Biochemistry : Citric Acid Cycle

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #11 : Citric Acid Cycle

Which of the following enzymes catalyzes a reaction within the citric acid cycle?

Possible Answers:

Isocitrate dehydrogenase

Phosphoglycerate kinase

Glyceraldehyde-3-phosphate dehydrogenase

Enolase

Pyruvate kinase

Correct answer:

Isocitrate dehydrogenase

Explanation:

The only enzyme listed in the answer choices that catalyzes a reaction within the citric acid cycle is isocitrate dehydrogenase, as it catalyzes the formation of alpha-ketoglutarate, carbon dioxide, NADH, and a proton, from isocitrate and .

Each of the other enzymes listed do not catalyze reactions within the citric acid cycle, but rather they catalyze reactions within glycolysis, as follows:

Pyruvate kinase catalyzes the conversion of phosphoenolpyruvate and ADP to pyruvate and ATP.

Glyceraldehyde-3-phosphate dehydrogenase catalyzes the conversion of glyceraldehyde-3-phosphate, , and inorganic phosphate to 1,3-bisphosphoglycerate, NADH, and a proton.

Phosphoglycerate kinase catalyzes the conversion of 1,3-bisphosphoglycerate and ADP to 3-phosphoglycerate and ATP.

Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate.

Example Question #12 : Citric Acid Cycle

Which of the following citric acid cycle enzymes catalyzes a reaction that results in the production of a molecule of ?

Possible Answers:

Aconitase

Fumarase

Succinate dehydrogenase

Isocitrate dehydrogenase

Pyruvate kinase

Correct answer:

Succinate dehydrogenase

Explanation:

The only enzyme listed that participates in the citric acid cycle and catalyzes a step producing  is succinate dehydrogenase. Succinate dehydrogenase catalyzes the conversion of succinate and FAD to fumarate and

Pyruvate kinase is incorrect as it neither participates in the citric acid cycle (it is part of glycolysis), nor catalyzes a reaction that produces

Isocitrate dehydrogenase catalyzes the conversion of isocitrate and  to alpha-ketoglutarate, NADH, , and CO2, but not .

Fumarase catalyzes the conversion of fumarate and  to malate, but not .

Aconitase catalyzes the conversion of cis-aconitate and  to isocitrate, but not

Example Question #13 : Citric Acid Cycle

Which of the following enzymes catalyzes the citric acid cycle step that directly produces succinyl-CoA?

Possible Answers:

Succinate dehydrogenase

Fumarase

Succinyl-CoA synthetase

Alpha-ketoglutarate dehydrogenase

Isocitrate dehydrogenase

Correct answer:

Alpha-ketoglutarate dehydrogenase

Explanation:

The citric acid cycle enzyme that catalyzes the reaction directly responsible for the production of succinyl-CoA is alpha-ketoglutarate dehydrogenase. Alpha-ketoglutarate dehydrogenase catalyzes the conversion of alpha-ketoglutarate, , and CoA-SH to succinyl-CoA, NADH, , and

Each of the other enzymes listed are enzymes that participate in the citric acid cycle, but not in the step the directly produces succinyl-CoA. Their general roles are as follows:

Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate.

Succinate dehydrogenase catalyzes the conversion of succinate to fumarate.

Isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate.

Fumarase catalyzes the conversion of fumarate to malate.

Example Question #14 : Citric Acid Cycle

Which of the following statements regarding the function of the enzyme, succinyl-CoA synthetase, is a true statement?

Possible Answers:

Succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinate.

Succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinyl-CoA.

Succinyl-CoA synthetase is produced by the reaction catalyzed by alpha-ketoglutarate dehydrogenase. 

Succinyl-CoA synthetase catalyzes the reaction responsible for the rate-limiting step of glycolysis.

Succinyl-CoA synthetase directly catalyzes a citric acid cycle reaction that produces .

Correct answer:

Succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinate.

Explanation:

The only correct statement within the answer choices is that succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinate. In this reaction, the citric acid cycle enzyme, succinyl-CoA synthetase, catalyzes the conversion of succinyl-CoA, GDP (or ADP) and inorganic phosphate to succinate, CoA-SH, and GTP (or ATP). 

The incorrect answer choices are explained below:

"Succinyl-CoA synthetase is produced by the reaction catalyzed by alpha-ketoglutarate dehydrogenase."

This is incorrect because it is succinyl-CoA that is produced by the reaction catalyzed by alpha-ketoglutarate dehydrogenase. Succinyl-CoA synthetase, the enzyme, is not produced by this reaction.

"Succinyl-CoA synthetase catalyzes the reaction responsible for the rate-limiting step of glycolysis."

Succinyl-CoA synthetase does not participate in glycolysis; it participates in the citric acid cycle. Furthermore, even in the citric acid cycle, it does not catalyze the reaction responsible for the rate-limiting step of the citric acid cycle.

"Succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinyl-CoA."

Alpha-ketoglutarate dehydrogenase catalyzes the reaction responsible for the formation of succinyl-CoA. Succinyl-CoA synthetase catalyzes the reaction responsible for the conversion of succinyl-CoA to succinate.

"Succinyl-CoA synthetase directly catalyzes a citric acid cycle reaction that produces ."

Succinate dehydrogenase directly catalyzes a citric acid cycle reaction that produces , not succinyl-CoA synthetase.

Example Question #13 : Citric Acid Cycle

Which of the following enzymes catalyzes the rate-limiting step of the citric acid cycle?

Possible Answers:

Phosphofructokinase-1 (PFK-1)

Succinyl-CoA synthetase

Fructose 1,6-bisphosphatase 

Isocitrate dehydrogenase

Pyruvate kinase

Correct answer:

Isocitrate dehydrogenase

Explanation:

The rate-limiting step of the citric acid cycle is catalyzed by the enzyme, isocitrate dehydrogenase. Isocitrate dehydrogenase catalyzes the conversion of isocitrate and  to alpha-ketoglutarate, NADH, a proton, and a molecule of carbon dioxide. 

Phosphofructokinase-1 (PFK-1) is incorrect, as it catalyzes the rate-limiting step of glycolysis, not the citric acid cycle.

Fructose 1,6-bisphosphatase is incorrect, as it catalyzes the rate-limiting step of gluconeogenesis, not the citric acid cycle.

Succinyl-CoA synthetase is incorrect, as it catalyzes a reaction within the citric acid cycle that is not the rate-limiting step.

Pyruvate kinase is incorrect as the reaction that it catalyzes is neither within the citric acid cycle, nor a rate-limiting step.

Example Question #11 : Citric Acid Cycle

Which molecule is not a citric acid cycle intermediate?

Possible Answers:

Citrate

Phosphoenolpyruvate

Succinate

Isocitrate

Correct answer:

Phosphoenolpyruvate

Explanation:

Phosphoenolpyruvate (PEP) is an intermediate in glycolysis, not the citric acid cycle. PEP is the product of the ninth reaction in glycolysis, which involves the enolase-catalyzed conversion of 2-phosphoglycerate into PEP. All other molecules are indeed intermediates in the citric acid cycle.

Example Question #12 : Citric Acid Cycle

Pyruvate enters the citric acid cycle after being converted to a molecule with how many carbons?

Possible Answers:

Correct answer:

Explanation:

The three-carbon molecule pyruvate produced from glycolysis is converted to the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA produces one . Acetyl-CoA has one less carbon than pyruvate; this third carbon from pyruvate was lost as carbon dioxide during its conversion to acetyl-CoA via the pyruvate dehydrogenase complex.

Example Question #2 : Citric Acid Cycle Carbohydrate Intermediates

What is the intermediate between citrate and isocitrate?

Possible Answers:

Oxaloacetate

Succinate

Cis-aconitate

Fumarate

Correct answer:

Cis-aconitate

Explanation:

The citric acid cycle begins when a four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) to produce the six-carbon molecule citrate. The enzyme citrate synthase carries out this reaction. Citrate then becomes the six-carbon molecule cis-aconitate via catalysis by aconitase. The same enzyme then converts cis-aconitate to isocitrate, which is an isomer of citrate.

Example Question #931 : Biochemistry

What vitamin does pyruvate dehydrogenase need in order to make pyruvate into acetyl-CoA for the citric acid cycle? 

Possible Answers:

Methylcobalamin (B12)

Niacin (B3)

Pyridoxine (B6)

Thiamine (B1)

Correct answer:

Thiamine (B1)

Explanation:

Thiamine (B1) acts as a cofactor to enable pyruvate dehydrogenase to convert pyruvate from glycolysis into acetyl-CoA so it can enter the citric acid cycle. 

Example Question #11 : Citric Acid Cycle

The citric acid cycle begins when the two-carbon acetyl group from acetyl-CoA combines with the four-carbon molecule __________ to form the six-carbon molecule citrate. 

Possible Answers:

Oxaloacetate

Pyruvate

Malate

Aspartate

Aspartate

Correct answer:

Oxaloacetate

Explanation:

Oxaloacetate combines with acetyl-CoA to form citrate. This is the first stage of the citric acid cycle. Eventually, citrate will lose two molecules of  to regenerate the four-carbon molecule oxaloacetate.

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