Citric Acid Cycle/TCA Cycle/KrebsCycle

Question 1

Where does the citric acid cycle/TCA/Krebs Cycle occur?

Answer 1

Mitochondrial matrix

(occurs aerobically)

Question 2

Pyruvate from glycolysis is oxidized into a two-carbon acetyl group. This acetyl group must be “activated” before entry into the citric acid cycle. How does this activation occur?

Answer 2

The acetyl group is bonded to the thiol group (-SH) of coenzyme A.

Acetyl Co-A is formed.

Question 3

What kind of bond is formed in acetyl Co-A?

Answer 3

High energy thioester bond

(between the acetyl group and coenzyme A)

Question 4

Which enzyme oxidizes pyruvate (from glycolysis) into acetyl Co-A for the citric acid cycle?

Answer 4

Pyruvate dehydrogenase (PDH)

Glycolysis → [PDH Complex] → Citric acid cycle

Question 5

What are the products of the pyruvate dehydrogenase (PDH) complex?

Answer 5

1. acetyl Co-A (2)
2. NADH (2)

Question 6

[Mnemonic]

Tender Loving Care For Nobody

(how to remember the coenzymes of the PDH complex)

Answer 6

Tender Loving Care For Nobody

TPP

Lipoic Acid

coenzyme A

FAD

NAD+

Question 7

What are the five coenzymes of the PDH complex?

Answer 7

1. TPP
2. Lipoic acid
3. coenzyme-A
4. FAD (reduced to FADH₂)
5. NAD^{<strong>+</strong>} (reduced to NADH)

Question 8

ATP is ONLY produced once in the citric acid cycle. When does this occur?

Answer 8

Succinyl-coA synthetase uses ATP to form succinate.

Question 9

[Mnemonic]

Please, Can I Keep Selling Shirts For Money, Officer?

(how to remember the nine substrates of the Citric Acid Cycle)

Answer 9

Please, Can I Keep Selling Shirts For Money, Officer?

Pyruvate

Citrate

Isocitrate

α-Ketoglutarate (α-KG)

Succinyl-CoA

Succinate

Fumarate

Malate

Oxaloacetate (OAA)

Question 10

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

Answer 10

Isocitrate dehydrogenase

Question 11

Which step of the citric acid cycle does NOT occur in the mitochondrial matrix?

Answer 11

Formation of fumarate

(occurs in the inner mitochondrial membrane because SDH is membrane-bound)

Question 12

Which two intermediates of the citric acid cycle undergo a decarboxylation reaction (removal of CO₂)?

Answer 12

1. isocitrate
2. α-ketoglutarate

Question 13

What are the nine substrates of the citric acid cycle?

Answer 13

1. pyruvate
2. citrate
3. isocitrate
4. α-ketoglutarate (α-KG)
5. succinyl-CoA
6. succinate
7. fumarate
8. L-malate
9. oxaloacetate (OAA)

Question 14

What are the three control points of the citric acid cycle?

Answer 14

1. citrate synthase
2. isocitrate dehydrogenase
3. α-ketoglutarate dehydrogenase complex

(They are all inhibited by ATP and NADH.)

Question 15

What is the difference between a synthase and synthetase?

Answer 15

Synthase: forms covalent bonds WITHOUT energy input

Example: Citrate synthase from the citric acid cycle.

Synthetase: forms covalent bonds WITH energy input

Example: Succinyl-CoA synthetase from the citric acid cycle.

Question 16

Which enzyme of the citric acid cycle uses FAD?

Answer 16

Succinate dehydrogenase (SDH)

SDH is considered a flavoprotein because it is covalently bound to FAD.

Question 17

Which enzyme functions in BOTH the citric acid cycle and electron transport chain (ETC)?

Answer 17

Succinate dehydrogenase (SDH)