Citric Acid Cycle

Question 1

Step 1

Citrate synthase

Answer 1

Converts oxaloacetate and acetyl-CoA to citrate.
Citrate synthase add an acetyl group to oxaloacetate.
This step is irreversible.
This step is highly exergonic.
Citrate contains three carboxylic acid groups.

Question 2

Step 2

Aconitase

Answer 2

Converts citrate to isocitrate.
This step is reversible.
This is an isomerization reaction.
Water is being released when citrate transitions to aconitate. Water is then introduced when aconitate becomes isocitrate.
The OH group on carbon 3 in citrate is moved to carbon 4. A hydrogen from carbon 4 in citrate is moved to Carbon three in isocitrate.

Question 3

Step 3

Isocitrate dehydrogenase

Answer 3

Converts isocitrate to alpha ketoglutarate.
This step is irreversible.
This is the first step that makes NADH plus CO2.
Isocitrate is oxidized to produce NADH first. The oxidized molecule then releases the first carbon dioxide. This is an oxidative decarboxylation step. This molecule is then reduced to make alpha ketoglutarate.

Question 4

Step 4

Alpha-ketoglutarate dehydrogenase

Answer 4

Converts alpha ketoglutarate to succinyl-CoA.
This step is irreversible.
The cofactor, Co-ASH, produces the second CO2. Also, the cofactor NAD+ produces NADH.
This is the second time in NADH + CO2 is generated.
In succiny-CoA, the CO2 group has been replaced by the CoA.
Until now, the two carbons released in the first half of the citric acid cycle did not come from acetyl-CoA.

Question 5

Step 5

Succinyl-CoA synthetase

Answer 5

Converts succinyl-CoA to succinate.
The step is reversible.
A phosphate group displaces CoA in succinyl-CoA. Succinyl phosphate donates its phosphoryl group to a His residue on the enzyme, producing a phospho-his intermediate and releasing succinate. Part of the enzyme will take up the phosphate. The phosphoryl group is then transferred to GDP to form GTP, which is high energy.
In the step GTP is generated by substrate level phosphorylation.

Question 6

Step 6

Succinate dehydrogenase

Answer 6

Converts succinate to fumarate.
This step is reversible.
Fumarate will have 4 carbons.
Reduces Q (ubiquinone) to QH2 (ubiquinol) -> enzyme-FADH2 to enzyme-FAD.

Question 7

Step 7

Fumarase

Answer 7

Converts fumarate to malate.
This step is reversible.
Fumarase catalyzes a hydration reaction.
When fumarate is catalyzed by adding H2O to the enzyme fumarase, a double bond on carbon 2 is broken and malate is formed. An OH group is added on carbon two; and a hydrogen is added on carbon three.

Question 8

Step 8

Malate dehydrogenase

Answer 8

Converts malate to oxaloacetate.
The step is reversible.
NAD+ is reduced to NADH + H+
This is the last step that generates NADH.
These eight steps of the citric acid cycle come from one unit of pyruvate.

Question 9

Net equation of a TCA

Answer 9

Acetyl-CoA + GDP + Pi + 3NAD+ + Q -> 2CO2 + CoA + GTP + 3NADH + QH2