Chapter 19- The Citric Acid Cycle

Question 1

What is the overview of the citric acid cycle?

Answer 1

Acetyl unit with two carbons enters the cycle, 2 carbons come out as carbon dioxide
8 reactions (4 are degydrogenases, oxidation/reduction)
Four carbon acceptor rotates through cycle continuously
Once acetyl CoA goes in, 2 carbons are released, 3 NADH produced, 1 FADH2 produced (8 electrons), and 1 atp produced
Slide 3-5

Question 2

How is acetyl CoA converted to citrate in the first step of the citric acid cycle?
What enzyme does this?
Slide 7

Answer 2

Slide 7
Citrate synthase condenses acetyl CoA and oxaloacetate to citrate
(Synthase is an enzyme that joins two units without use of ATP)
H2O is used to converted to CoA

Question 3

How is citrate converted to isocitrate in the second step of citric acid cycle?
What enzyme does this?
Slide 8

Answer 3

Slide 8
Aconitase catalyzes the formation of isocitrate from citrate
Simply changed isomer by switching OH and H

Question 4

How is isocitrate converted to α-Ketoglutarate in the 3rd reaction of the citric acid cycle?
What enzyme does this?
Slide 9

Answer 4

Slide 9
Isocitrate dehydrogenase (oxidative decarboxylation of isocitrate)
Reduction to NADH + H from NAD+ then uses H+ to form first CO2 of pathway

6 carbon to 5 carbon

Question 5

How is α-ketoglutarate converted to succinyl CoA in the 4th reaction of the citric acid cycle?
What enzyme does this?
Slide 10

Answer 5

Slide 10
α-Ketoglutarate dehydrogenase
Carboxyl group comes off as second carbon dioxide
Coenzyme A brought in with NAD+ and creates CO2 and NADH
5 carbon to 4 carbon

Question 6

How is succinyl CoA converted to succinate in the 5th reaction of the citric acid cycle?
What enzyme does this?
Slide 11

Answer 6

Slide 11
Succinyl CoA synthetase cleaves thioester linkage and forms ATP
(Synthetase because it needs ATP in the reverse direction and this reaction was discovered in the reverse direction)
First step to produce ATP

Question 7

How is succinate converted to fumerate in the 6th reaction of the citric acid cycle?
What enzyme does this? what inhibits this enzyme????
Slide 12

Answer 7

Slide 12
Succinate dehydrogenase, inhibited by malonate
FAD brought in to produce FADH2
Oxidation of succinate produces fumerate

Question 8

How is fumerate converted to malate in the 7th reaction of the citric acid cycle?
What enzyme does this?
Slide 12

Answer 8

Slide 12
Fumerase
Adds water and convert to malate

Question 9

How is malate converted to oxaloacetate in the 8th reaction of the citric acid cycle?
What enzyme does this?
Slide 12

Answer 9

Slide 12
Malate dehydrogenase
Two hydrogen atoms used to convert NAD to NADH + H+

Question 10

What is the net reaction of the citric acid cycle?

What is the ATP produced breakdown?

Answer 10

Acetyl CoA + 3 NAD+ + FAD + ADP + Pi + 2H2O -> 2CO2 + 3NADH + FADH2 + ATP + 2H + CoA

Each Acetyl CoA produces 10 ATP (NADH is 2.5, FADH2 is 1.5)
2 acetyl CoA from 2 pyruvate = 20 ATP
2 pyruvate to acetyl CoA= 5ATP

Pyruvate to citric acid cycle= 25 ATP
add glucose molecules= 32ATP Produced total so far!!!

Question 11

Study the break down of citric acid cycle on slide 6

Answer 11

Okay

Question 12

How is the citric acid cycle regulated? (Inhibitor and activators at pyruvate-acetyl CoA, isocitrate-α-ketoglutarate, and α-ketogkutarate-succinyl CoA)

Answer 12

Pyruvate->acetyl CoA
activators: ADP and pyruvate
Inhibitors: ATP, Acetyl CoA, and NADH
Isocitrate->α-ketoglutarate
Activators: ADP
Inhibitors: ATP and NADH
α-ketoglutarate->Succinyl CoA
Activators: none
Inhibitors: ATP, succinyl CoA, and NADH
Slide 15 overview

Question 13

What is an inhibitor of succinate dehydrogenase?

How is succinate related to malonate?

Answer 13

Malonate
Slide 14
Succinate has two CH2 compared to malonate having one
FAD is prosthetic group of succinate

Question 14

What happens if the citric acid cycle is inhibited but we still need oxaloacetate for other amino acids, purines, and pyrimidines?

Answer 14

Pyruvate can be directly converted to oxaloacetate without producing more energy the body doesn’t need through the citric acid cycle

Slide 4 Chapter 17 picture
Enzyme pyruvate carboxylase

Question 15

What are anaplerotic reactions?

Answer 15

Replenishing reactions that restore the citric acid cycle components used as precursors for biosynthesis if the precursors are needed but the energy from the cycle isn’t

Pyruvate to oxaloacetate is an example

Question 16

What is the glyoxylate cycle?

Answer 16

Similar to citric acid cycle but bypasses the two decarboxylation steps which allows synthesis of carbohydrates from fats
Succinate is converted to oxaloacetate and then to glucose
Only found in plants!
Fats to carbohydrates
Carbons stay in this cycle so there is a net production of oxaloacetate (from 2 acetyl CoA)
Slide 20