biochem chapter 19

1) Which of the following situations or phrases most accurately describes the citric acid cycle? - "All roads lead to Rome." - It is like a chaotic, unregulated traffic circle. - The shortest distance between two points is a straight line.

- "All roads lead to Rome."

4) Which intermediates in the citric acid cycle are also used for the biosynthesis of amino acids? - Citrate - -ketoglutarate and oxaloacetate - Succinyl CoA

- -ketoglutarate and oxaloacetate

3) Which enzyme of the citric acid cycle most closely resembles the pyruvate dehydrogenase complex in terms of its structure, organization, and the reaction it performs? - Isocitrate dehydrogenase - -ketoglutarate dehydrogenase - Succinate dehydrogenase

- -ketoglutarate dehydrogenase

6) Assuming that pyruvate was labeled with radioactive 14C in its middle (keto) carbon, where would the radiolabel appear after one turn of the citric acid cycle? - In one carboxyl group of oxaloacetate - Equally divided between the two carboxyl groups of oxaloacetate - As CO2

- Equally divided between the two carboxyl groups of oxaloacetate

2) Which two enzymes within the citric acid cycle are allosterically inhibited by ATP? - Citrate synthase and aconitase - Fumarase and malate dehydrogenase - Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase

- Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase

8) Of the enzymes of the citric acid cycle, which is not a freely soluble enzyme? - Citrate synthase - Succinate dehydrogenase - Malate dehydrogenase

- Succinate dehydrogenase

5) In going from acetyl-CoA to succinate, two carbons have entered the cycle and two have been released as CO2. Why is the cycle not considered as complete at this point? - Not enough NADH has been generated. - The cell requires FADH2 produced in subsequent reactions of the cycle. - The oxaloacetate used to initiate the cycle must be regenerated.

- The oxaloacetate used to initiate the cycle must be regenerated.

7) In the reaction catalyzed by succinyl-CoA synthetase: - GDP (or ADP) is phosphorylated by a succinyl phosphate. - a succinylphosphate transfers its phosphate to a histidyl residue. - a phosphohistidyl residue is used to displace the CoA from succinate.

- a succinylphosphate transfers its phosphate to a histidyl residue.

10) Although not present in animals, many plants and microorganisms can use a modified form of the citric acid cycle known as the glyoxylate cycle. This is beneficial in that: - it provides a source of glyoxylate which is an essential molecule in biosynthetic pathways in these organisms. - it allows the organisms which possess it to convert acetyl CoA into glucose precursors. - it does not release CO2 which is toxic to these organisms.

- it allows the organisms which possess it to convert acetyl CoA into glucose precursors.

9) The purpose of anaplerotic reactions is: - to replenish the citric acid cycle if it becomes depleted of intermediates by biosynthetic demands. - to divert materials out of the citric acid cycle for use in biosynthesis. - to produce pyruvate to initiate the cycle when glucose degradation is not occurring.

- to replenish the citric acid cycle if it becomes depleted of intermediates by biosynthetic demands.

The citric acid cycle has a 8 step mechanism

1. Acetyl CoA + oxaloacetate +H2O --> citrate + CoA +H+ 2. Citrate <--> cis-aconitate + H2O 3. cis Aconitate+ H2O <--> Isocitrate 4. Isocitrate + NAD+ <--> α-ketoglutarate + CO2+ NADH 5. α-Ketoglutarate + NAD+ + CoA <--> succinyl CoA + CO2 + NADH 6. Succinyl CoA + Pi + ADP <--> succinate + ATP + CoA 7. Succinate + FAD(enzyme-bound) <--> fumarate + FADH2 (enzyme bound) 8. Fumarate + H2O <--> L-malate 9. L-Malate + NAD+ <--> oxaloacetate + NADH + H+ net reaction: Acetyl CoA +3NAD+ + FAD + ADP + Pi + 2H2O --> 2CO2 + 3NADH + FADH2 + ATP + 2H+ + CoA

What are the five enzymes (including regulatory enzymes) that constitute the pyruvate dehydrogenase complex? Which reactions do they catalyze?

1. Pyruvate Dehydrogenase (E1)- catalyzes the oxidative decarboxylation of pyruvate and the formation of acetyllipoamide. 2. Dihydrolipoyl transacetylase (E2)- catalyzes the transfer of acetyl group to CoA. 3. Dihydrolipoyl Dehydrogenase (E3)- catalyzes the regeneration of the oxidized form of lipoamide. 4. PDH Kinase- phosphorylates and switches off the activity of the complex. 5. PDH Phosphatase- reverses the deactivation by dephosphorylating and activating the complex.

33. Approximately how many ATP or GTP equivalents are produced during one turn of the citric acid cycle? A) 10 B) 6 C) 9 D) 12 E) None of the above.

A) 10

32. Succinate dehydrogenase deficiency or genetic mutation results in: A) cancer through HIF-1. B) decreased rate of gylcolysis. C) depletion of succinate in mitochondria. D) A and B. E) A, B, and C.

A) cancer through HIF-1.

29. In which reaction is ATP directly formed in the citric acid cycle? A) conversion of succinyl CoA to succinate B) decarboxylation of α-ketoglutarate C) conversion of isocitrate to α-ketoglutarate D) All of the above. E) None of the above.

A) conversion of succinyl CoA to succinate

25. Which of the following vitamins are precursors to coenzymes that are necessary for the formation of succinyl CoA from α-ketoglutarate? A) thiamine, riboflavin, niacin, lipoic acid, and pantothenic acid B) thiamine, riboflavin, niacin, lipoic acid, pantothenic acid, and biotin C) thiamine, riboflavin, niacin, and biotin D) thiamine, riboflavin, and lipoic acid E) None of the above.

A) thiamine, riboflavin, niacin, lipoic acid, and pantothenic acid

26. Isomerization of citrate is catalyzed by: A) citrate synthase. B) aldolase. C) α-ketogutarate dehydrogenase. D)---

Answer is D 19.2

31. Which of these compounds is oxidized by a multienzyme complex that requires five different coenzymes? A) -O2C C H C CO2 - H B) -O2C CH2 CH2 C CO2 - O C) -O2C CH2 C CO2 - O D) -O2C CH2 CH OH CO2 - E) CH3 C SCoA

B) -O2C CH2 CH2 C CO2

35. The glyoxylate cycle enables plants to survive using only: A) pyruvate. B) acetate. C) oxaloacetate. D) All of the above. E) None of the above.

B) acetate.

22. What molecule initiates the citric acid cycle by reacting with oxaloacetate? A) pyruvate B) acetyl CoA C) oxaloacetate D) All of the above. E) None of the above.

B) acetyl CoA

24. The direct channeling of proteins from one enzyme to the next will happen in a(n): A) protein complex. B) metabolon. C) linker coenzyme. D) cell with sufficient available water. E) electron acceptor.

B) metabolon.

21. A mutation in the active site of succinyl CoA synthetase where His is converted to a lysine would result in which? A) increased stable folding B) the loss of a succinyl phosphate intermediate C) the loss of a positive-charged amino acid necessary for catalysis D) All of the above E) None of the above

B) the loss of a succinyl phosphate intermediate

27. Formation of citrate from acetyl CoA and oxaloacetate is a(n) _________ reaction. A) oxidation B) reduction C) condensation D) ligation E) None of the above.

C) condensation

30. In which step of the citric acid cycle is FADH2 formed? A) the conversion of succinate to malate B) the conversion of succinate to oxaloacetate C) the conversion of succinate to fumarate D) the conversion of malate to oxaloacetate E) None of the above.

C) the conversion of succinate to fumarate

28. What is/are the chemical change(s) involved in the conversion of citrate into isocitrate? A) hydration followed by dehydration B) oxidation C) oxidation followed by reduction D) dehydration followed by hydration E) A and B

D) dehydration followed by hydration

23. In muscle, the enzyme used to catalyze a substrate-level phosphorylation is: A) nucleoside triphosphate transferase. B) protein kinase C. C) GTP kinase. D) succinyl CoA synthetase. E) ATP-GTP transferase.

D) succinyl CoA synthetase.

34. In addition to pyruvate dehydrogenase, what other enzymes are key regulatory sites in the citric acid cycle? A) isocitrate dehydrogenase B) α-ketoglutarate dehydrogenase C) citrate synthase (in bacteria) D) A and B E) A, B, and C

E) A, B, and C

Citrate synthase is well suited to the hydrolysis of citryl CoA but not acetyl CoA. How is this discrimination accomplished?

First, acetyl CoA does not bind to the enzyme until oxaloacetate is bound and ready for condensation. Second, the catalytic residues crucial for the hydrolysis of the thioester linkage are not appropriately positioned until citryl CoA is formed. Induced fit prevents an undesirable side reaction.

What signals regulate the rate of the cycle?

Isocitrate dehydrogenase is allosterically stimulated by ADP, which signifies the need for more energy. NADH inhibits A second control site in the citric acid cycle is a-ketoglutarate dehydrogenase, inhibited by high levels of ATP. The rate of the cycle is decreased when the cell has high levels of ATP and NADH. Other enzymes in the cycle require NAD+ or FAD. These enzymes will function only when the energy charge is low, because only then will NAD+ and FAD be available.

What reaction serves to link glycolysis and the citric acid cycle, and what is the enzyme that catalyzes the reaction?

The irreversible conversion of pyruvate into acetyl CoA by pyruvate dehydrogenase complex links the two reactions together. Acetyl CoA is the fuel for the citric acid cycle. Pyruvate + CoA + NAD+ --> acetyl CoA + NADH + H+ + CO2

glyoxylate cycle

a metabolic pathway that allows the conversion of acetyl CoA generated from fat stores into glucose. important difference is that two molecules of acetyl CoA enter per turn Citrate is then isomerized to isocitrate. isocitrate is cleaved by isocitrate lyase into succinate and glyoxylate. acetyl CoA condenses with glyoxylate to form malate in a reaction catalyzed by malate synthase, and then malate is oxidized to oxaloacetate,

The citric acid cycle is part of aerobic respiration, but no O2 is required for the cycle. Explain this paradox.

the citric acid is a part of aerobic respiration because the NAD+ that is required for the cycle is generated when NADH reacts with O2. Without the O2 in place to regenerate NAD+ there can be no citric acid cycle

How is this energy utilized in the citric acid cycle?

the cleavage of the thioester bond of succinyl CoA is coupled to the phosphorylation of a purine nucleoside diphosphate, usually ADP. This reaction is catalyzed by succinyl CoA synthetase (succinate thiokinase).

regulation to outside CAC

the inhibition of isocitrate dehydrogenase leads to a buildup of citrate, (because the interconversion of isocitrate/citrate.) Citrate can be transported to the cytoplasm where it signals phosphofructokinase to halt glycolysis The a-ketoglutarate that accumulates when a-ketoglutarate dehydrogenase is inhibited can be used as a precursor for several amino acids and the purine bases

The key catabolic function of the citric acid cycle

the production of high-energy electrons in the form of NADH and FADH2.