The Citric Cycle Flashcards
The citric acid cycle is the only metabolic pathway that can be used both as an anabolic and as
a catabolic pathway.
a. True
b. False
b. False
Which of the following statements concerning the citric acid cycle as the central metabolic
pathway is true?
a. It is involved in the metabolism of sugars and amino acids.
b. It is involved in the metabolism of amino acids and lipids.
c. It links anaerobic metabolism to aerobic metabolism.
d. Many of its intermediates are starting points for synthesis of a variety of compounds.
e. All of these are reasons why the citric acid cycle is considered to be the central
pathway.
e. All of these are reasons why the citric acid cycle is considered to be the central
pathway.
The citric acid cycle is amphibolic, meaning
a. it plays a role in both anabolism and catabolism.
b. it is essentially irreversible.
c. it can operate both in the presence and absence of
oxygen.
d. it can oxidize carbons and nitrogens equally well.
e. none of these
a. it plays a role in both anabolism and catabolism.
Roughly how many more ATP’s can be produced via the complete aerobic oxidation of glucose
compared to that produced by glycolysis alone?
a. twice as much
b. five times as much
c. fifteen times as much
d. thirty-two times as
much
e. none of these
c. fifteen times as much
Which of the following does not play a role in the overall aerobic metabolism of glucose?
a. citric acid cycle
b. electron transport
c. oxidative phosphorylation
d. urea cycle
e. all of these play a role in overall aerobic metabolism of
glucose
d. urea cycle
Which enzymes in the citric acid cycle catalyze oxidative decarboxylation reactions?
a. isocitrate dehydrogenase and the α-ketoglutarate dehydrogenase
complex
b. aconitase and succinate dehydrogenase
c. the α-ketoglutarate dehydrogenase complex and succinate thiokinase
d. fumarase and succinate dehydrogenase
a. isocitrate dehydrogenase and the α-ketoglutarate dehydrogenase
complex
In which cellular location do the majority of the reactions of the citric acid cycle take place?
a. the cytosol.
b. the mitochondrial matrix.
c. the endoplasmic
reticulum.
d. lysosomes.
e. none of these
b. the mitochondrial matrix.
The immediate electron acceptor for the majority of the oxidative reactions of the citric acid
cycle is
a. ATP.
b. NAD.
c. FAD.
d. coenzyme A.
e. none of these
b. NAD.
The citric acid cycle produces one high energy phosphate molecule per cycle directly in the
form of ____
a. ATP
b. GTP
c. CTP
d. AMP
e. none of these
b. GTP
The reaction of the citric acid cycle that does not take place in the mitochondrial matrix is the
one catalyzed by:
a. fumarase
b. citrate synthase
c. isocitrate dehydrogenase
d. succinate dehydrogenase
e. All of these reactions take place in the
matrix
d. succinate dehydrogenase
Which of the following is not a component of the pyruvate dehydrogenase complex?
a. pyruvate dehydrogenase
b. dihydrolipoyl transacetylase
c. dihydrolipoyl dehydrogenase
d. pyruvate dehydrogenase kinase
e. aconitase
e. aconitase
Thiamine pyrophosphate carries a ____ carbon unit.
a. 1
b. 2
c. 3
d. 4
b. 2
The acetyl group is carried on lipoic acid as
a. an alcohol.
b. a thioester.
c. a phosphoanhydride.
d. an amide.
b. a thioester.
Which of the following vitamins and enzyme cofactors are used by the pyruvate
dehydrogenase complex during oxidative decarboxylation?
a. Lipoic Acid.
b. Niacin.
c. Pantothenic
Acid.
d. Thiamine.
e. All of these
e. All of these
Which of the following is not a reaction occurring during oxidative decarboxylation of
pyruvate?
a. Removal of CO2.
b. Oxidation of an acetate group.
c. Addition of Coenzyme A to a 2-carbon fragment.
d. Reduction of NAD+
e. All of these reactions take place during oxidative
decarboxylation.
b. Oxidation of an acetate group.
The enzymes involved in the pyruvate dehydrogenase complex are
a. physically separated from each other
b. crosslinked to each other by lipoic acid linkers
c. covalently bonded to coenzyme A
d. associated with each other in an ordered and complex array
d. associated with each other in an ordered and complex array
Lipoic acid is a required vitamin in the human diet.
a. True
b. False
b. False
Each of the enzymes of the pyruvate dehydrogenase complex requires a different vitamin.
a. True
b. False
a. True
Which group of small molecules best fit the boxes associated with the reaction shown?
(see photo)
a. I
b. II
c. III
d. IV
b. II
The reactions in which succinate is converted to oxaloacetate are, in order
a. an oxidation, a dehydration, and an oxidation
b. three successive oxidation reactions
c. an oxidative decarboxylation, a dehydration, and a
condensation
d. a condensation, a dehydration, and an oxidative decarboxylation
a. an oxidation, a dehydration, and an oxidation
All but one of the enzymes of the citric acid cycle are found in this part of the mitochondrion:
a. The outer membrane.
b. The inner membrane.
c. The mitochondrial matrix.
d. The intermembrane space.
e. It is not known where these enzymes are located.
c. The mitochondrial matrix.
Which of the following enzymes contains a non-heme iron?
a. citrate synthase
b. succinyl-CoA
synthetase
c. succinate
dehydrogenase
d. fumarase
c. succinate
dehydrogenase
Which group of small molecules best fit the boxes associated with the reaction shown?
(see photo)
a. I
b. II
c. III
d. IV
d. IV
Which group of small molecules best fit the boxes associated with the reaction shown?
(see photo)
a. I
b. II
c. III
d. IV
a. I
Which of the following reactions involves substrate-level phosphorylation?
a. isocitrate → α-ketoglutarate
b. citrate → isocitrate
c. succinate → fumarate
d. succinyl-CoA → succinate
e. acetyl-CoA + oxaloacetate → citrate
d. succinyl-CoA → succinate
Which enzyme catalyzes the reaction shown?
(see photo)
a. isocitrate dehydrogenase
b. pyruvate dehydrogenase
c. fumarase
d. succinate
dehydrogenase
e. none of these
e. none of these
Which coenzyme listed below is not associated with the α-ketoglutarate dehydrogenase
complex?
a. thiamine
pyrophosphate
b. lipoic acid
c. biotin
d. NAD+
c. biotin
Which of the following enzymes is the only membrane-bound enzyme in the citric acid cycle?
a. Aconitase.
b. IsoCitrate Dehydrogenase.
c. Succinate Dehydrogenase.
d. Malate Dehydrogenase.
e. Alpha-Ketoglutarate Dehydrogenase
complex.
c. Succinate Dehydrogenase.
The iron ion, which is part of succinate dehydrogenase, is bonded to heme.
a. True
b. False
b. False
Which enzyme catalyzes the reaction shown?
(see photo)
a. isocitrate dehydrogenase
b. malate dehydrogenase
c. fumarase
d. succinate
dehydrogenase
e. none of these
d. succinate
dehydrogenase
Which enzyme catalyzes the reaction shown?
(see photo)
a. succinyl-CoA synthetase
b. succinate dehydrogenase
c. pyruvate dehydrogenase
d. α-ketoglutarate
dehydrogenase
a. succinyl-CoA synthetase
When acetyl-CoA reacts with oxaloacetate to form citrate
a. a new carbon-carbon bond is formed
b. an oxidative decarboxylation reaction takes place
c. a dehydration reaction takes place
d. a rearrangement takes place
e. none of these
a. a new carbon-carbon bond is formed
The order of compounds and intermediates found in the citric acid cycle is as follows:
a. IsoCitrate → Aconitate → α-Ketoglutarate → Fumarate → Malate → Oxaloacetate
b. Aconitate → IsoCitrate → Oxaloacetate → α-Ketoglutarate → Malate → Fumarate
c. Aconitate → IsoCitrate → α-Ketoglutarate → Fumarate → Malate → Oxaloacetate
d. Aconitate → IsoCitrate → α-Ketoglutarate → Malate → Fumarate → Oxaloacetate
e. IsoCitrate → Aconitate → α-Ketoglutarate → Malate → Oxaloacetate → Fumarate
c. Aconitate → IsoCitrate → α-Ketoglutarate → Fumarate → Malate → Oxaloacetate
Release of succinate from succinyl-CoA can be coupled to GTP synthesis because:
a. The amide bond between succinate and CoA has a large −ΔG of hydrolysis.
b. The thioester bond between succinate and CoA has a large −ΔG of hydrolysis.
c. The link between succinate and CoA involves an acid anhydride to
phosphate.
d. Coenzyme A is a “high energy” compound, just like GTP.
e. None of these explains why GTP can be formed during this reaction.
b. The thioester bond between succinate and CoA has a large −ΔG of hydrolysis.
The only difference between succinate and fumarate is the geometry around their double bonds, one contains a cis double bond and the other contains a trans double bond.
a. True
b. False
b. False
The enzyme “aconitase” is also known as “condensing enzyme”
a. True
b. False
b. False
Fluorine is related to the citric acid cycle because:
a. fluoroacetyl-CoA is also a substrate for citrate synthase
b. fluoroacetate, found in poisonous plants, acts as an inhibitor of aconitase
c. fluorocitrate acts as a potent inhibitor of the citric acid cycle
d. all of these
d. all of these
Which of the reactions of the citric acid cycle requires FAD as a coenzyme?
a. the conversion of isocitrate to α-ketoglutarate
b. the conversion of citrate to isocitrate
c. the conversion of succinate to fumarate
d. the conversion of malate to oxaloacetate
e. none of these
c. the conversion of succinate to fumarate
The conversion of citrate to isocitrate is remarkable because
a. it is a condensation reaction.
b. a chiral center is introduced in a molecule that did not have one previously.
c. a dehydration reaction is involved.
d. the enzyme that catalyzes it has very little specificity.
b. a chiral center is introduced in a molecule that did not have one previously.
In the conversion of succinyl-CoA to succinate, GTP is produced from GDP in a reaction in
which the source of the added phosphate is
a. ATP.
b. ADP.
c. phosphenolpyruvate.
d. inorganic phosphate ion.
d. inorganic phosphate ion.
Which of the following enzymes does not use NAD+ for oxidation?
a. Alpha-Ketoglutarate Dehydrogenase
complex.
b. IsoCitrate Dehydrogenase.
c. Succinate Dehydrogenase.
d. Malate Dehydrogenase.
e. All of these enzymes use NAD+
c. Succinate Dehydrogenase.
Which of these enzymes is most similar to pyruvate dehydrogenase?
a. Alpha-Ketoglutarate Dehydrogenase complex.
b. IsoCitrate Dehydrogenase.
c. Succinate Dehydrogenase.
d. Malate Dehydrogenase.
a. Alpha-Ketoglutarate Dehydrogenase complex.
Refer to Exhibit 19A. Which intermediate releases CO2 concurrent with oxidation?
a. 1
b. 2
c. 3
d. Both 1 and 3
e. Both 2 and 3
e. Both 2 and 3
Refer to Exhibit 19A. Which intermediate is formed from acetyl-CoA and oxaloacetate.
a. 1
b. 2
c. 3
d. 4
e. 5
a. 1
Refer to Exhibit 19A. Which intermediate does FAD oxidize?
a. 2
b. 3
c. 4
d. 5
e. More than one of these is oxidized by FAD.
c. 4
Refer to Exhibit 19A. Which intermediate is formed from fumarate?
a. 1
b. 2
c. 3
d. 4
e. 5
e. 5
Refer to Exhibit 19A. Which intermediate becomes bonded to Coenzyme A during the
cycle?
a. 1
b. 2
c. 3
d. 4
e. 5
c. 3
In muscle cells, the following reaction proceeds as written, i.e., from left to right, despite having ΔG°’ ≈ +30 kJ/mol. How can this occur?
malate + NAD+ → oxaloacetate + NADH + H+
a. It is obviously thermodynamically favored under standard conditions.
b. In the cell, it is kinetically favored, even though it’s thermodynamically unfavored.
c. The concentration of malate must be higher than oxaloacetate for this reaction to occur in the cell.
d. [H+] must be higher in muscle than under standard conditions, thus altering ΔG°’ to
ΔG°.
c. The concentration of malate must be higher than oxaloacetate for this reaction to occur in the cell.
One round of the citric acid cycle generates about ten equivalents of ATP.
a. True
b. False
b. False
The conversion of malate to oxaloacetate has a high +ΔG (it is endergonic). It can take place
because:
a. It is coupled to hydrolysis of the GTP produce earlier in the cycle.
b. It is coupled to hydrolysis of ATP from other sources.
c. It involves a substrate level phosphorylation.
d. The oxaloacetate product is used up in the subsequent reaction.
e. It is coupled to a strong reduction reaction.
d. The oxaloacetate product is used up in the subsequent reaction
Which of the following enzymes is not a control point of the citric acid cycle?
a. citrate synthase
b. isocitrate dehydrogenase
c. aconitase
d. the α-ketoglutarate dehydrogenase
complex
c. aconitase
Which of the following enzymes is allosterically inhibited by ATP?
a. pyruvate dehydrogenase complex
b. succinyl-CoA synthetase
c. succinate dehydrogenase
d. fumarase
a. pyruvate dehydrogenase complex
Which of the following enzymes is allosterically activated by NAD+?
a. isocitrate dehydrogenase
b. succinyl-CoA
synthetase
c. succinate
dehydrogenase
d. fumarase
e. none of these
a. isocitrate dehydrogenase
The reaction in which malate is oxidized to oxaloacetate is not thermodynamically favored. It takes place because
a. it is coupled to ATP hydrolysis.
b. it involves substrate-level phosphorylation.
c. the product is continuously used up in the next reaction of the cycle, which is
thermodynamically favored.
d. it is coupled to a strong reduction.
c. the product is continuously used up in the next reaction of the cycle, which is
thermodynamically favored.
A control point outside the citric acid cycle is the reaction catalyzed by
a. the pyruvate dehydrogenase complex.
b. citrate synthetase.
c. isocitrate dehydrogenase.
d. the α-ketoglutarate dehydrogenase complex.
a. the pyruvate dehydrogenase complex.
A cell in an active metabolic state has
a. a high (ATP/ADP) and a high (NADH/NAD+) ratio.
b. a high (ATP/ADP) and a low (NADH/NAD+) ratio.
c. a low (ATP/ADP) and a low (NADH/NAD+) ratio.
d. a low (ATP/ADP) and a high (NADH/NAD+) ratio.
c. a low (ATP/ADP) and a low (NADH/NAD+) ratio.
The “energy charge” in a cell is important in the control of metabolism.
a. True
b. False
a. True
“Energy charge” in a cell is a measure of
a. ATP/NAD+ ratios.
b. ATP/NADH ratios.
c. ATP/ADP ratios.
d. NADH/NAD+ ratios.
e. NAD+/ADP ratios.
c. ATP/ADP ratios.
Which of the following is true regarding the control of pyruvate dehydrogenase?
a. It is inhibited by ATP
b. It is inhibited by NAD+
c. It is activated by acetyl-CoA
d. It is inhibited by
succinyl-CoA
e. none of these are true
a. It is inhibited by ATP
The glyoxylate cycle occurs in
a. plants and animals.
b. bacteria and animals.
c. plants and bacteria.
d. plants, animals, and
bacteria.
c. plants and bacteria.
The intracellular site of the glyoxylate cycle is
a. glyoxysomes only.
b. glyoxysomes and lysosomes.
c. glyoxysomes and Golgi apparatus.
d. glyoxysomes and smooth endoplasmic
reticulum.
a. glyoxysomes only.
A unique feature of the glyoxylate cycle is that it allows the organisms that possess this
pathway to
a. produce fats from carbohydrates.
b. produce carbohydrates from fats.
c. convert acetyl-CoA to pyruvate.
d. do all of the above.
b. produce carbohydrates from fats.
The glyoxylate pathway bypasses part of the citric acid cycle by converting isocitrate to
glyoxylate and
a. α-ketoglutarate
b. fumarate
c. succinyl-CoA
d. succinate
d. succinate
In the glyoxylate cycle, acetyl-CoA reacts with glyoxylate to produce
a. succinyl-CoA
b. succinate
c. fumarate
d. malate
d. malate
Glyoxysomes are named for the fact that they contain the glyoxylate pathway.
a. True
b. False
a. True
An organism that undergoes the glyoxylate cycle can make sugar from fat because:
a. there is a specific isomerase that converts a six carbon fatty acid to glucose
b. the unique reactions of the glyoxylate cycle bypass the two decarboyxlation reactions of the citric acid cycle
c. glyoxysomes lack succinate dehydrogenase
d. none of these
b. the unique reactions of the glyoxylate cycle bypass the two decarboyxlation reactions of the citric acid cycle
The production of malate in the glyoxylate pathway is important, since it can be readily
converted to phosphoenolpyruvate and then to sugars.
a. True
b. False
a. True
Which of the following statements concerning the glyoxylate pathway is false?
a. It utilizes one mole of acetyl-CoA per cycle.
b. It can produce a net synthesis of 4-carbon fragments that are intermediates of the citric
acid cycle.
c. It usually occurs in the mitochondria
d. It is the main pathway that allows for synthesis of sugars from acetyl-CoA.
c. It usually occurs in the mitochondria
Most of the products of the catabolism of sugars, fats and amino acids enter the citric acid
cycle as:
a. pyruvate
b. acetyl-CoA
c. malate
d. all of these
e. none of these
d. all of these
The citric acid cycle uses anaplerotic reactions to get rid of the many intermediates of the
cycle that accumulate during catabolism of amino acids.
a. True
b. False
b. False
When the citric acid cycle is not functioning, the most common fate of acetyl-CoA from sugar
metabolism in humans is the formation of fatty acids or cholesterol.
a. True
b. False
a. True
Which of the following cannot cross the inner mitochondrial membrane?
a. malate
b. phosphoenolpyruvat e
c. succinyl-CoA
d. oxaloacetate
d. oxaloacetate
Which of the following is a source of NADPH?
a. the pentose phosphate pathway
b. a series of reactions in which oxaloacetate is reduced to malate followed by oxidative
decarboxylation of the malate to pyruvate
c. both of the above
d. neither of these
c. both of the above
There is a cyclic reaction in which pyruvate becomes oxaloacetate. The oxaloacetate is
converted to malate and then back to pyruvate. This cycle is important because:
a. There is no net use or fixation of CO2 in this cycle.
b. NADH is converted to NADPH in this cycle.
c. There is no net oxidation or reduction in this cycle.
d. NADPH is converted to NADH in this cycle.
e. This is actually a wasteful pathway with no practical
use.
b. NADH is converted to NADPH in this cycle.
The anaplerotic reactions associated with the citric acid cycle are the result of
a. the oxidative nature of the citric acid cycle
b. the use of many of the citric acid cycle intermediates in anabolism
c. the decarboxylation reactions
d. the production of GTP and reduced coenzymes
b. the use of many of the citric acid cycle intermediates in anabolism
Weight loss in humans can be difficult to achieve, since we lack the ability to convert our fats
to sugars, and it is difficult to change our metabolism to using fats as a primary energy source.
a. True
b. False
a. True
Which of the following describes a use for acetyl-CoA as an important intermediate in
metabolism?
a. Breakdown to CO2 and water, yielding much energy.
b. Synthesis of terpenes and steroids.
c. Synthesis of oxaloacetate in plants.
d. Synthesis of fatty acids.
e. All of these are reasons why acetyl-CoA is a central molecule in metabolism.
e. All of these are reasons why acetyl-CoA is a central molecule in metabolism.
Intermediates of the citric acid cycle are especially important in the synthesis of fatty acids
and amino acids.
a. True
b. False
b. False
The citric acid cycle is considered part of aerobic metabolism even though oxygen does not
appear explicitly in any reaction because
a. the NADH and FADH2 produced are reoxidized in the electron transport chain linked to oxygen
b. the reoxidation of NADH and FADH2 leads to the production of considerable quantities of ATP
c. it takes place in the mitochondrion
d. it contains oxidation reactions
a. the NADH and FADH2 produced are reoxidized in the electron transport chain linked to oxygen
In the classical equation for respiration:
Glucose + 6O2 –> 6CO2 + 6H2O, the following molecules are found directly in the citric acid
cycle:
a. O2
b. Glucose and O2
c. CO2 and H2O
d. all of these arae found directly in the citric acid cycle
c. CO2 and H2O
The citric acid cycle is also known as the _____.
a. Calvin cycle
b. Krebs cycle
c. Embden-Meyerhof
cycle
d. Kolb’s cycle
b. Krebs cycle
Which of the following is true of the inner membrane of a mitochondrion?
a. It is situated between the cytosol and the outer membrane.
b. It is situated between the mitochondrial matrix and the cytosol.
c. It is permeable to a large number of compounds without a transport protein.
d. It is the site where the Embden–Meyerhof–Parnas pathway takes place.
b. It is situated between the mitochondrial matrix and the cytosol.
The pyruvate produced by glycolysis is first oxidized to _____.
a. one carbon dioxide molecule and one acetyl group
b. two carbon dioxide molecules and four acetyl groups
c. three carbon dioxide molecules and three acetyl
groups
d. four carbon dioxide molecules and six acetyl groups
a. one carbon dioxide molecule and one acetyl group
Identify a true statement about lipoic acid.
a. It contains two disulfide groups in its oxidized form and one sulfhydryl group in its
reduced form.
b. It contains two disulfide groups in its reduced form and one sulfhydryl group in its
oxidized form.
c. It contains one disulfide group in its reduced form and two sulfhydryl groups in its
oxidized form.
d. It contains one disulfide group in its oxidized form and two sulfhydryl groups in its
reduced form.
d. It contains one disulfide group in its oxidized form and two sulfhydryl groups in its
reduced form.
Isocitrate has _____ possible isomers, out of which only one is produced in the isomerizationof citrate to isocitrate.
a. three
b. two
c. four
d. six
c. four
