Tricarboxylic acid cycle KREBS

What is the first step of the Krebs cycle?

• A. Cleavage of succinyl-CoA

• B. Formation of citrate

• C. Decarboxylation of isocitrate

• D. Oxidation of succinate

• Answer: B. Formation of citrate

Which enzyme catalyzes the formation of GTP in the Krebs cycle?

• A. Citrate synthase

• B. Isocitrate dehydrogenase

• C. Succinate thiokinase

• D. Fumarase

Answer: C. Succinate thiokinase

Cleaves Succinyl-CoA to Succinate

Reaction is coupled to phosphorylation of guanosine diphosphate (GDP) to guanosine triphosphate (GTP).

an example of substrate level phosphorylation

What inhibits isocitrate dehydrogenase?

• A. ADP

• B. Ca²⁺

• C. ATP

• D. FADH₂

Answer: C. ATP

Isocitrate dehydrogenase catalyzes irreversible oxidative decarboxylation of isocitrate (6-C) into α-ketoglutarate (5-C).

Activated by ADP (a low-energy signal) & Ca2+

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

• A. 1

• B. 2

• C. 3

• D. 4

Answer: C. 3

Which intermediate is formed after fumarate in the cycle?

• A. Citrate

• B. Malate

• C. Oxaloacetate

• D. Succinate

Answer: B. Malate

What coenzyme is required for succinate dehydrogenase activity?

• A. NAD⁺

• B. FAD

• C. CoA

• D. TPP

Answer: B. FAD

Succinate (4-C) is oxidized to fumarate (4-C) by succinate dehydrogenase, as FAD (its coenzyme) is reduced to FADH2

What is the end product of malate oxidation?

• A. Succinate

• B. Citrate

• C. Oxaloacetate

• D. Acetyl-CoA

Answer: C. Oxaloacetate

Malate (4-C) is oxidized to oxaloacetate (4-C) by malate dehydrogenase.  This reaction produces 1 NADH.

Which enzyme of the TCA cycle is also part of the electron transport chain?

• A. Malate dehydrogenase

• B. Succinate dehydrogenase

• C. Isocitrate dehydrogenase

• D. α-Ketoglutarate dehydrogenase

Answer: B. Succinate dehydrogenase

Which step produces FADH₂ in the TCA cycle?

• A. Oxidation of succinate

• B. Oxidation of isocitrate

• C. Cleavage of succinyl-CoA

• D. Oxidation of malate

Answer: A. Oxidation of succinate to fumarate

What molecule combines with acetyl-CoA to begin the TCA cycle?

• A. Citrate

• B. Oxaloacetate

• C. Pyruvate

• D. Succinate

Answer: B. Oxaloacetate

The TCA cycle occurs in the mitochondrial matrix.

Answer: True

ATP is directly generated in the TCA cycle.

Answer: False
Explanation: GTP, which is energetically equivalent to ATP, is produced.

FADH₂ is produced during the conversion of succinate to fumarate.

Answer: True

Citrate synthase catalyzes the only reversible reaction in the TCA cycle.

Answer: False
Explanation: Citrate synthase catalyzes an irreversible step.

Condensation of acetyl CoA & oxaloacetate to form citrate (6-C) (a tricarboxylic acid) is catalyzed by citrate synthase.

Isocitrate dehydrogenase produces CO₂.

Answer: True

Isocitrate dehydrogenase catalyzes irreversible oxidative decarboxylation of isocitrate (6-C) into α-ketoglutarate (5-C).

This step yield 1 NADH molecules & 1 CO2

Oxygen is directly used in the reactions of the TCA cycle.

Answer: False
Explanation: Oxygen is used indirectly as the final electron acceptor in ETC.

Succinate dehydrogenase is an enzyme located in the mitochondrial membrane.

Answer: True

Malate is oxidized to oxaloacetate, producing NADH.

Answer: True

Pyruvate dehydrogenase is part of the TCA cycle.

Answer: False
Explanation: It is a preparatory step producing acetyl-CoA.

The TCA cycle contributes intermediates for biosynthesis.

Answer: True

example : NADH , FADH2

Which enzyme catalyzes the first reaction of the TCA cycle?

Answer: Citrate synthase.

What is the product of the oxidative decarboxylation of α-ketoglutarate?

Answer: Succinyl-CoA.

Name one coenzyme required for the α-ketoglutarate dehydrogenase complex.

Answer: Thiamine pyrophosphate (TPP).

• Which reaction in the TCA cycle produces FADH₂?
  

Answer: Oxidation of succinate to fumarate.

How many molecules of ATP are generated per NADH in oxidative phosphorylation?

Answer: 3 ATP.