PDH Complex

Question 1

Where does the PDH complex reaction take place

Answer 1

Mitochondrial matrix

After pyruvate is shuttled from cytosol —> matrix

Question 2

Characteristics of outer mitochondrial membrane

Answer 2

Contains porin molecules —> large channels that allow free diffusion of most all low MW metabolits

Cytosolic content is essentially contiguous with the IMS content

Question 3

Characteristics of inner mitochondrial membrane (IMM)

Answer 3

Very impermeable to charged molecules

Transporters are required

Question 4

Enzymes for with processes are contained in the matrix

Answer 4

PDH-complex

CAC

FA oxidation

Question 5

Pyruvate transport from cytosol —> matrix

Answer 5

Pyruvate is a charged molecule … so cannot cross IMM

Active symport transport with H+ ions

H+ = move down gradient

pyruvate = moves against gradient

Question 6

Effect of pyruvate transport into matrix on ATP production

Answer 6

H+ gradient is crucial for ATP production

With each pyruvate transport…[H+] in cytosol decreases

Thus, ATP production is decreased

Question 7

The 5 vitamins that are required as cofactors in the PDH complex

Answer 7

1. Niacin (B3)
2. Riboflavin (B2)
3. Thiamine (B1)
4. Lipoic acid (lipoate)
5. Pantothenic acid (B5)

Question 8

Niacin (B3)

Answer 8

Structural component of NAD+

NADH is produced in the final enzyme reaction and is the end product of the PDH complex reaction

Question 9

Riboflavin (B2)

Answer 9

Structural component of FAD

4th reaction = FADH2 produced

Ultimately will become oxidized in the next reaction and NADH is made

FAD/FADH2 = also cofactor for CAC enzyme (succinate dehydrogenase)

Question 10

Thiamine (B1)

Answer 10

Thiamine pyrophosphate (TPP)

Cofactor covalently bound to PDH complex

Involved in the acetyl-transferase function the first 2 reactions

**also cofactor in transketolase reaction of PPP

Question 11

Lipoic acid (lenoate)

Answer 11

Bioactive form = covalently conjugated to the complex via amide bond (lipoamide)

Oxidation/reduction acetyl-transferase function of the 2nd and 3rd reactions

Question 12

Pantothenic acid (B5)

Answer 12

Structural component of CoA

Used in 3rd reaction to generate Acetyl-CoA

**CoA used in FAS and oxidation as well

Question 13

Overall reaction that the PDH complex catalyzes

Answer 13

Pyruvate —> Acetyl-CoA

In mitochondrial matrix

Three subunits E1, E2, E3

The reaction is thermodynamically favorable

Question 14

As the PDH reaction progresses…the substrate is linked to the complex and channeled …..

Answer 14

Towards the center of the complex

Moving from E1 —> E3

Question 15

Pyruvate dehydrogenase subunit (E1)

Answer 15

1st reaction

Pyruvate becomes oxidatively decarboxylated

CO2 produced

Resulting 2C hydroxyl-ethyl group is transferred to the TPP cofactor (which is covalently attached to E1

—> forms hydroxyethyl-TPP

Question 16

Second reaction of PDH complex

Answer 16

Hydroxyethyl group is oxidized to an acetyl group by (E1)

Dihydrolipoyl transacetylase (E2) then catalyzes the transfer of the acetyl group from TTP —> lipollysine (cofactor lipoate covalently attached to a lysine molecule in the transacetylase)

Electrons donated = reduce a disulfide bone in lipoate

Acetyl group is transferred to lipoate through a thioester linkage —>

Producing acyl-lipollysine

Question 17

3rd reaction of PDH complex

Answer 17

Acetyl group is transferred from the thioester linkage in lipollysine to another thioester linkage in CoA

—> Acetyl-CoA

(Lipollysine is still in reduced state —> disulfide bond is still separated)

Question 18

After the production of acetyl-CoA in the 3rd reaction…what is the purpose of the remaining PDH reactions?

Answer 18

Reoxidizing lipollysine…so that it can accept electrons from antoher hydroxyethyl-TPP

Question 19

4th reaction of PDH complex (post Acetyl-CoA)

Answer 19

Dihydrolipoyl dehydrogenase (E3)

Reoxidizes lipollysine

FAD —> FADH2

Question 20

Final (5th) reaction of PDH complex

Answer 20

E3 subunit

Electrons transferred from FADH2 —> NAD+

To make NADH

Question 21

Regulation of PDH complex

Answer 21

Phosphorylation/dephosphorylation regulation

PDH kinase phosphorylates and INACTIVATES PDH….a phosphatase does the opposite

Build up of end products (Acetyl-CoA and NADH) —> stimulate PDH kinase

Pyruvate —> inactivate PDH kinase

High ADP, NAD+, CoA—> inactivate PDH kinase

LCFAs —> stimulant PDH kinase

Phosphatase activty:

Primarily activated by Ca2+ —> leads to activated PDH

Think of muscle cells - increase Ca influx…need ATP for muscle contraction

Question 22

Beriberi

Answer 22

Most common PDH disorder

Deficiency in thiamine

Can be caused by malnourishment

Also in alcoholics

No thiamine —> disrupt PDH complex, and CAC enzyme a-KG dehydrogenase

Question 23

Disruption in PDH complex and alpha-KGDH —>

Answer 23

Neuro and cardio disorders

Nerves prefer glucose…no PDH complex and cannot get a lot of energy from glucose anymore

No PDH complex —> less Acetyl-Coa —> less FAS

Lactic acidosis

Treatment = eating ketogenic amino acids (leucine and lysine) and fat…bypass the
PDH complex and generate Acetyl-CoA by other mechanisms

Question 24

Arsenite and Mercury poisoning

Answer 24

Heavy metals that bind tightly to SH groups in PDH complex

Disrupts function of E2 lipoate cofactor (lipollysine) in PDH and a-KGDH

Symptoms like beriberi

Question 25

Leigh Disease

Answer 25

Defects in one of the genes encoding the PDH complex (usually E1)

Progressive CNS degeneration

Can’t metabolize glucose —>—> CO2

Hypotonia, seizures, eye issues, ataxia

Treatment = no carbs, increase ketogenic aas, supplement TPP