Ch. 16 Citric Acid Cycle

Question 1

Where do glycolysis and the citric acid cycle happen, respectively?

Answer 1

Glycolysis happens in the cytosol. Citric acid cycle happens in the mitochondrial matrix.

Question 2

What is the difference between the inner and outer mitochondrial membrane?

Answer 2

Outer membrane is leaky while the inner membrane is a barrier.

Question 3

What do we get out of glycolysis that can feed into the citric acid cycle?

Answer 3

two pyruvate from glucose

Question 4

What needs to happen to pyruvate before the citric acid cycle can occur?

Answer 4

pyruvate must be moved into the mitochondrial matrix

Question 5

How is pyruvate moved into the mitochondrial matrix? (4)

Answer 5

Pyruvate (3C) is 1. oxidized and 2. decarboxylated to acetyl-CoA by pyruvate dehydrogenase. 3. CoA is covalently attached to two carbons from pyruvate → acetyl-CoA.

Question 6

Why does the conversion of pyruvate to acetyl-CoA occur (other than for transport into the mito)?

Answer 6

It is a decarboxylation reaction which is favorable.

Question 7

What else is involved in the conversion of pyruvate to acetyl-CoA and what happens to it?

Answer 7

NAD⁺ is reduced to NADH (energy input)

Question 8

What is the significance of acetyl-CoA for the citric acid cycle? (2)

Answer 8

It is how carbon enters the citric acid cycle. It also has a thioester bond which is very high in energy (about -30.5 kJ/mol)

Question 9

What drives the formation of the thioester bond and reduction of NAD⁺ in acetyl-CoA formation?

Answer 9

The decarboxylation of pyruvate drives both thioester bond formation and NAD reduction.

Question 10

Why is acetyl-CoA known as in metabolism?

Answer 10

it is the most central metabolite

Question 11

What is the first step of the citric acid cycle and what occurs?

Answer 11

Citrate Synthase Step.
acetyl-CoA (2C) condenses with oxaloacetate (4C) to produce citrate (6C) via citrate synthase.
(cooperative binding happening)

Question 12

What is the ΔG of citrate synthase and what does this mean about the reaction (2)?

Answer 12

ΔG = -54 kJ/mol which means products are massively favored keeping [oxaloacetate] EXCEEDINGLY low.
Regulatory enzyme!!

Question 13

What drives the citrate synthase step forward?

Answer 13

The breaking of the thioester bond drives the reaction forward.

Question 14

What is citrate synthase allosterically inhibited by? (2)

Answer 14

ALLOSTERICALLY inhibited by succinyl-CoA and NADH (feedback inhibition)

Question 15

What happens to coenzyme A in the citrate synthase step?

Answer 15

CoA is produced by this step and recycled.

Question 16

What is the net reaction of the conversion of pyruvate to acetyl-CoA for transport into the mitochondria?

Answer 16

pyruvate + CoA + NAD⁺ → acetyl-CoA + NADH + H⁺ + CO₂
(via pyruvate dehydrogenase)

Question 17

What is the second step of the citric acid cycle and what happens?

Answer 17

Aconitase Step.
citrate is isomerized to isocitrate via aconitase.

Question 18

What is the ΔG of aconitase and what does this mean?

Answer 18

ΔG = 0.8 kJ/mol so it is bidirectional

Question 19

What is the rationale for the aconitase step?

Answer 19

Citrate has a tertiary alcohol that CANNOT be oxidized without breaking a C-C bond :( . This step sets up for an oxidation step.

Question 20

What is the third step of the citric acid cycle and what happens?

Answer 20

Isocitrate Dehydrogenase Step.
isocitrate is converted to ɑ-ketoglutarate, NADH, and CO₂ by isocitrate dehydrogenase.
(NAD⁺ is reduced)

Question 21

Where does the energy to carry out the isocitrate dehydrogenase step come from?

Answer 21

The reaction is a decarboxylation reaction which is energetically favorable and drives the reaction. (go from 6C → 5C)

Question 22

What two chemical reactions/phenomenon happen to isocitrate in the isocitrate dehydrogenase step?

Answer 22

Isocitrate is both oxidized and decarboxylated.

Question 23

What is the ΔG of isocitrate dehydrogenase and what does this mean?

Answer 23

ΔG = -17 kJ/mol which means it is unidirectional and it PULLS the aconitase reaction forward.
Regulatory enzyme!

Question 24

What is isocitrate dehydrogenase inhibited by? (2)

Answer 24

Inhibited by NADH and ATP (high energy molecules)

Question 25

What is isocitrate dehydrogenase stimulated by?

Answer 25

Stimulated by ADP (low energy molecule)

Question 26

Does ɑ-ketoglutarate stay in the citric acid cycle?

Answer 26

Not always; there is divergence. It is a precursor to many amino acids, so the 5Cs can and do leave the citric acid cycle.

Question 27

What is the fourth step of the citric acid cycle and what happens?

Answer 27

ɑ-Ketoglutarate Dehydrogenase Step. ɑ-ketoglutarate decarboxylated and oxidized (5C → 4C) via ɑ-ketoglutarate dehydrogenase. Reaction produces succinyl-CoA, NADH, and CO₂. (energy input)

Question 28

Why does the ɑ-ketoglutarate dehydrogenase step require an energy input? (2)

Answer 28

You are producing two high energy bonds. 1. You are making NADH from NAD⁺. 2. You are forming a thioester bond in succinyl-CoA.

Question 29

Where does the energy for the fourth step come from?

Answer 29

The reaction is a decarboxylation, which is always favorable. The decarboxylation provides the energy.

Question 30

What is the ΔG of ɑ-ketoglutarate dehydrogenase and what does this mean?

Answer 30

ΔG = -44 kJ/mol so it is unidirectional. Regulatory enzyme!

Question 31

What is ɑ-ketoglutarate dehydrogenase inhibited by? (2)

Answer 31

Inhibited by NADH and succinyl-CoA. (feedback inhibition)

Question 32

What is ɑ-ketoglutarate dehydrogenase stimulated by?

Answer 32

Stimulated by AMP.

Question 33

What is the fifth step of the citric acid cycle and what happens?

Answer 33

Succinyl-CoA Synthetase Step. the succinyl-CoA thioester bond is broken and the energy is used to phosphorylate GDP → GTP (SLP). Produces GTP (only phosphoanhydride bond made in the citric acid cycle) , succinate, and CoA.

Question 34

What is the ΔG of succinyl-CoA synthetase and what does this mean?

Answer 34

ΔG = 0 kJ/mol so it is bidirectional.

Question 35

What is the sixth step of the citric acid cycle and what happens?

Answer 35

Succinate Dehydrogenase Step. succinate (4C) is oxidized to fumarate (4C) via succinate dehydrogenase. FAD is reduced to FADH₂

Question 36

What is succinate dehydrogenase?

Answer 36

It is an integral membrane protein in the inner mitochondrial membrane that is also part of the electron transport chain.

Question 37

What is the ΔG of succinate dehydrogenase and what does this mean?

Answer 37

ΔG = 0 kJ/mol so it is bidirectional.

Question 38

What does FAD carry and how does it differ from NADH? (1)

Answer 38

FAD carries two electrons to become FADH₂. It is a high energy molecule, but not as high in energy as NADH.

Question 39

What is the seventh step of the citric acid cycle and what happens?

Answer 39

Fumarase Step. fumarate is converted to malate via fumarase

Question 40

What is the ΔG of fumarase and what does this mean?

Answer 40

ΔG = 0 kJ/mol so it is bidirectional.

Question 41

What is the eighth and final step of the citric acid cycle and what happens?

Answer 41

Malate Dehydrogenase Step. malate and NAD⁺ are converted to oxaloacetate and NADH by malate dehydrogenase.

Question 42

What is the ΔG for malate dehydrogenase and what does this mean?

Answer 42

ΔG = +30 kJ/mol so it shouldn't occur but it does.

Question 43

How does the malate dehydrogenase step occur with a positive ΔG?

Answer 43

The concentration of oxaloacetate is essentially 0M because of citrate synthase. Citrate synthase PULLS the reaction forward.

Question 44

What is the citric acid cycle net reaction?

Answer 44

acetyl-CoA + 3NAD⁺ + FAD + GDP + Pᵢ + 2H₂O → 2CO₂ + 3NADH + 3H⁺ + CoA + FADH₂ + GTP Catabolic Process!

Question 45

How many times does the citric acid cycle happen per glucose molecule?

Answer 45

Twice. One acetyl-CoA goes through the cycle, but glucose produces two acetyl-CoA so we get TWO REVOLUTIONS per glucose.

Question 46

What does it mean the the citric acid cycle can be both catabolic and anabolic?

Answer 46

We know that it is catabolic. But, carbon frequently leaves the cycle as precursors to nucleotide, aa, fatty acids, etc (divergence). In this sense it can be anabolic.

Question 47

If anabolism is happening during the citric acid cycle what must happen to the precursors?

Answer 47

They must get out of the mitochondrial matrix, so specific transport proteins are needed.

Question 48

How does the citric acid cycle continue to occur if carbon leaves breaking the cycle?

Answer 48

Metabolites are in pools, so some can leave without completely breaking the cycle. (anaplerotic reactions)

Question 49

What are anaplerotic reactions?

Answer 49

"filling up" reactions. Reactions that produce metabolites in the citric acid cycle.

Question 50

What are three examples of anaplerotic reactions? Which one is the most important?

Answer 50

1. Malate Dehydrogenase: oxaloacetate ⇌ malate 2. PEP Carboxylase: PEP → oxaloacetate 3. Pyruvate Carboxykinase: pyruvate → oxaloacetate which requires CO₂ Most important one is pyruvate carboxykinase!

Question 51

In a global sense, what inhibits the CA Cycle and what stimulates it?

Answer 51

inhibit = high energy stimulate = low energy

Question 52

What kind of energy designation does succinyl-CoA have?

Answer 52

It is seen as high energy, so it INHIBITS. (Anabolism is occurring, so high [succinyl-CoA] = high [all citric acid metabolites] = anabolic pathways are NOT drawing carbon out = inhibit)

Question 53

What does Ca²⁺ do to the citric acid cycle (regulation)?

Answer 53

It stimulates the citric acid cycle.

Question 54

What is the glyoxalate cycle?

Answer 54

A way for organisms to oxidize acetate to CO₂ and produce NADH.

Question 55

What are the basic steps of the glyoxalate cycle? (3)

Answer 55

1. acetate (2C) easily converted to acetyl-CoA 2. acetyl-CoA enters glyoxalate cycle 3. NADH produced

Question 56

Where does the glyoxalate cycle occur?

Answer 56

ONLY occurs in seeds/seed plants like corn, soybeans, wheat...