Biochemistry - Citric Acid Cycle - MCAT Practice Questions

The Citric Acid Cycle is also known by which two other names?

A. Calvin Cycle and Pentose Phosphate Pathway
B. Electron Transport Chain and Oxidative Phosphorylation
C. Krebs Cycle and Tricarboxylic Acid Cycle (TCA)
D. Glycolysis and Gluconeogenesis

What is the starting substrate that condenses with acetyl-CoA to begin the citric acid cycle?

A. Citrate
B. Oxaloacetate
C. Isocitrate
D. α-Ketoglutarate

Which enzyme catalyzes the first step of the citric acid cycle?

A. Citrate synthase
B. Aconitase
C. Isocitrate dehydrogenase
D. α-Ketoglutarate dehydrogenase

How many main enzymatic steps are there in the citric acid cycle?

A. Six steps
B. Seven steps
C. Eight steps
D. Ten steps

Which enzyme is often considered the rate-limiting enzyme of the citric acid cycle?

A. Citrate synthase
B. Aconitase
C. Isocitrate dehydrogenase
D. Malate dehydrogenase

How many molecules of NADH are produced per turn of the citric acid cycle?

A. 1 NADH
B. 2 NADH
C. 3 NADH
D. 4 NADH

How many molecules of FADH₂ are produced per turn of the citric acid cycle?

A. 1 FADH₂
B. 2 FADH₂
C. 3 FADH₂
D. 0 FADH₂

How many molecules of CO₂ are released per turn of the citric acid cycle?

A. 1 CO₂
B. 2 CO₂
C. 3 CO₂
D. 4 CO₂

Which enzyme produces FADH₂ in the citric acid cycle?

A. Isocitrate dehydrogenase
B. α-Ketoglutarate dehydrogenase
C. Succinate dehydrogenase
D. Malate dehydrogenase

Which enzyme produces GTP (or ATP) via substrate-level phosphorylation?

A. Citrate synthase
B. Succinyl CoA synthetase
C. Succinate dehydrogenase
D. Fumarase

Which enzyme converts citrate to isocitrate?

A. Citrate synthase
B. Aconitase
C. Isocitrate dehydrogenase
D. Fumarase

Which enzyme adds water (H₂O) to fumarate to form malate?

A. Succinate dehydrogenase
B. Aconitase
C. Malate dehydrogenase
D. Fumarase

Which enzyme regenerates oxaloacetate from malate to complete the cycle?

A. Fumarase
B. Succinate dehydrogenase
C. Malate dehydrogenase
D. Citrate synthase

How many GTP (or ATP) molecules are produced per turn of the citric acid cycle via substrate-level phosphorylation?

A. 1 GTP/ATP
B. 2 GTP/ATP
C. 3 GTP/ATP
D. 0 GTP/ATP

What are the three main regulatory enzymes of the citric acid cycle?

A. Aconitase, Fumarase, Malate dehydrogenase
B. Citrate synthase, Isocitrate dehydrogenase, α-Ketoglutarate dehydrogenase complex
C. Succinate dehydrogenase, Succinyl CoA synthetase, Fumarase
D. Hexokinase, Phosphofructokinase, Pyruvate kinase

What is the primary purpose of the citric acid cycle?

A. To directly produce large amounts of ATP
B. To synthesize glucose from non-carbohydrate precursors
C. To funnel high-energy electrons via NADH and FADH₂ to the electron transport chain
D. To break down fatty acids into acetyl-CoA

Which step produces the first CO₂ molecule in the citric acid cycle?

A. Citrate synthase reaction
B. Isocitrate dehydrogenase reaction (Isocitrate → α-Ketoglutarate)
C. α-Ketoglutarate dehydrogenase reaction
D. Succinate dehydrogenase reaction

Which step produces the second CO₂ molecule in the citric acid cycle?

A. Aconitase reaction
B. Isocitrate dehydrogenase reaction
C. α-Ketoglutarate dehydrogenase reaction (α-Ketoglutarate → Succinyl CoA)
D. Malate dehydrogenase reaction

The citric acid cycle can oxidize acetyl-CoA derived from which macromolecules?

A. Only carbohydrates
B. Only fats
C. Carbohydrates and fats only
D. Carbohydrates, fats, and proteins

What happens to the NADH and FADH₂ produced by the citric acid cycle?

A. They are sent to the electron transport chain for ATP production
B. They are used directly to synthesize glucose
C. They are converted back to NAD⁺ and FAD in the cytoplasm
D. They are stored as energy reserves in the cell