Krebs citric acid cycle- Lecture 63

Question 1

Overall, the Citric Acid Cycle is _______ and occurs in the _______.

Answer 1

irreversible

mitochondrial matrix compartment of all cells

Question 2

What is the net reaction of the TCA cycle?

Answer 2

Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O –> 2CO2 + 3NADH + FADH2 + GTP + 2H+ + CoA

Question 3

The NADH and FADH2 produced in the TCA cycle will provide _____ to the ______.

Answer 3

electrons

mitochondrial respiratory chain

Question 4

What is step one of the TCA cycle?

Answer 4

Acetyl CoA + OAA –> citric acid

via citrate synthase

Question 5

What regulates citrate synthase?

Answer 5

inhibited by ATP and NADH

regulated by [OAA]

Question 6

What is step two of the TCA cycle?

Answer 6

citric acid –> Cis-aconitate isocitrate

via aconitase

Question 7

What is the third step of the TCA cycle?

Answer 7

isocitrate + NAD+ –> alpha-ketpglutarate + CO2 + NADH

via isocitrate dehydrogenase

Question 8

What is the rate limiting enzyme of the TCA cycle?

Answer 8

isocitrate dehydrogenase

Question 9

What regulates isocitrate dehydrogenase?

Answer 9

energy charge
stimulated by ADP
inhibited by ATP adn NADH

Question 10

What is step four of the TCA cycle?

Answer 10

alpha-KG + NAD+ + CoADH –> Succinyl CoA + NADH + H+ + CO2

via alpha-KG dehydrogenase

Question 11

Describe the alpha-KG enzyme and its components.

Answer 11

E1 (alpha-KG decarboxylase): thiamine PP cofactor; functions to decarboxylate alpha-KG
E2: uses lipoic acid to transfer four carbon succinyl product to CoASH
E3: uses FAD and NAD to regenerate oxidized lipoic acid for E2

Question 12

What inhibits each of the components of the alpha-KG enzyme?

Answer 12

E1: ATP
E2: succinyl CoA
E3: NADH

Question 13

What is step five of the TCA cycle?

Answer 13

succinyl CoA + GDP + Pi –> succinate + CoASH + GTP

via succinyl CoA synthase (thiokinase)

Question 14

Describe the last three steps of the TCA cycle?

Answer 14

designed to convert succinate to OAA

catalyzed by succinic dehydrogenase (producing FADH2), cumarase, and malate dehydrogenase (producing NADH)

Question 15

What is the overall yield of the ATP from G6P –> 6CO2?

Answer 15

Total 32
Gucose --> 2 pyruvate + 2NADH = 2 ATP
2NADH --> mito = 5 ATP
2 pyruvate --> 2 acetyl CoA + 2NADH
2 NADH = 5 ATP
2 Acetyl CoA = 20 ATP

Question 16

What regulates the TCA cycle?

Answer 16

respiratory control (oxidation of NADH and FADH2)
energy charge
concentration of and regeneration of OAA

Question 17

What are anaplerotic reactions?

Answer 17

intermediates of a pathway (most often the TCA cycle) that can be diverted into other pathways or those that replenish the necessary substrates after they have been pulled out

Question 18

What are the ways OAA can be replenished?

Answer 18

pyr carboxylase reaction

PEPCK reaction

Question 19

What is the pyruvate carboxylase reaction as it applies to the TCA cycle?

Answer 19

enzyme contains biotin (vitamin B used to carry CO2)
requires ATP
stimulated by acetyl CoA

Question 20

What is PEPCK reaction as it applies to the TCA cycle?

Answer 20

converts glycolytic intermediate PEP to OAA

makes GTP