Lecture 20

Question 1

CAC- overview

Answer 1

1. Hub of cellular metabolism.
2. The net reaction of the citric acid cycle is:
   Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O —>
   2CO2 + 3NADH + FADH2 + GTP + 2H+ + CoA
3. Regulation occurs at steps with largest free energy drop
4. The two carbons from Acetyl CoA are fully oxidized to CO
5. Citric Acid Cycle intermediates are precursors for other metabolites
6. 4 pairs of electrons are transferred to oxidize Acetyl CoA to CO2.
7. Shuttles transfer reducing power from to the cytosol to the mitochondrial matrix

Question 2

what type of phosphorylation is succinyl-CoA

Answer 2

substrate-level phosphorylation

Question 3

Substrate-level phosphorylation

Answer 3

is a type of chemical reaction in which ATP is formed by the direct transfer of a phosphate group to ADP from a substrate or reactive intermediate.

Question 4

Succinate dehydrogenase

Answer 4

The only enzyme in this pathway that is membrane associated

Question 5

citrate synthase ΔGº & ΔG

Answer 5

-31.5

negative

Question 6

Isocitrate dehydrogenase ΔGº & ΔG

Answer 6

-21

negative

Question 7

α- ketoglutarate dehydrogenase ΔGº & ΔG

Answer 7

-33

negative

Question 8

malate dehydrogenase ΔGº & ΔG

Answer 8

+29.7

~0

Question 9

Regulation of the Citric Acid Cycle

Answer 9

• Citrate synthase
– Inhibited by citrate, NADH, succinyl CoA, ATP
– Activated by ADP

• Isocitrate dehydrogenase:
– Inhibited by: ATP
– Activated by: Ca2+ ADP

• α-ketoglutarate dehydrogenase
– Inhibited by succinyl CoA, NADH
– Activated by Ca2+

Substrate availability; product inhibition; competitive feedback inhibition; Allosteric regulation

Question 10

Why is the citric acid cycle so effective at extracting energy from Acetyl CoA?

Answer 10

Because the two carbons from Acetyl CoA are fully oxidized to CO2

Calculate the oxidation numbers of the two carbons of acetyl CoA (from the acetyl group) before and after they have gone through the citric acid cycle.

Also notice that carbon atoms of 2CO2 generated in one round are not directly from the acetyl group that entered in the same round.

Question 11

Cataplerotic

Answer 11

depletes intermediates

Question 12

Anaplerotic

Answer 12

builds up intermediates

Question 13

How many NADH made in mitochondria?

how many in cytosol?

Answer 13

8

2

Question 14

glycerophosphate shuttle

Answer 14

is a mechanism that regenerates NAD+ from NADH, a by-product of glycolysis. Its importance in transporting reducing equivalents is secondary to the malate-aspartate shuttle.

Question 15

The malate-aspartate shuttle

Answer 15

This shuttle is particularly important in heart and liver. It is a shuttle for NADH that does NOT cost ATP. The malate dehydrogenase (MDH) occurs as separate enzymes in mitochondria and cytosol (cMDH & mMDH).

Question 16

Aminotransferase

Answer 16

enzyme that catalyzes a type of reaction between an amino acid and an α-keto acid. They are important in the synthesis of amino acids, which form proteins. In medicine, they are an important indicator of liver damage.

Question 17

Which one of the following is the part of the citrate synthase that is most exergonic and provides the driving force for the reaction?

Answer 17

Thioester hydrolysis

Question 18

Which one of the following enzymes is most similar to pyruvate dehydrogenase?

Answer 18

Succinate dehydrogenase

Question 19

Which one of the following statements is correct?

Answer 19

Neither or the two carbons that enter the citric acid cycle as acetyl CoA are removed as CO2 in the first round of citric acid cycle

Question 20

Which one of the following reactions in the citric acid cycle produces ATP in heart muscle by substrate level phosphorylation?

Answer 20

Succinyl-CoA synthetase

Question 21

Which one of the following enzymes of the citric acid cycle is an integral membrane protein?

Answer 21

Succinate dehydrogenase

Question 22

Which one of the following intermediates of the citric acid cycle is least abundant in mitochondria?

Answer 22

Oxaloacetate

Question 23

Which one of the following inhibits many steps in the citric acid cycle?

Answer 23

NADH