Citric acid cycle

Question 1

What is the purpose of aerobic catabolism?

Answer 1

To capture energy in biological fuel molecules and use it to make ATP.

Question 2

What is the function of the citric acid cycle?

Answer 2

To oxidize acetyl-CoA to CO2, generate high-energy products like NADH, FADH2/QH2, and GTP (NTP)

NTP= Nucleoside triphosphates
QH2=ubiquinol

Question 3

Where does the citric acid cycle occur?

Answer 3

In the mitochondrial matrix (eukaryotes).

Question 4

Is the citric acid cycle aerobic or anaerobic?

Answer 4

Aerobic. Oxygen is needed to reoxidize NADH and FADH2.

Question 5

What is the term for a metabolic pathway that can be used for both catabolic and anabolic processes?

Answer 5

Amphibolic. The citric acid cycle is an example of an amphibolic pathway

Question 6

What are some examples of molecules that can be metabolized to generate acetyl-CoA for the citric acid cycle?

Answer 6

Carbohydrates, fats, and proteins.

Question 7

What is the first step in the citric acid cycle?

Answer 7

Acetyl-CoA (2C) combines with oxaloacetate (4C) to make citrate (6C).

Question 8

What enzyme catalyzes the first step of the citric acid cycle and is it regulated?

Answer 8

Citrate synthase. This reaction is not regulated.

Question 9

What happens to the citrate molecule in the citric acid cycle?

Answer 9

It undergoes a series of reactions, including two oxidative decarboxylations, ultimately leading to the regeneration of oxaloacetate.

Question 10

How many NADH molecules are generated per acetyl-CoA in the citric acid cycle?

Answer 10

3 NADH molecules.

Question 11

How many FADH2/QH2 molecules are generated per acetyl-CoA in the citric acid cycle?

Answer 11

1 FADH2/QH2 molecule

Question 12

How many GTP molecules are generated per acetyl-CoA in the citric acid

Answer 12

1 GTP molecule.

Question 13

Which steps in the citric acid cycle are oxidative decarboxylations?

Answer 13

The steps catalyzed by isocitrate dehydrogenase (step 3) and α-ketoglutarate dehydrogenase (step 4)

Question 14

What is significant about the enzyme α-ketoglutarate dehydrogenase?

Answer 14

It catalyzes an oxidative decarboxylation step in the citric acid cycle that is similar to the pyruvate dehydrogenase (PDH) reaction. It is also regulated.

Question 15

Which step in the citric acid cycle involves substrate-level phosphorylation?

Answer 15

The step catalyzed by succinyl-CoA synthetase (step 7) where GTP is generated.

Question 16

Which enzyme in the citric acid cycle is FAD-dependent?

Answer 16

Succinate dehydrogenase, which is also an integral membrane protein and part of Complex II in the electron transport chain.

Question 17

What happens to the FADH2 generated by succinate dehydrogenase?

Answer 17

It is reoxidized by donating electrons to coenzyme Q (ubiquinone), which is then reduced to ubiquinol (QH2).

Question 18

How is the citric acid cycle regulated?

Answer 18

By the NAD+/NADH ratio and the concentrations of intermediates. Regulated enzymes include isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

Question 19

What molecules inhibit isocitrate dehydrogenase and α-ketoglutarate dehydrogenase?

Answer 19

NADH and ATP

Question 20

What molecules activate isocitrate dehydrogenase and α-ketoglutarate dehydrogenase?

Answer 20

ADP and Ca++

Question 21

Why is the citric acid cycle considered amphibolic?

Answer 21

Because its intermediates can be used in the synthesis of other molecules like amino acids, carbohydrates, fats, and nucleotides.

Question 22

What are anaplerotic reactions?

Answer 22

Reactions that replenish citric acid cycle intermediates that may be consumed in other processes.

Question 23

What is an important anaplerotic reaction and what enzyme catalyzes it?

Answer 23

he formation of oxaloacetate from pyruvate, catalyzed by pyruvate carboxylase

Question 24

What molecules activate and inhibit pyruvate carboxylase?

Answer 24

Acetyl-CoA activates it, while ADP inhibits it.

Question 25

What is the approximate ATP yield from the complete oxidation of one glucose molecule under aerobic conditions?

Answer 25

~ 32 ATP

Question 26

How much ATP does anaerobic glycolysis generate?

Answer 26

2 ATP (net)

Question 27

Why does the addition of oxygen to an anaerobic culture of yeast decrease glucose consumption?

Answer 27

Yeast cells can produce more ATP from glucose under aerobic conditions via the citric acid cycle and oxidative phosphorylation than through anaerobic fermentation.

Question 28

Can lactate be used as a metabolic fuel in aerobic metabolism?

Answer 28

Yes, lactate can be oxidized to pyruvate, which then enters the citric acid cycle.

Question 29

What are the energy capture steps in the citric acid cycle?

Answer 29

Steps 3, 4, 5, 6, and 8 generate NADH, FADH2, or GTP.

Question 30

What are the oxidation reactions in the citric acid cycle?

Answer 30

Steps 3, 4, 6, and 8 involve oxidation.

Question 31

Which reaction links glycolysis to the citric acid cycle?

Answer 31

The reaction catalyzed by the pyruvate dehydrogenase complex (Step 9).

Question 32

What are the regulated steps in the citric acid cycle?

Answer 32

Steps 1, 3, 4, and 9 are regulated.

Question 33

What is the substrate level phosphorylation step in the citric acid cycle?

Answer 33

Step 5, where GTP is generated.