Stage 2 - acetyl-CoA oxidation - CAC

Question 1

Where does the CAC take place?

Answer 1

mitochondria

Question 2

What does the CAC do?

Answer 2

CAC takes acetyl CoA and oxidizes it to CO2

Question 3

Is energy released conserved or not conserved? As what?

Answer 3

Conserved as recused cofactors
NADH and FADH2 - this is then captured to form ATP through stage 3 (oxidative phosphorylation)

Question 4

What did Szent discover?

Answer 4

Addition of succinic acid = burst of O2 consumption

Question 5

What did Knoop disocver?

Answer 5

B oxidation and a few pathways in krebs cycle
Citrate-a-ketoglutarate-succinate reaction

Question 6

What did Krebs discover?

Answer 6

• Linked in cyclic pathways
• Initiation of respiration by malate and accumulation of succinate

Question 7

First step in CAC
cofactor
enzyme
what happens

Answer 7

acetyl coA (nucleophile)->oxaloacetate(electrophile)->citrate (tertiary alcohol = bad substrate)
H2O->CoASH
citrate synthase
condensation of acetyl CoA with oxaloacetate
forms a C-C bond

Question 8

Second step in CAC pt 1
cofactor
enzyme
what happens

Answer 8

citrate->cis aconitate
->H2O
aconitase

Question 9

Second step in CAC pt 2
cofactor
enzyme
what happens

Answer 9

cis aconitate->isocitrate (secondary alcohol = good substrate)
H2O->
aconitase
isomerization of citrate by cis aconitate

Question 10

Third step in CAC
cofactor
enzyme
what happens

Answer 10

isocitrate->a-ketoglutarate+CO2
NAD+->NADH + H+
isocitrate dehydrogenase
oxidative decarboxylation of isocitrate (alcohol->ketone) through transfer of hydride to NAD+
loss of -COO as CO2

Question 11

Fourth step in CAC
cofactor
enzyme
what happens

Answer 11

a-ketoglutarate->succinyl CoA
NAD+->NADH + H+ and CoASH->CO2
a-ketoglutarate dehydrogenase
oxidative decarboxylation of a-ketoglutarate
converts a-ketoglutarate to high energy thioester succinyl CoA

Question 12

Fifth step in CAC
cofactor
enzyme
what happens

Answer 12

succinyl CoA->succinate
->CoASH and GDP+Pi->GTP
succinyl CoA synthetase
substrate level phosphorylation
high energy thioester bond of succinyl CoA is hydrolysed in a reaction coupled to either GTP or ATP synthesis

Question 13

Sixth step in CAC
cofactor
enzyme
what happens

Answer 13

succinate->fumarate
FAD->FADH2
succinate dehydrogenase - ONLY membrane bound enzyme
oxidation of succinate by FAD to fumarate (analogous to acyl-CoA dehydrogenase in B-oxidation)

Question 14

Seventh step in CAC
cofactor
enzyme
what happens

Answer 14

fumarate->malate
->H2O
fumarase
hydration of fumarate to malate (analogous to enol CoA hydration reaction in B-oxidation) alkene->alcohol

Question 15

Eighth step in CAC
cofactor
enzyme
what happens

Answer 15

malate->oxaloacetate
NAD->NADH+H+
oxidation of malate to oxaloacetate (analogous to hydroxyacyl CoA dehydrogenase reaction of B-oxidation)

Question 16

What is the overall stoichiometry of CAC (CO2, CoASH, ATP)

Answer 16

2 co2
1 coash
1 atp - succinyl coA step

Question 17

What are the major products with respect to bioenergetics?

Answer 17

Reduced cofactors
3 NADH and 1 FADH2 per turn

Question 18

Oxaloacetate is formed in the final step of the TCA cycle, by oxidation of malate. Can the net synthesis of oxaloacetate from acetyl-CoA be carried out via the action of the TCA cycle? Explain

Answer 18

No. In each turn of the TCA cycle, two carbon atoms are introduced as acetate and two carbon atoms are lost as carbon dioxide, so there is no net synthesis of oxaloacetate (or any other TCA cycle intermediate)

Question 19

Oxygen (O2) does not appear as a reactant or product in the TCA cycle. Nevertheless, the TCA cycle does not occur under anaerobic conditions. Explain.

Answer 19

The TCA cycle requires a supply of oxidized cofactors, FAD and NAD+. Under anaerobic conditions, the cofactors would accumulate in their reduced states, because the electron transport chain is not operating