Deck 17: The Citric Acid Cycle

Question 1

Carbons from carbohydrates enter the citric acid cycle in the form of _______________.

Answer 1

acetyl CoA

Question 2

E2 of the pyruvate dehydrogenase complex contains a lipoyl group that is covalently attached to a _______________ residue of the enzyme.

Answer 2

lysine

Question 3

Beri-beri is caused by a deficiency of __________________.

Answer 3

thiamine

Question 4

_______________ is a citric acid cycle enzyme that is also an example of an iron-sulfur protein.

Question 5

What is the enzyme-bound intermediate in the conversion of citrate and isocitrate?

A) oxalosuccinate
B) dihydrocitrate
C) cis-aconitate
D) hydroxyethyl-TPP
E) fumarate

Question 6

________________ is the first citric acid cycle intermediate to be oxidized.

Question 7

When intermediates of the citric acid cycle are depleted by biosynthetic pathways,_____ reactions are used to replenish the citric acid cycle intermediates.

A) anaplerotic
B) amphibolic
C) amphoteric
D) biosynthetic
E) None of the answers is correct.

Question 8

Which of the following citric acid cycle intermediates is found at the traditional start and finish of the cycle?

A) citrate
B) isocitrate
C) $\alpha$ -ketoglutarate
D) succinate
E) oxaloacetate

Question 9

What type of enzymes are tightly associated with FAD or FMN?

A) transferases
B) cytochrome P450 enzymes
C) metalloenzymes
D) flavoproteins
E) None of the answers is correct.

Question 10

Where does the citric acid cycle take place in the cell?

A) cytosol
B) mitochondria
C) nucleus
D) endoplasmic reticulum
E) lysosome

Question 11

The ____________ cycle is a process by which plants and some bacteria can convert two-carbon acetyl units into four-carbon units (succinate)for glucose synthesis,energy production,and biosynthesis.

Question 12

Which of the following is the primary mode of regulation for the pyruvate dehydrogenase complex?

A) feedback inhibition by citrate
B) feed-forward activation by glucose
C) allosteric activation by ATP and NADH
D) allosteric inhibition by ADP and CO₂
E) inhibition by phosphorylation

Question 13

In general,the citric acid cycle is inhibited under ________ (high,low)energy conditions.

Question 14

Where in the cell is succinate dehydrogenase located?

A) inner mitochondrial membrane
B) outer mitochondrial membrane
C) mitochondrial matrix
D) intermembrane space
E) cytosol

Question 15

What cycle allows for the net synthesis of glucose from acetyl-CoA?

A) Calvin cycle
B) glyoxylate cycle
C) C₄ pathway
D) urea cycle
E) None of the answers is correct.

Question 16

E1 of the pyruvate dehydrogenase complex requires the coenzyme ________________ for proper activity.

Question 17

Which of the following is considered a product of the citric acid cycle?

A) acetate
B) $\alpha$ -ketoglutarate
C) NAD⁺
D) oxaloacetate
E) carbon dioxide

Question 18

Most organisms cannot convert ______________________ into glucose because of the two decarboxylations in the citric acid cycle.

Question 19

In the citric acid cycle,one __________ is produced by a substrate-level phosphorylation.

Question 20

Which of the following is toxic due to its ability to react with neighboring sulfhydryls of dihydrolipoyl groups,thus blocking their reoxidation to lipoamide?

A) fluoroacetate
B) carbon monoxide
C) arsenite
D) rotenone
E) cyanide

Question 21

What enzyme(s)is (are)responsible for the following reaction?
Pyruvate + CoA + NAD⁺ $\rarr$ acetyl CoA + NADH + H⁺ + CO₂

A) acetyl CoA synthetase
B) pyruvate decarboxylase
C) pyruvate dehydrogenase complex
D) pyruvate carboxylase
E) None of the answers is correct.

Question 22

Which of the following conditions will activate pyruvate dehydrogenase kinase,which catalyzes the phosphorylation and inactivation of E₁ in the pyruvate dehydrogenase complex?

A) elevated concentrations of NADH and ATP
B) elevated concentrations of NAD⁺ and ADP
C) Ca²⁺
D) glucagon
E) elevated concentrations of coenzyme A

Question 23

Why is it necessary that there be a mechanism to replenish oxaloacetate?

Question 24

Which of the following functions as a "flexible swinging arm" when it transfers the reaction intermediate from one active site to the next?

A) FAD
B) NAD⁺
C) lipoamide
D) thiamine pyrophosphate
E) coenzyme A

Question 25

The glyoxylate cycle enables plants to survive using only

A) pyruvate.
B) acetate.
C) oxaloacetate.
D) lactate.
E) None of the answers is correct.

Question 26

In addition to pyruvate dehydrogenase,which of the following enzymes is a key regulatory site in the citric acid cycle?

A) malate dehydrogenase
B) $\alpha$ -ketoglutarate dehydrogenase
C) succinyl CoA synthetase
D) succinate dehydrogenase
E) None of the answers is correct.

Question 27

Give the net equation of the citric acid cycle.

Question 28

Why is the isomerization of citrate to isocitrate a necessary step of the citric acid cycle?

Question 29

In which reaction is GTP (or ATP)directly formed in the citric acid cycle?

A) conversion of succinyl CoA to succinate
B) decarboxylation of $\alpha$ -ketoglutarate
C) conversion of isocitrate to $\alpha$ -ketoglutarate
D) condensation of acetyl-CoA and oxaloacetate
E) None of the answers is correct.

Question 30

What are the steps involved (in order)in the conversion of pyruvate to acetyl-CoA?

A) decarboxylation, oxidation, transfer to CoA
B) decarboxylation, transfer to CoA, oxidation
C) oxidation, decarboxylation, transfer to CoA
D) oxidation, transfer to CoA, decarboxylation
E) None of the answers is correct.

Question 31

Explain why a GTP is energetically equivalent to an ATP in metabolism.

Question 32

Which of the following vitamins are precursors to coenzymes that are necessary for the formation of acetyl CoA from pyruvate?

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 folic acid
E) None of the answers is correct.

Question 33

The citric acid cycle is also known as the

A) Krebs cycle.
B) Cori cycle.
C) tricarboxylic acid cycle.
D) acetate cycle.
E) Krebs cycle and tricarboxylic acid cycle.

Question 34

Approximately how many ATP or GTP equivalents will be produced from energy molecules produced during one turn of the citric acid cycle?

A) 10
B) 6
C) 9
D) 12
E) None of the answers is correct.

Question 35

Give the reaction in the citric acid cycle by which the energy is conserved in the formation of a phosphoanhydride bond by substrate level phosphorylation.Give the name of the enzyme that catalyzes this reaction and give the structures of the reactants and products of this reaction.

Question 36

Formation of citrate from acetyl CoA and oxaloacetate is a(n)_________ reaction.

A) oxidation
B) reduction
C) condensation
D) ligation
E) None of the answers is correct.

Question 37

Which of the following enzymes are found in the glyoxylate cycle but not in the citric acid cycle?

A) malate synthase and isocitrate dehydrogenase
B) glyoxylate synthase and malate synthase
C) isocitrate lyase and malate dehydrogenase
D) malate synthase and isocitrate lyase
E) succinyl CoA synthetase and glyoxylate synthase

Question 38

What molecule initiates the citric acid cycle by reacting with oxaloacetate?

A) pyruvate
B) acetyl CoA
C) oxaloacetate
D) glucose
E) None of the answers is correct.

Question 39

What is/are the chemical change(s)involved in the conversion of citrate into isocitrate?

A) hydration followed by dehydration
B) dehydration followed by hydration
C) oxidation followed by reduction
D) reduction followed by oxidation
E) None of the answers is correct.

Question 40

List the five coenzymes that are required for the oxidative decarboxylation of pyruvate and α-ketoglutarate,and give the essential nutrient (vitamin)that is required for each.

Question 41

How does the term "mad as a hatter" realistically reflect the condition?

Question 42

What is the energy source that drives the condensation of oxaloacetate and acetyl CoA to produce citrate?

Question 43

For a single rotation of the Krebs cycle,are the acetyl carbons that enter the citric acid cycle the exact same carbons that leave as CO₂? Briefly explain.

Question 44

How is succinate dehydrogenase unique when compared to the other enzymes in the citric acid cycle?

Question 45

How does the decarboxylation of $\alpha$ -ketoglutarate resemble that of pyruvate decarboxylation?

Question 46

How many ATP equivalents are produced from the total oxidation of one pyruvate to three CO₂?

Question 47

The ?G??=-21 kJ mol^-1 for the reaction catalyzed by isocitrate dehydrogenase,yet the ?G?? = +29.7 kJ mol^-1 for the reaction catalyzed by malate dehydrogenase.Both of these reactions involve the oxidation of a secondary alcohol.Give an explanation as to why the oxidation of isocitrate is so exergonic.

Question 48

Starting with oxaloacetate in the glyoxylate cycle,identify what molecules enter and exit the glyoxylate cycle.