CAC

Question 1

Where does the citric acid cycle occur?

Answer 1

The mitochondrial matrix

Question 2

How does the cycle use electrons?

Answer 2

It removes electrons and passes them on to form NADH and FADH2.

Question 3

How is the CAC very efficient?

Answer 3

It is cyclical, small number of citric acid cycle molecules can make loads of NADH and FADH2.

Question 4

What % of aerobic cell energy does the CAC in collaboration with oxidative phosphorylation produce?

Answer 4

90%

Question 5

How much ATP is produced through fermentation?

Answer 5

2 mol ATP per Glc molecule

Question 6

How did the citric acid cycle evolve?

Answer 6

The CAC evolved to harvest electrons that could then be used to completely oxidise food molecules to CO2 and H2O.

Question 7

Acetyl CoA in the cycle

Answer 7

Pyruvate from glycolysis and fatty acids are oxidised further to acetyl CoA in the mitochondrial matrix.

Question 8

How is Acetyl CoA made?

Answer 8

Made from pyruvate, through the action of the enzyme pyruvate dehydrogenase. Very complicated series of reactions involving decarboxylation of the pyruvate molecule,then oxidation,followed by the transfer of the CoA complex.

Question 9

How many electrons are produced during the decarboxylation step?

Answer 9

The decarboxylation step releases two electrons (in the form of 2 H ions), which can pass to O2 to produce more ATP through NADH intermediates.

Question 10

Pyruvate dehydrogenase

Answer 10

It contains tens of copies of each enzyme sub-unit (E1, E2, E3). Each sub-unit catalyses a different part of the reaction to convert pyruvate to acetyl CoA.

Question 11

Pyruvate dehydrogenase - action of E1

Answer 11

E1 catalysis the first decarboxylation of pyruvate.

Question 12

Pyruvate dehydrogenase - action of E2

Answer 12

E2 transfers the acetyl group to coenzyme A.

Question 13

Pyruvate dehydrogenase - action of E3

Answer 13

E3 recycles the lipoyllysine through the reduction of FAS, which is recycled by passing electrons to NAD+.

Question 14

Acetyl CoA in the citric acid cycle

Answer 14

Acetyl CoA (C2) enters the “black box” of the citric acid cycle. Each turn, 2C’s enter (acetyl CoA) followed by removal of two different C’s as 2x CO2.

Question 15

How is pyruvate dehydrogenase regulated?

Answer 15

It is regulated by it’s immediate products and the end point of cellular respiration, ATP.
It’s regulated depending on the needs of the cell.

Question 16

2 other points of control in CAC

Answer 16

1st one - isocitrate dehydrogenase
2nd one - a-ketoglutarate dehydrogenase
Both points are at non-reversible reactions (exergonic steps).
These control points allow re-direction of cellular resources.

Question 17

What is an anaplerotic reaction?

Answer 17

A reaction that replenishes the concentration of an intermediate in a metabolic pathway

Question 18

How can pyruvate be converted to oxaloacetate?

Answer 18

Pyruvate can be converted to oxaloacetate by the enzyme pyruvate carboxylase.
Pyruvate carboxylase is only active when acetyl CoA is present, so a build up of acetyl CoA triggers this reaction.