Question 1

Which compound is NOT required for the oxidative decarboxylation of the carbon skeleton of isoleucine?

Accepted Answer

All the listed compounds are required for the oxidative decarboxylation of isoleucine, as they are involved in the enzymatic reactions of the branched-chain alpha-keto acid dehydrogenase complex.

Question 2

Thyroxine is a competitive inhibitor of malate dehydrogenase. If thyroxine is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which compound would you expect to decrease in concentration FIRST?

Accepted Answer

Thyroxine inhibits malate dehydrogenase, which catalyzes the conversion of malate to oxaloacetate. Therefore, the concentration of oxaloacetate would decrease first.

Question 3

Which citric acid cycle intermediate is chiral?

Accepted Answer

Malate is the correct answer because it has a chiral center, making it capable of existing in different enantiomeric forms.

Question 4

Which statement is FALSE with respect to the pyruvate dehydrogenase complex?

Accepted Answer

The proximity of the catalytic enzymes in the pyruvate dehydrogenase complex actually facilitates the overall reaction rate by reducing the distance that intermediates must travel between enzymatic steps, thereby increasing the efficiency of the process.

Question 5

Shown below is a list of pairs of compounds. In which pair is the second compound produced by a reduction of the first compound?

Accepted Answer

Fumarate is reduced to succinate in the citric acid cycle by the enzyme succinate dehydrogenase.

Question 6

Which cofactor is required for the conversion of fumarate to succinate?

Accepted Answer

The conversion of fumarate to succinate is catalyzed by the enzyme fumarate reductase, which requires FADH2 as a cofactor.

Question 7

Why is the citric acid cycle considered to be part of aerobic metabolism, even though oxygen is not a substrate in any reaction?

Accepted Answer

Although oxygen is not directly involved, the citric acid cycle produces reduced electron carriers (NADH and FADH2), which transfer their electrons to the electron transport chain for oxidative phosphorylation, ultimately using oxygen as the final electron acceptor. Therefore, the citric acid cycle is considered part of aerobic metabolism.

Question 8

Which statement about the oxidative decarboxylation of pyruvate in animal cells is CORRECT?

Accepted Answer

The oxidative decarboxylation of pyruvate in animal cells only occurs under aerobic conditions, as it requires oxygen as the final electron acceptor in the electron transport chain.

Question 9

How much carbon dioxide is produced from the complete aerobic catabolism of one molecule of 2-phosphoglycerate via catabolic pathways?

Accepted Answer

The complete aerobic catabolism of one molecule of 2-phosphoglycerate through glycolysis, the citric acid cycle, and oxidative phosphorylation ultimately produces 3 molecules of CO2. This includes the conversion of 2-phosphoglycerate to pyruvate in glycolysis, which then enters the citric acid cycle as acetyl-CoA, leading to the release of CO2 during the cycle.

Question 10

When is the consumption of acetyl-CoA by the citric acid cycle is decreased?

Accepted Answer

When the mitochondrial ATP synthase is inactive, ATP production is hindered, leading to an increase in the ADP/ATP ratio. This signals the cell to decrease energy production processes, including the citric acid cycle, thereby reducing the consumption of acetyl-CoA.

Question 11

Which reaction in the citric acid cycle will NOT proceed in the absence of inorganic phosphate ions?

Accepted Answer

The reaction catalyzed by succinyl-CoA synthetase in the citric acid cycle requires inorganic phosphate ions for the conversion of succinyl-CoA to succinate, generating GTP (or ATP) in the process. This is the only step among the options provided that directly involves inorganic phosphate.

Question 12

Arsenic inhibits the pyruvate dehydrogenase complex by inactivating the dihydrolipoamide component of dihydrolipoyl transacetylase. Which enzyme is also MOST likely to be inhibited by arsenic?

Accepted Answer

Arsenic inhibits the dihydrolipoamide component of dihydrolipoyl transacetylase in the pyruvate dehydrogenase complex. The dihydrolipoamide components of other dehydrogenase complexes, such as the alpha-ketoglutarate dehydrogenase complex, share structural similarity with the dihydrolipoamide component of the pyruvate dehydrogenase complex. Therefore, it is likely that the alpha-ketoglutarate dehydrogenase complex would also be inhibited by arsenic.

Question 13

Which statement is FALSE regarding the citric acid cycle?

Accepted Answer

Acetyl-CoA is an activator for the citric acid cycle, not an inhibitor. It is a substrate for the cycle, entering the cycle by reacting with oxaloacetate to form citrate, thus promoting the cycle rather than inhibiting it.

Question 14

Which cofactors associated with the pyruvate dehydrogenase complex are classified as prosthetic groups? 1) TPP 2) lipoate 3) CoA-SH 4) NAD⁺ 5) FAD

Accepted Answer

Prosthetic groups are non-protein components that are tightly or covalently bound to a protein or enzyme. In the pyruvate dehydrogenase complex, Thiamine pyrophosphate (TPP) and Flavin adenine dinucleotide (FAD) are tightly bound to the enzyme complex, making them prosthetic groups. Lipoate, although not covalently attached to the enzyme, plays a crucial role as a swinging arm for substrate channeling, and is considered a prosthetic group for its essential function and tight association. CoA-SH is a substrate rather than a prosthetic group, and "NAD⁺" appears to be a typographical error and does not represent a known cofactor or prosthetic group.

Question 15

Which enzymes catalyze oxidation reactions in the citric acid cycle? 1) succinate dehydrogenase
2) pyruvate dehydrogenase
3) citrate dehydrogenase
4) $\alpha$-ketoglutarate dehydrogenase

Accepted Answer

Succinate dehydrogenase and α-ketoglutarate dehydrogenase are directly involved in the citric acid cycle, catalyzing oxidation reactions. Pyruvate dehydrogenase, while not a part of the citric acid cycle itself, catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, linking glycolysis to the citric acid cycle. Citrate dehydrogenase is not a recognized enzyme in the context of the citric acid cycle.

Question 16

Which enzyme catalyzes a reversible reaction under normal cellular conditions?

Accepted Answer

Succinate dehydrogenase catalyzes the reversible conversion of succinate to fumarate in the citric acid cycle.

Question 17

Which molecule is an $\alpha$-keto acid?

Accepted Answer

Oxaloacetate and α-ketoglutarate are both keto acids, with oxaloacetate being a 4-carbon molecule and α-ketoglutarate a 5-carbon molecule, both involved in the citric acid cycle.

Question 18

Which high-energy bond is associated with the succinyl-CoA synthetase reaction?

Accepted Answer

Succinyl-CoA synthetase catalyzes the reversible conversion of succinate to succinyl-CoA, coupled to the hydrolysis of ATP. The reaction proceeds via a thioester intermediate, which is a high-energy bond.

Question 19

Which reaction of the citric acid cycle requires a water molecule?

Accepted Answer

The answer of Which reaction of the citric acid cycle...

Question 20

Which reaction of the citric acid cycle produces a net of one water molecule?

Accepted Answer

The answer of Which reaction of the citric acid cycle...

Quiz 16: The Citric Acid Cycle

Which compound is NOT required for the oxidative decarboxylation of the carbon skeleton of isoleucine?

Thyroxine is a competitive inhibitor of malate dehydrogenase. If thyroxine is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which compound would you expect to decrease in concentration FIRST?

Which citric acid cycle intermediate is chiral?

Which statement is FALSE with respect to the pyruvate dehydrogenase complex?

Shown below is a list of pairs of compounds. In which pair is the second compound produced by a reduction of the first compound?

Which cofactor is required for the conversion of fumarate to succinate?

Why is the citric acid cycle considered to be part of aerobic metabolism, even though oxygen is not a substrate in any reaction?

Which statement about the oxidative decarboxylation of pyruvate in animal cells is CORRECT?

How much carbon dioxide is produced from the complete aerobic catabolism of one molecule of 2-phosphoglycerate via catabolic pathways?

When is the consumption of acetyl-CoA by the citric acid cycle is decreased?

Which reaction in the citric acid cycle will NOT proceed in the absence of inorganic phosphate ions?

Arsenic inhibits the pyruvate dehydrogenase complex by inactivating the dihydrolipoamide component of dihydrolipoyl transacetylase. Which enzyme is also MOST likely to be inhibited by arsenic?

Which statement is FALSE regarding the citric acid cycle?

Which cofactors associated with the pyruvate dehydrogenase complex are classified as prosthetic groups? 1) TPP 2) lipoate 3) CoA-SH 4) NAD⁺ 5) FAD

Which enzymes catalyze oxidation reactions in the citric acid cycle? 1) succinate dehydrogenase 2) pyruvate dehydrogenase 3) citrate dehydrogenase 4) $\alpha$ -ketoglutarate dehydrogenase

Which enzyme catalyzes a reversible reaction under normal cellular conditions?

Which molecule is an $\alpha$ -keto acid?

Which high-energy bond is associated with the succinyl-CoA synthetase reaction?

Which reaction of the citric acid cycle requires a water molecule?

Which reaction of the citric acid cycle produces a net of one water molecule?

Quiz 16: The Citric Acid Cycle

Which compound is NOT required for the oxidative decarboxylation of the carbon skeleton of isoleucine?

Thyroxine is a competitive inhibitor of malate dehydrogenase. If thyroxine is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which compound would you expect to decrease in concentration FIRST?

Which citric acid cycle intermediate is chiral?

Which statement is FALSE with respect to the pyruvate dehydrogenase complex?

Shown below is a list of pairs of compounds. In which pair is the second compound produced by a reduction of the first compound?

Which cofactor is required for the conversion of fumarate to succinate?

Why is the citric acid cycle considered to be part of aerobic metabolism, even though oxygen is not a substrate in any reaction?

Which statement about the oxidative decarboxylation of pyruvate in animal cells is CORRECT?

How much carbon dioxide is produced from the complete aerobic catabolism of one molecule of 2-phosphoglycerate via catabolic pathways?

When is the consumption of acetyl-CoA by the citric acid cycle is decreased?

Which reaction in the citric acid cycle will NOT proceed in the absence of inorganic phosphate ions?

Arsenic inhibits the pyruvate dehydrogenase complex by inactivating the dihydrolipoamide component of dihydrolipoyl transacetylase. Which enzyme is also MOST likely to be inhibited by arsenic?

Which statement is FALSE regarding the citric acid cycle?

Which cofactors associated with the pyruvate dehydrogenase complex are classified as prosthetic groups? 1) TPP 2) lipoate 3) CoA-SH 4) NAD+ 5) FAD

Which enzymes catalyze oxidation reactions in the citric acid cycle? 1) succinate dehydrogenase 2) pyruvate dehydrogenase 3) citrate dehydrogenase 4) α\alphaα -ketoglutarate dehydrogenase

Which enzyme catalyzes a reversible reaction under normal cellular conditions?

Which molecule is an α\alphaα -keto acid?

Which high-energy bond is associated with the succinyl-CoA synthetase reaction?

Which reaction of the citric acid cycle requires a water molecule?

Which reaction of the citric acid cycle produces a net of one water molecule?

Which cofactors associated with the pyruvate dehydrogenase complex are classified as prosthetic groups? 1) TPP 2) lipoate 3) CoA-SH 4) NAD⁺ 5) FAD

Which enzymes catalyze oxidation reactions in the citric acid cycle? 1) succinate dehydrogenase 2) pyruvate dehydrogenase 3) citrate dehydrogenase 4) $\alpha$ -ketoglutarate dehydrogenase

Which molecule is an $\alpha$ -keto acid?