Ch 16 citric acid cycle

Question 1

What are the three names for the citric acid cycle?

Answer 1

Citric Acid Cycle, Krebs Cycle, Tricarboxylic Acid (TCA) Cycle.

Question 2

What is the main function of the citric acid cycle?

Answer 2

Oxidation of acetyl groups to CO₂ and electron transfer to cofactors.

Question 3

Where does the citric acid cycle take place in eukaryotic cells?

Answer 3

Mitochondrial matrix.

Question 4

What enzyme complex converts pyruvate to acetyl CoA?

Answer 4

Pyruvate dehydrogenase complex.

Question 5

What are the three subunits of the pyruvate dehydrogenase complex?

Answer 5

E1 (Pyruvate dehydrogenase), E2 (Dihydrolipoamide acetyltransferase), E3 (Dihydrolipoamide dehydrogenase).

Question 6

What vitamin is required for the E1 subunit of pyruvate dehydrogenase?

Answer 6

Vitamin B1 (Thiamine diphosphate, TDP).

Question 7

What enzyme catalyzes the first step of the cycle?

Answer 7

Citrate synthase.

Question 8

What is the first product of the citric acid cycle?

Answer 8

Citrate.

Question 9

What enzyme catalyzes the conversion of citrate to isocitrate?

Answer 9

Aconitase.

Question 10

What type of reaction does aconitase catalyze?

Answer 10

Isomerization.

Question 11

Which enzyme catalyzes the first decarboxylation step in the cycle?

Answer 11

Isocitrate dehydrogenase.

Question 12

What is the unstable intermediate formed by isocitrate dehydrogenase?

Answer 12

Oxalosuccinate.

Question 13

What enzyme catalyzes the conversion of α-ketoglutarate to succinyl CoA?

Answer 13

α-Ketoglutarate dehydrogenase complex.

Question 14

What is the energy-rich molecule formed in step 5?

Answer 14

GTP (or ATP).

Question 15

What enzyme catalyzes the formation of GTP/ATP?

Answer 15

Succinyl-CoA synthetase.

Question 16

What is the only enzyme in the citric acid cycle embedded in the inner mitochondrial membrane?

Answer 16

Succinate dehydrogenase.

Question 17

What reaction does succinate dehydrogenase catalyze?

Answer 17

Oxidation of succinate to fumarate.

Question 18

What enzyme catalyzes the hydration of fumarate?

Answer 18

Fumarase.

Question 19

What is the final step of the cycle?

Answer 19

Oxidation of malate to oxaloacetate.

Question 20

What enzyme catalyzes the final step of the citric acid cycle?

Answer 20

Malate dehydrogenase.

Question 21

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

Answer 21

Three NADH.

Question 22

How many FADH2 molecules are produced per cycle?

Answer 22

One FADH2.

Question 23

What is the total ATP equivalent yield from one turn of the cycle?

Answer 23

10 ATP equivalents.

Question 24

What enzyme controls the rate of acetyl CoA entering the cycle?

Answer 24

Pyruvate dehydrogenase complex.

Question 25

What molecule inhibits isocitrate dehydrogenase?

Answer 25

ATP and NADH

Question 26

What molecule activates isocitrate dehydrogenase?

Answer 26

ADP and Ca²⁺.

Question 27

What inhibits α-ketoglutarate dehydrogenase complex?

Answer 27

NADH and succinyl CoA.

Question 28

What activates α-ketoglutarate dehydrogenase complex?

Answer 28

Ca²⁺.

Question 29

What is the role of the pyruvate transporter?

Answer 29

Moves pyruvate into the mitochondrial matrix.

Question 30

How does oxaloacetate exit the mitochondria for gluconeogenesis?

Answer 30

It is converted to phosphoenolpyruvate (PEP).

Question 31

What is the function of the malate-aspartate shuttle?

Answer 31

Transfers electrons from NADH in the cytosol into the mitochondria.

Question 32

How is acetyl CoA transported out of the mitochondria?

Answer 32

As citrate via the tricarboxylic acid transporter.

Question 33

What enzyme converts citrate back to acetyl CoA in the cytosol?

Answer 33

ATP-citrate lyase.

Question 34

What is the glyoxylate pathway?

Answer 34

A modified TCA cycle that bypasses CO₂ loss.

Question 35

What organisms use the glyoxylate pathway?

Answer 35

Bacteria, plants, fungi, protists, and some animals (not mammals).

Question 36

What two enzymes are unique to the glyoxylate pathway?

Answer 36

Isocitrate lyase and malate synthase.

Question 37

What does isocitrate lyase do?

Answer 37

Splits isocitrate into glyoxylate and succinate.

Question 38

What is the product of malate synthase?

Answer 38

L-malate.

Question 39

What happens to NADH and FADH2 produced in the TCA cycle?

Answer 39

They donate electrons to the electron transport chain.

Question 40

How many ATP are produced per NADH in oxidative phosphorylation?

Answer 40

Approximately 2.5 ATP.

Question 41

How many ATP are produced per FADH2?

Answer 41

Approximately 1.5 ATP.

Question 42

What is the net ATP yield from glucose oxidation via glycolysis and TCA cycle?

Answer 42

About 32 ATP.

Question 43

What molecule acts as a feedback inhibitor of citrate synthase?

Answer 43

ATP, NADH, succinyl-CoA

Question 44

What intermediate links the TCA cycle to amino acid metabolism?

Answer 44

α-Ketoglutarate.

Question 45

What is the importance of succinyl-CoA outside the TCA cycle?

Answer 45

It is used in heme and chlorophyll biosynthesis.

Question 46

What is a key inhibitor of succinate dehydrogenase?

Answer 46

Malonate.

Question 47

What process ensures oxaloacetate is available for the TCA cycle?

Answer 47

Anaplerotic reactions (e.g., pyruvate carboxylation).

Question 48

Why does the TCA cycle require oxygen?

Answer 48

It depends on NAD+ and FAD, which are regenerated in oxidative phosphorylation.

Question 49

What is the overall purpose of the TCA cycle?

Answer 49

Energy production and biosynthesis of key intermediates.

Question 50

What molecule acts as an activator of citrate synthase?

Answer 50

ADP

Question 51

What molecule acts as a feedback inhibitor of pyruvate dehydrogenase?

Answer 51

ATP, NADH, and acetyl-CoA

Question 52

What molecule acts as an activator of pyruvate dehydrogenase?

Answer 52

ADP, NAD+, and pyruvate

Question 53

What are the coenzymes required by the pyruvate dehydrogenase complex?

Answer 53

Thiamine diphosphate, Lipoamide (E2), Coenzyme A (HS-CoA) in E2, FAD (reduced to FADH2 in E3), NAD+ (forms NADH in E3)