, linking glycolysis and citric acid cycle:

   Pyruvate + CoA + NAD⁺ → acetyl CoA + NADH + H⁺ + CO₂

2. Pyruvate dehydrogenase catalyzes the decarboxylation of pyruvate and the formation of acetyllipoamide. Dihydrolipoyl transacetylase catalyzes the formation of acetyl CoA. Dihydrolipoyl dehydrogenase catalyzes the reduction of the oxidized lipoic acid. PDH kinase associated with the complex phosphorylates and inactivates the complex, whereas PDH phosphatase dephosphorylates and activates the complex.

3. The remaining steps regenerate oxidized lipoamide, which is required to begin the next reaction cycle. Moreover, this regeneration results in the production of high-energy electrons in the form of NADH.

4. Decarboxylation, oxidation, and transfer of the resultant acetyl group to CoA

5. Oxidation to CO₂ by the citric acid cycle or incorporation into lipids

6. Thiamine pyrophosphate plays a role in the decarboxylation of pyruvate. Lipoic acid (as lipoamide) transfers the acetyl group. Coenzyme A accepts the acetyl group from lipoic acid to form acetyl CoA. FAD accepts the electrons and hydrogen ions when oxidized lipoic acid is reduced. NAD⁺ accepts electrons and a proton from FADH₂.

7. Catalytic coenzymes (TPP, lipoic acid, and FAD) are modified but regenerated in each reaction cycle. Thus, they can play a catalytic role in the processing of many molecules of pyruvate. Stoichiometric coenzymes (coenzyme A and NAD⁺) are used in only one reaction because they are the components of products of the reaction.

8. The electrons are transferred from reduced lipoamide to FAD initially and then to NAD⁺. This transfer is unusual because the electrons are passed to NAD⁺ from FADH₂. The transfer is usually in the other direction.

9. Complete the interactive matching exercise to see answers.

10. In muscle, the acetyl CoA generated by the complex is used for energy generation. Consequently, signals that indicate an energy-rich state (high ratios of ATP/ADP and NADH/NAD⁺) inhibit the complex, whereas the reverse conditions stimulate the enzyme. Calcium as the signal for muscle contraction (and, hence, energy need) also stimulates the enzyme. In liver, acetyl CoA derived from pyruvate is used for biosynthetic purposes, such as fatty acid synthesis. Insulin, the hormone denoting the fed state, stimulates the complex.

11. 1. Enhanced kinase activity will result in a decrease in the activity of the PDH complex because phosphorylation by the kinase inhibits the complex.

    2. Phosphatase activates the complex by removing a phosphoryl group. If the phosphatase activity is diminished, the activity of the PDH complex also will decrease.

12. She might have been ingesting, in some fashion, the arsenite from the wallpaper. The arsenite would have inhibited enzymes that require lipoic acid—notably, the pyruvate dehydrogenase complex.

13. Acetyllipoamide and acetyl CoA

14. The mercury was inhibiting the PDH complex, specifically the transacetylase, by binding to lipoic acid. Many of the symptoms are neurological because the brain uses only glucose as a fuel.

15. Acetyl CoA will inhibit the complex. The metabolism of glucose to pyruvate will be slowed because acetyl CoA is being derived from an alternative source.

16. A thioester is a key intermediate in the formation of 1,3-bisphosphoglycerate from glyceraldehyde 3-phosphate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase. 1,3-Bisphosphoglycerate is subsequently metabolized to pyruvate.

    C19

17. (a) A decrease in the amount of O₂ will necessitate an increase in anaerobic glycolysis for energy production, leading to the generation of a large amount of lactic acid. (b) Under conditions of shock, the kinase inhibitor is administered to ensure that pyruvate dehydrogenase is operating maximally.

18. 1. DCA inhibits pyruvate dehydrogenase kinase.

    2. The fact that inhibiting the kinase results in more dehydrogenase activity suggests that there must be some residual activity that is being inhibited by the kinase.

19. Pyruvate dehydrogenase kinase phosphorylates and inhibits the pyruvate dehydrogenase component of the pyruvate dehydrogenase complex. Inhibiting pyruvate dehydrogenase kinase may be an effective mechanism to increase glucose utilization, thereby lowering blood-glucose levels. Furthermore, increasing pyruvate oxidation may contribute to the lowering of the blood-glucose level by decreasing the supply of gluconeogenic substrates.

20. Thiamine thiazolone pyrophosphate is a transition-state analog. The sulfur-containing ring of this analog is uncharged, and so it closely resembles the transition state of the normal coenzyme in thiamine-catalyzed reactions (e.g., the uncharged resonance form of hydroxyethyl-TPP).