lecture 30 - the citric acid cycle

Question 1

Where does the citric acid cycle occur?

Answer 1

In the mitochondrial matrix, except for one enzyme

Question 2

What is the molecule that enters the citric acid cycle?

Answer 2

Acetyl-CoA

Question 3

How many carbons enter the citric acid cycle, and in what form?

Answer 3

2 carbons (as acetyl-CoA)

Question 4

How many carbons leave the citric acid cycle, and in what form?

Answer 4

2 carbons as 2 CO2 (when oxidation is complete)

Question 5

In what forms is energy captured in the CAC?

Answer 5

1GTP (equivalent to 1 ATP), 3NADH and 1 FADH2 in each turn of the citric acid cycle.

Question 6

What are the 2 key parts of the CAC?

Answer 6

Release of Carbon, Regeneration of the starting molecule. Energy is captured in both

Question 7

What is the 2 carbon acetyl-CoA molecule entering the CAC attached to to produce a 6 carbon molecule?

Answer 7

Attached to a 4 carbon oxaloacetate to form a 6 carbon citrate molecule. reaction also involves water and This uses the citrate synthase molecule

Question 8

Where does the energy for the condensation of acetyl-CoA with oxaloacetate come from?

Answer 8

the hydrolysis of CoA from acetyl CoA to produce a free CoA molecule.

Question 9

What is the first reaction in the CAC that makes a 2C acetyl-CoA into a 6C citrate?

Answer 9

Condensation reaction with oxaloacetate, a 4 carbon molecule

Question 10

What happens to citrate when it is formed at the beginning of the citric acid cycle, and why?

Answer 10

2 step Isomerisation involving dehydration then hydration. To make isocitrate that is susceptible to decarboxylation. Catalysed by aconitase (will be shown on exam diagram)

Question 11

How does fluoroacetate interrupt the CAC?

Answer 11

Takes the place of acetyl-CoA and forms fluorocitrate which interrupt the aconitase enzyme

Question 12

How is the first carbon removed during the CAC?

Answer 12

An oxidative decarboxylation reaction catalysed by isocitrate dehydrogenase (see exam diagram ) with NAD capturing energy

Question 13

How is the second carbon removed in the CAC?

Answer 13

Second oxidative decarboxylation, where an CoA-SH group is attached to form succinyl CoA, a 4 carbon molecule. Energy captured in NADH. (similar to the pyruvate dehydrogenase reaction)

Question 14

How is energy captured during the conversion of succinyl-CoA to succinate in the CAC?

Answer 14

Removal of CoA releases enough energy to drive the synthesis of GTP from GDP, which leads to ATP production due to equilibrium

Question 15

What is the energy equivelant of GTP + ADP, which forms an equilibrium in cellular pathways?

Answer 15

GTP + ADP ⇌ GDP + ATP

Question 16

What type of reaction is succinyl to succinate?

Answer 16

Substrate level phosphorylation

Question 17

What are the 3 reactions of the last part of the CAC that are the same as the first 3 steps of Beta-oxidation?

Answer 17

Oxidation, Hydration, Oxidation

Question 18

What happens during the first oxidation of succinate in the last part of the CAC?

Answer 18

Single bond is made to double bond. FAD to FADH

Question 19

What happens during the hydration reaction of fumarate in the last part of the CAC?

Answer 19

Water splits across the double bond to form a hydroxyl group.

Question 20

What happens during the oxidation of malate at the last part of the citric acid cycle?

Answer 20

Hydroxyl group converted to carboxyl group, with NAD+ forming NADH

Question 21

What enzyme in the CAC is located outside of the mitochondrial matrix in the inner mitochondrial membrane?

Answer 21

SDH - succinate dehydrogenase

Question 22

What coenzyme does SDH use in the CAC?

Answer 22

FAD - tightly binds to the protein

Question 23

SDH is part of the CAC and what other metabolic process?

Answer 23

Electron transport chain (ETC)

Question 24

What are the reactants of the CAC?

Answer 24

Acetyl-CoA + 3NAD+ + FAD + 2H2O + GDP + Pi (see exam diagram)

Question 25

What are the products of the CAC?

Answer 25

2CO2 + CoASH + 3NADH + 3H+ + FADH2 + GTP (see exam diagram)