Citric Acid Cycle

Question 1

Citric Acid Cycle role

Answer 1

Catalyze the net oxidation of acetyl-CoA to CO2

Question 2

Amphibolic

Answer 2

Both catabolism and anabolism

Question 3

Pyruvate dehydrogenase complex

Answer 3

Multienzyme that links glycolysis to the CAC

Question 4

What are the advantages of a multienzyme?

Answer 4

-Intermediates are never released into solution because the product of one active site quickly moves to the next
-Easier to regulate all enzymes
-Minimizes side rxns

Question 5

What are the 3 active sites?

Answer 5

E1(steps 1,2), E2 (step 3) and E3 (steps 4,5)

Question 6

Where is the Pyruvate dehydrogenase complex located?

Answer 6

The mitochondrial matrix (pyruvate from glycolysis must move from the cytosol to the mitochondrion

Question 7

E 1 (steps 1,2)

Answer 7

Decarboxylation of pyruvate and transfer of the acetyl group to lipoamine

Question 8

E2 step 3

Answer 8

Transfer of the acetyl group to CoA

Question 9

E3 (step 4,5)

Answer 9

Dihydrolipoamide os reoxidized using NAD+ and FAD

Question 10

Lipoamide linkage

Answer 10

Linked to a long flexible chain of lysine in the E2 active site

Question 11

How does Acetly-CoA enter the CAC

Answer 11

Acetyl-CoA enters from the pyruvate dehydrogenase complex, amino acid catabolism or fatty acid oxidation

Question 12

What steps of the CAC are irreversible?

Answer 12

1,3,4

Question 13

Where does step 6 occur vs all the other steps

Answer 13

Step 6 occurs in the inner membrane and the rest occur in the mitochondrial matrix

Question 14

Result of each acetyl entered

Answer 14

2 molecules of fully oxidized CO2 are produced

Question 15

Step 1 Citrate synthase

Answer 15

Formation of citrate; 4c to 6c; large negative free energy= -31.5

Question 16

Why is the CAC cyclic?

Answer 16

The large negative delta G from 1. Citrate synthase can pull the 8. malate dehydrogenase reaction forward as it has a positive delta G

Question 17

Step 2 Aconitase

Answer 17

Hydroxyl is moved to the carbon originating from step 8. Oxaloacetate and not from the carbon from acetyl CoA

Question 18

Where are the electrons going in step 6

Answer 18

Electrons are passed to the FAD enzyme prosthetic cofactor to form FADH2; FADH2 then passes 2 e- to Q producing QH2

Question 19

Energy yield from the aerobic breakdown of glucose to CO2

Answer 19

6-NADH —> 15 ATP
2 QH ——> 3 ATP
2 GTP
4 CO2

Question 20

How is the kreb cycle regulated

Answer 20

At the irreversible steps (1,3,4)

Question 21

How does step 1 regulate

Answer 21

Substrate availability of Acetyl CoA and oxaloacetate;
Product and feedback Inhibition by NADH, citrate and succinyl-CoA

Question 22

How does step 1 regulate

Answer 22

Substrate availability of Acetyl CoA and oxaloacetate;
Product and feedback Inhibition by NADH, citrate and succinyl-CoA

Question 23

How does step 3 regulate

Answer 23

Allosteric Inhibition by NADH and ATP; allosteric activated by Ca2+ and ADP

Question 24

How’s does step 4 regulate

Answer 24

Allosterically Inhibited by succinyl-CoA and NADH
Allosterically Activated by Ca2+

Question 25

Where does Ca2+ come from to Allosterically activate the CAC

Answer 25

Influx of Ca2+ in response to epinephrine

Question 26

Why is the citric acid cycle considered to be a central metabolic pathway

Answer 26

It’s intermediates serve as a precursor for anabolic pathways through cataplerotic reactions

Question 27

Anaplerotic reactions

Answer 27

Replenish citric acid cycle intermediates

Question 28

Cataplerotic reactions

Answer 28

Disposal of citric acid cycle intermediates

Question 29

How are the carbons lost as CO2 in the kreb cycle

Answer 29

Through decarboxylation; each pyruvate loses one carbon to CoA to form the 2 carbon molecule acetyl-CoA; the carbon atom removed takes 2 Oxygens with it to exit the body as CO2