Final: Chapter 10 The Citrate Cycle

Question 1

Why is the TCA considered the “hub” of cellular metabolism? (Three reasons)

Answer 1

• Is central to aerobic metabolism and ATP production
• Links oxidation of various metabolites to ATP synthesis through shared intermediates
• Provides metabolism for a number of biosynthetic pathways

Question 2

What is the primary function of the citrate cycle?

Answer 2

To oxidize acetyl CoA

Question 3

What is pyruvate dehydrogenase and what does it do?

Answer 3

It is a complex that converts pyruvate to acetyl-CoA

Question 4

What five coenzymes are required for pyruvate dehydrogenase activity?

Answer 4

NAD+, FAD, CoA, thiamine pyrophosphate (TPP), and alpha-lipoic acid

Question 5

What causes pellagra (“rough skin”)?

Answer 5

Niacin deficiency (usually from diet)

Question 6

Inflamed skin, diarrhea, dementia, and sores in the mouth are the symptoms of what disease?

Answer 6

Pellagra/niacin deficency

Question 7

What are the common coenzymes in oxidation-reduction reactions?

Answer 7

NAD+ and FAD

Question 8

What is NAD+ derived from?

Answer 8

Niacin (vitamin B3)

Question 9

What is FAD derived from?

Answer 9

Riboflavin (vitamin B2)

Question 10

What is coenzyme A (CoA) derived from and what is it used for?

Answer 10

Derived from vitamin B5 and is used to generate acetyl CoA

Question 11

What is thiamine pyrophosphate (TPP) derived from and what is it used for?

Answer 11

Derived from vitamin B1 and is required for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase activty

Question 12

What does alpha-lipoic acid do?

Answer 12

Provides a reactive disulfide that participates in redox reactions

Question 13

What regulates pyruvate dehydrogenase?

Answer 13

Phosphorylation, NADH levels, ATP levels, and acetyl-CoA levels

Question 14

How does arsenic inhibit pyruvate dehydrogenase?

Answer 14

It irreversibly blocks catalytic activity of lipoamide-containing enzymes

Question 15

How many reactions are in the citrate cycle?

Answer 15

Eight reactions

Question 15

How many reactions are in the citrate cycle?

Answer 15

Eight reactions

Question 16

What is the first reaction of the citrate cycle and what enzyme is involved?

Answer 16

Condensation reaction catalyzed with citrate synthase to convert oxaloacetate and acetyl-CoA to citrate

Question 17

What is the second reaction of the citrate cycle and what enzyme is involved?

Answer 17

• Citrate undergoes isomerization to form aconitase
• Citrate first dehydrated along with aconitase to form cis-Aconitate
• cis-Aconitate is hydrated along with aconitase to form isocitrate
• Aconitase contain an iron-sulfur cluster (acts a lewis acid)

Question 18

What is the third reaction of the citrate cycle and what enzyme is involved?

Answer 18

• Oxidative decarboxylation using isocitrate dehydrogenase to convert isocitrate to alpha-ketoglutarate and produces NADH
• Isocitrate first converted to oxalosuccinate and then converted to alpha-ketoglutarate (each step using isocitrate dehydrogenase)

Question 19

What is the fourth reaction of the citrate cycle and what enzyme is involved?

Answer 19

Alpha-ketoglutarate undergoes oxidative decarboxylation using alpha-ketoglutarate dehydrogenase to produce succinyl-CoA and NADH

Question 20

What is the fifth reaction of the citrate cycle and what enzyme is involved?

Answer 20

Succinyl-CoA undergoes substrate level phosphorylation using succinyl-CoA synthetase to produce succinate and GTP

Question 21

What is the sixth reaction of the citrate cycle and what enzyme is involved?

Answer 21

Succinate undergoes a redox reaction using succinate dehydrogenase to produce fumarate and FADH2

Question 22

What is the seventh reaction of the citrate cycle and what enzyme is involved?

Answer 22

• Fumarate undergoes hydration using fumarase to form malate
• This reaction is stereospecific (only the L-isomer)

Question 23

What is the eight reaction of the citrate cycle and what enzyme is involved?

Answer 23

Malate undergoes oxidation using malate dehydrogenase to form oxaloacetate and NADH

Question 24

What three main regulatory enzymes regulate the citrate cycle?

Answer 24

Citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase

Question 25

What are anaplerotic reactions?

Answer 25

Reactions that replenish citrate cycle intermediates that have been shunted to other metabolic pathways (ex: pyruvate carboxylase converts pyruvate to oxaloacetate with the help of biotin)

Question 26

What does phosphoenolpyruvate carboxylase do?

Answer 26

This enzyme is involved in an anaplerotic reaction that replenishes oxaloacetate

Question 27

Since the citrate cycle is an aerobic process,
where does it occur?

A) Cytoplasm
B) Mitochondria
C) Ribosome
D) Endoplasmic reticulum
E) None of the above

Answer 27

B) Mitochondria

Question 28

High levels of succinate would result from a
deficiency of which enzyme?

A) Citrate synthase
B) Isocitrate dehydrogenase
C) α-ketoglutarate dehydrogenase
D) Succinyl-CoA dehydrogenase
E) Succinate dehydrogenase

Answer 28

E) Succinate dehydrogenase

Question 29

Citric acid is found in high levels in fruit juices.
How would drinking several glasses of orange
juice effect the citrate cycle?

A) The citrate cycle would not occur, since step 1
produces citrate.
B) Elevated levels of citrate would feed into the cycle
to make more ATP equivalents.
C) Step 1 (citrate synthase) would be bypassed.
D) The cycle would go in the reverse manner.
E) B and C

Answer 29

E) B and C

Question 30

Why does the citrate cycle produce more
energy than glycolysis?

A) It is an aerobic process.
B) It indirectly forms ATP through GTP, NADH, and FADH2 .
C) It is a cyclic pathway.
D) A and B
E) B and C

Answer 30

E) B and C