Citric Acid Cycle Flashcards
- Which statement is NOT true of the reaction catalyzed by the pyruvate dehydrogenase complex?
A) Biotin participates in the decarboxylation.
B) Both NAD+ and a flavin nucleotide act as electron carriers.
C) The reaction occurs in the mitochondrial matrix.
D) The substrate is held by the lipoyl-lysine “swinging arm.”
E) Two different cofactors containing —SH groups participate.
A) Biotin participates in the decarboxylation.
Which statement about the oxidative decarboxylation of pyruvate in aerobic conditions in animal cells is CORRECT?
A) One of the products of the reactions of the pyruvate dehydrogenase complex is a
thioester of acetate.
B) The methyl (—CH3) group is eliminated as CO2.
C) The process occurs in the cytosolic compartment of the cell.
D) The pyruvate dehydrogenase complex uses all of the following as cofactors: NAD ,
lipoic acid, pyridoxal phosphate (PLP), and FAD.
E) The reaction is so important to energy production that pyruvate dehydrogenase
operates at full speed under all conditions.
A) One of the products of the reactions of the pyruvate dehydrogenase complex is a
thioester of acetate.
Glucose labeled with 14C in C-3 and C-4 is completely converted to acetyl-CoA via glycolysis and the pyruvate dehydrogenase complex. What percentage of the acetyl- CoA molecules formed will be labeled with 14C, and in which position of the acetyl moiety will the 14C label be found?
A) 100% of the acetyl-CoA will be labeled at C-1 (carboxyl).
B) 100% of the acetyl-CoA will be labeled at C-2.
C) 50% of the acetyl-CoA will be labeled, all at C-2 (methyl).
D) No label will be found in the acetyl-CoA molecules.
E) Not enough information is given to answer this question.
D) No label will be found in the acetyl-CoA molecules.
Which statement is NOT true of the citric acid cycle?
A) All enzymes of the cycle are located in the cytoplasm, except succinate
dehydrogenase, which is bound to the inner mitochondrial membrane.
B) In the presence of malonate, one would expect succinate to accumulate.
C) Oxaloacetate is used as a substrate but is not consumed in the cycle.
D) Succinate dehydrogenase channels electrons directly into the electron transfer
chain.
E) The condensing enzyme is subject to allosteric regulation by ATP and NADH.
A) All enzymes of the cycle are located in the cytoplasm, except succinate
dehydrogenase, which is bound to the inner mitochondrial membrane.
Acetyl-CoA labeled with 14C in both of its acetate carbon atoms is incubated with unlabeled oxaloacetate and a crude tissue preparation capable of carrying out the reactions of the citric acid cycle. After one turn of the cycle, oxaloacetate would have 14C in:
A) all four carbon atoms.
B) no pattern that is predictable from the information provided.
C) none of its carbon atoms.
D) the keto carbon and one of the carboxyl carbons.
E) the two carboxyl carbons.
A) all four carbon atoms.
Malonate is a competitive inhibitor of succinate dehydrogenase. If malonate is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which compound would you expect to decrease in concentration? A) citrate B) fumarate C) isocitrate D) pyruvate E) succinate
B) fumarate
In mammals, what process does NOT occur during the citric acid cycle?
A) formation of -ketoglutarate
B) generation of NADH and FADH2
C) metabolism of acetate to carbon dioxide and water
D) net synthesis of oxaloacetate from acetyl-CoA
E) oxidation of acetyl-CoA
D) net synthesis of oxaloacetate from acetyl-CoA
Oxaloacetate uniformly labeled with 14C (i.e., with equal amounts of 14C in each of its carbon atoms) is condensed with unlabeled acetyl-CoA. After a single pass through the citric acid cycle back to oxaloacetate, what fraction of the original radioactivity will be found in the oxaloacetate?
A) All B) 1/2 C) 1/3 D) 1/4 E) 3/4
B) 1/2
Conversion of 1 mol of acetyl-CoA to 2 mol of CO2 and CoA via the citric acid cycle results in the net production of: A) 1 mol of citrate. B) 1 mol of FADH2. C) 1 mol of NADH. D) 1 mol of oxaloacetate. E) 7 mol of ATP.
B) 1 mol of FADH2.
Which factor is NOT associated with the oxidation of substrates by the citric acid cycle?
A) CO2 production
B) flavin reduction
C) the presence of lipoic acid in some of the enzyme systems
D) pyridine nucleotide oxidation
E) All of these factors are associated with the oxidation of substrates by the citric acid
cycle.
D) pyridine nucleotide oxidation
The oxidative decarboxylation of -ketoglutarate proceeds utilizing multistep reactions. Which cofactor is NOT required? A) ATP B) coenzyme A C) lipoic acid D) NAD+ E) thiamine pyrophosphate
A) ATP
The reaction of the citric acid cycle that is most similar to the pyruvate dehydrogenase complex-catalyzed conversion of pyruvate to acetyl-CoA is the conversion of: A) citrate to isocitrate. B) fumarate to malate. C) malate to oxaloacetate. D) succinyl-CoA to succinate. E) alpha-ketoglutarate to succinyl-CoA.
E) alpha-ketoglutarate to succinyl-CoA.
Which enzymatic activity would be decreased by thiamine deficiency?
A) fumarase
B) isocitrate dehydrogenase
C) malate dehydrogenase
D) succinate dehydrogenase
E) alpha-ketoglutarate dehydrogenase complex
E) alpha-ketoglutarate dehydrogenase complex
The standard reduction potentials (E’°) for the following half-reactions are given. Fumarate + 2H+ + 2e– → succinate E’° = +0.031 V
FAD + 2H+ + 2e– → FADH2 E’° = –0.219 V
If succinate, fumarate, FAD, and FADH2, all at l M concentrations, were mixed in the presence of succinate dehydrogenase, which of the following would happen initially?
A) Fumarate and succinate would become oxidized; FAD and FADH2 would become
reduced.
B) Fumarate would become reduced; FADH2 would become oxidized.
C) No reaction would occur because all reactants and products are already at their
standard concentrations.
D) Succinate would become oxidized; FAD would become reduced.
E) Succinate would become oxidized; FADH2 would be unchanged because it is a
cofactor, not a substrate.
B) Fumarate would become reduced; FADH2 would become oxidized.
For the following reaction, G’° = 29.7 kJ/mol. L-Malate + NAD+ → oxaloacetate + NADH + H+
The reaction as written:
A) can never occur in a cell.
B) can only occur in a cell if it is coupled to another reaction for which G’° is
positive.
C) can only occur in a cell in which NADH is converted to NAD+ by electron
transport.
D) may occur in cells at certain concentrations of substrate and product.
E) would always proceed at a very slow rate
D) may occur in cells at certain concentrations of substrate and product.
The oxidative step of the citric acid cycle that is NOT linked to the reduction of NAD+ is the reaction catalyzed by:
A) isocitrate dehydrogenase.
B) malate dehydrogenase.
C) pyruvate dehydrogenase
D) succinate dehydrogenase.
E) the -ketoglutarate dehydrogenase complex.
D) succinate dehydrogenase.
Which cofactor is required for the conversion of succinate to fumarate in the citric acid cycle? A) ATP B) biotin C) FAD D) NAD+ E) NADP+
C) FAD
In the citric acid cycle, a flavin coenzyme is required for:
A) condensation of acetyl-CoA and oxaloacetate.
B) oxidation of fumarate.
C) oxidation of isocitrate.
D) oxidation of malate.
E) oxidation of succinate.
E) oxidation of succinate.
Which intermediate of the citric acid cycle is prochiral? A) citrate B) isocitrate C) malate D) oxaloacetate E) succinate
A) citrate
Intermediates in the citric acid cycle are used as precursors in the biosynthesis of: A) amino acids. B) nucleotides. C) fatty acids. D) sterols. E) All of the answers are correct.
E) All of the answers are correct.
During the reaction of pyruvate carboxylase, CO2 is NOT covalently attached to: A) phosphate. B) biotin. C) pyruvate. D) lysine. E) All of the answers are correct.
D) lysine.
The entry of acetyl-CoA into the citric acid cycle is DECREASED when:
A) [AMP] is high.
B) NADH is rapidly oxidized through the respiratory chain.
C) the ratio of [ATP]/[ADP is low
D) the ratio of [ATP]/[ADP] is high.
E) the ratio of [NAD+]/[NADH] is high.
D) the ratio of [ATP]/[ADP] is high.
Citrate synthase and the NAD+-specific isocitrate dehydrogenase are two key regulatory enzymes of the citric acid cycle. These enzymes are inhibited by: A) acetyl-CoA and fructose 6-phosphate. \+ B) AMP and/or NAD+ C) AMP and/or NADH D) ATP and/or NAD+ E) ATP and/or NADH
E) ATP and/or NADH.
During seed germination, the glyoxylate pathway is important to plants because it enables them to:
A) carry out the net synthesis of glucose from acetyl-CoA.
B) form acetyl-CoA from malate.
C) get rid of isocitrate formed from the aconitase reaction.
D) obtain glyoxylate for cholesterol biosynthesis.
E) obtain glyoxylate for pyrimidine synthesis.
A) carry out the net synthesis of glucose from acetyl-CoA.
