CAC- PDC

Question 1

What is the CAC?

Answer 1

A series of eight reactions, oxidising the acetyl groups of acetyl CoA to two CO2, converting the liberated free energy into the reduced compounds NADH and FADH2.

Question 2

What does one complete cycle of CAC yield?

Answer 2

Two CO2, Three NADH, one FADH2 and one GTP.

Question 3

What is the reaction pathway of CAC?

Answer 3

3 NAD+ + FAD + GDP + Pi + Acetyl CoA —-> 3NADH+ FADH2 + GTP + CoA + 2 CO2

Question 4

What happens to the oxaloacetate used in the first step?

Answer 4

It is regenerated in the last one, thus can oxidise unlimited number of acetyl gruop.

Question 5

Where are all CAC enzymes found?

Answer 5

In the mitochondria.

Question 6

What is required of acetyl group oxidation to the two CO2?

Answer 6

Transfer of four electron pairs, accounted for by reduction of 3 NAD+ to 3 NADH and reduciton of FAD to FADH2.

Question 7

Where is the free energy from acetyl group oxidation conserved?

Answer 7

NADH and FADH2

Question 8

What is the end-product of anaerobic and aerobic glycolysis?

Answer 8

Anaerobic is lactate, whilst pyruvate is aerobic.

Question 9

Multienzyme Complexes

Answer 9

Protein complexes where two or more sequential steps are catalysed.

Question 10

Why are enzymatic reaction rates increased in multienzyme complexes?

Answer 10

Usually enzyme reaction rates ae limited by frequency in which enzymes collide with their substrates, in ME , the distance the sequential steps occur is reduced, limining need for chance collision.

Question 11

How do ME complexes minimise side reactions?

Answer 11

Reduced distance between enzymes reduces chance for metabolic intermediates produced reaction with other molecules.

Question 12

Pyruvate Dehydrogenase Complex

Answer 12

Forms acetyl-CoA from pyruvate through oxidative carboxylation

Question 13

Oxidative Decarboxylation

Answer 13

A reaction where a carboxylate group is removed, forming CO2.

Question 14

What are the three enzymes in the PDC?

Answer 14

Pyruvate Dehydrogenase, Dihydrolipoyl Transacetylase and Dihydrolipoyl dehydrogenase.

Question 15

Stoichiometry of PDC?

Answer 15

Pyruvate + CoA + NAD+ —-> acetyl-CoA + CO2 + NADH

Question 16

What are the 5 coenzymes the PDC requires?

Answer 16

Thiamine Pyrophosphate, Lipoic Acid, Coenzyme A, FAD and NAD+

Question 17

Thiamine Pyrophosphate

Answer 17

Decarboxylates pyruvate, yielding a hydroxyethyl TPP carbanion

Question 18

Lipoic Acid

Answer 18

This accepts the TPP formed hydroxyethyl carbanion as an acetyl group.

Question 19

What is the first step in PDC?

Answer 19

E1, utilising TPP, decarboxylates pyruvate with forming of hydroxyethyl-TPP intermediate.

Question 20

What is the second PDC step?

Answer 20

Hydroxyetyl formed from E1 transfers to E2, which attacks lipoamide disulfide, eliminating TPP, thus HE oxidised to an acetyl group as LAD is reduced.

Question 21

What does the lipoamide group consist of?

Answer 21

Lipoic acid, linked via amide bonds to the amino group of a lysine residue.

Question 22

Structure of Lipoamide

Answer 22

Has a reactive center, being a cylci disulfide that can be reversible reduced to yield dihydrolipoamide

Question 23

What is the third step in the PDC?

Answer 23

E2 catalyses transesterfication, where acetyl is transferred to CoA, yielding acetyl-CoA and dihydrolipoamide-E2.

Question 24

What is the fourth step in PDC?

Answer 24

E3 reoxidises dihydrolipoamide to regenerate the lipoamide group, completing the cycle.

Question 25

What is step 5 of PDC?

Answer 25

The reduced E3 sulfhydryl groups are reoxidesd through FAD funelling electrons to NAD+, yielding NADH.

Question 26

How does the PDC channel molecules between its enzymes?

Answer 26

A lipoyllysyl arm that swings disulfide group from E2, ‘picking up’ hydroxyethyl group to the E2 active site, where HE forms acetyl, then transferred to E3 where reduced disulfide is reoxidised.

Question 27

Why are lipoamide containing enzymes like PDC susceptible to arsenic poisoning?

Answer 27

It contains two sulfur, of which arsenic binds, removing its hydrogens, halting cellular respiration.

Question 28

Lipoamide structure.

Answer 28

A hexagon with the left two bonds removd, a H-S replacing C1, an R group at C4 and H-S replacing C5.