Which of the following enzymes in the Krebs cycle catalyzes a substrate-level phosphorylation reaction?

• Aconitase

• Isocitrate dehydrogenase

• α-Ketoglutarate dehydrogenase

• Succinyl-CoA synthetase

• Fumarase

Answer explanation

Succinyl-CoA synthetase catalyzes a substrate-level phosphorylation reaction in the Krebs cycle, converting succinyl-CoA to succinate while generating GTP or ATP, unlike the other enzymes listed.

Which of the following statements about the Krebs cycle is TRUE?

• It occurs in the cytosol

• It produces 4 molecules of NADH per cycle

• It is the primary site of oxidative phosphorylation

• It directly generates ATP via oxidative phosphorylation

• It supplies intermediates for amino acid synthesis

Answer explanation

The Krebs cycle, also known as the citric acid cycle, provides intermediates for amino acid synthesis, making the statement true. It occurs in the mitochondria, produces NADH, and does not directly generate ATP via oxidative phosphorylation.

Tags

NGSS.HS-LS1-7

Fluoroacetate is a metabolic poison that inhibits the Krebs cycle by blocking which enzyme?

• Citrate synthase

• Aconitase

• Isocitrate dehydrogenase

• Succinate dehydrogenase

• Malate dehydrogenase

Answer explanation

Fluoroacetate inhibits aconitase, an enzyme in the Krebs cycle, preventing the conversion of citrate to isocitrate. This blockage disrupts energy production, making aconitase the correct answer.

Which of the following is NOT a product of one turn of the Krebs cycle?

• NADH

• FADH₂

• ATP

• Oxygen

• Carbon dioxide

Answer explanation

Oxygen is not a product of one turn of the Krebs cycle. The cycle produces NADH, FADH₂, ATP, and carbon dioxide, but oxygen is not generated; it is used in cellular respiration.

Which coenzyme is required for the α-ketoglutarate dehydrogenase complex in the Krebs cycle?

• NADP+

• Coenzyme A

• Biotin

• Pyridoxal phosphate

• Tetrahydrofolate

Answer explanation

Coenzyme A is essential for the α-ketoglutarate dehydrogenase complex in the Krebs cycle, facilitating the conversion of α-ketoglutarate to succinyl-CoA, a key step in energy production.

Tags

NGSS.HS-LS1-7

All of the following enzymes are involved in the Krebs cycle EXCEPT:

• Citrate synthase

• Pyruvate dehydrogenase

• Isocitrate dehydrogenase

• Malate dehydrogenase

• Fumarase

Answer explanation

Pyruvate dehydrogenase is not part of the Krebs cycle; it converts pyruvate to acetyl-CoA before entering the cycle. The other enzymes listed are directly involved in the Krebs cycle reactions.

Tags

NGSS.HS-LS1-7

Which of the following reactions in the Krebs cycle generates FADH₂?

• Isocitrate → α-ketoglutarate

• α-Ketoglutarate → Succinyl-CoA

• Succinate → Fumarate

• Fumarate → Malate

• Malate → Oxaloacetate

Answer explanation

In the Krebs cycle, the reaction that generates FADH₂ is Succinate → Fumarate. This step involves the oxidation of succinate, leading to the reduction of FAD to FADH₂.

Which enzyme catalyzes the first committed step of the Krebs cycle?

• Citrate synthase

• Isocitrate dehydrogenase

• Aconitase

• Succinyl-CoA synthetase

• Malate dehydrogenase

Answer explanation

Citrate synthase catalyzes the first committed step of the Krebs cycle by combining acetyl-CoA and oxaloacetate to form citrate, marking the beginning of the cycle.

Tags

NGSS.HS-LS1-7

The conversion of malate to oxaloacetate is an oxidation reaction that produces which molecule?

• NADH

• FADH₂

• ATP

• GTP

• CO₂

Answer explanation

The conversion of malate to oxaloacetate involves the oxidation of malate, which reduces NAD+ to NADH. Therefore, the correct answer is NADH, as it is produced during this reaction.

Which of the following is the correct sequence of intermediates in the Krebs cycle?

• Acetyl-CoA → Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate

• Acetyl-CoA → Citrate → α-Ketoglutarate → Isocitrate → Succinyl-CoA → Succinate → Malate → Fumarate → Oxaloacetate

• Acetyl-CoA → Citrate → Succinyl-CoA → Isocitrate → α-Ketoglutarate → Fumarate → Succinate → Malate → Oxaloacetate

• Acetyl-CoA → Citrate → Isocitrate → Succinate → α-Ketoglutarate → Succinyl-CoA → Fumarate → Malate → Oxaloacetate

• Acetyl-CoA → Citrate → Isocitrate → α-Ketoglutarate → Succinate → Succinyl-CoA → Fumarate → Malate → Oxaloacetate

Answer explanation

The correct sequence of intermediates in the Krebs cycle is Acetyl-CoA → Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate, as it follows the established pathway.