Pyruvate Dehydrogenase (PDH)

Question 1

What happens to pyruvate in the mitochondria?

Answer 1

It is oxidized to form acetyl CoA + CO2

Pyruvate (3 carbon molecule) → Acetyl CoA (2 carbon molecule) + CO2

Question 2

Describe the reaction that coverts pyruvate to acetyl CoA

Answer 2

Oxidative decarboxylation
(oxidative reduction + decarboxylation)

Question 3

How is pyruvate transported into the mitochondria?

Answer 3

• The mitochondria has 2 membranes
  1. Outer
  2. Inner
• Pyruvate = generated int the cytosol
• Pyruvate can diffuse through the outer mitochondrial membrane
• Mitochondrial pyruvate carrier transports pyruvate past the inner membrane and into the inner mitochondrial matrix

Question 4

Thioester

Answer 4

Acyl group covalently linked to a thiol (SH) group

Question 5

Why do thioester’s have high acyl group transfer potential

Answer 5

Thioester’s have a high negative free energy and are able to donate their acyl group to different acceptor molecules in an energetically favorable reaction (they are “activated”)

Question 6

How many enzymes and coenzymes are involved in pyruvate dehydrogenase complex?

Answer 6

• 3 enzymes
• 5 coenzymes

Question 7

What are the five different enzymes involved in the PDH complex?

Answer 7

1. NAD/NADH
2. TPP
3. Lipoate (lipoic acid)
4. CoA (coenzyme A)
5. FAD/FADH

Question 8

What are the three enzymes of the PDH complex?

Answer 8

• E1
• E2
• E3
  There are multiple copies of these enzymes

Question 9

NAD+/NADH & FAD+/FADH

Answer 9

• Electron carriers
• Ring structure is important for accepting electrons and protons

Question 10

TPP

Answer 10

Electronegative Nitrogen helps make / break carbon-carbon bonds by stabilizing the carbanion intermediate

Question 11

Lipoate

Answer 11

• Covalently bound to E2 by amide linkage to a lysine residue
• Serves as both an electron carrier and an acyl carrier

Question 12

Coenzyme A (CoA)

Answer 12

• Carrier of the activated acyl group (-SH)

Question 13

Is the number of copies of each enzyme in the PDH complex the same for all species?

Answer 13

NO!
- The number of copies of each enzyme (and therefore the size of the complex) varies among species

Question 14

What enzyme makes up the core of the PDH complex?

Answer 14

• E2
• The core is surrounded by variable copies of E1 and E3 enzymes

Question 15

How many reactions does the PDH complex undergo?

Answer 15

5
This is essentially the oxidative decarboxylation of pyruvate into acetyl CoA broken down into 5 smaller steps

Question 16

Describe step 1 of the PDH complex?

Answer 16

• The active site of E1 is covalently bound to TPP which facilitates the decarboxylation of pyruvate to acetate (CO2 = removed)
• Hydroxy ethyl intermediate = formed

Question 17

Describe step 2 of the PDH complex?

Answer 17

• TPP will hand over the hydroxy ethyl intermediate
• This is an oxidation reaction coupled with a transfer reaction
• TPP will first oxidize the hydroxy ethyl intermediate reducing the oxidized lipoic acid group of the E2 enzyme

Question 18

Describe step 3 of the PDH complex

Answer 18

• The transfer part of the reaction from step 2
• The reduced lipoic acid covalently combines to the acetate group
• The acetate group can now be converted to acetyl CoA
• The end product of this step is reduced lipoic acid

Question 19

Describe steps 4 and 5 of the PDH complex?

Question 20

Which step is the rate limiting step in the reactions of the PDH complex?

Answer 20

Step 1
Once pyruvate is present the reaction proceeds very quickly