Pyruvate Dehydrogenase Complex (10)

Glc:Pyr:NADH ratio

1:2:2

How does pyruvate translocate from the cytoplasm to the mitochondria?

Pyruvate translocase (H⁺ symport)

Comment on the size of the PDC

Massive (MegaDaltons vs Daltons for pyruvate)

What are the 4 vitamins that can bind to this complex?

1. Thiamine

2. Panthotenic acid

3. Riboflavin

4. Niacin

What is the overall reaction of the PD reaction?

pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH

Reaction to enzyme ratio

5:3

How is substrate channelling happening within the PDC?

By having the active sites in close proximity of each other

What does the E1 subunit do to the pyruvate molecule?

Removes CO₂ from its C1

How is that decarboxylation reaction possible?

TPP group from E1 does a nucleophilic attack on it

What is the new molecule formed after decarboxylation?

Hydroxyethyl-TPP*E1

Is this first reaction reversible or irreversible?

Irreversible bc CO₂ diffuses out

What is E2’s role after that?

Oxidation of the hydroxyethyl group by the lipoamide subunit of E2

What is formed after E2’s action?

Acetyl-dihydrolipoamide which consequently regenerates TPP → Acetyl-CoA

Is this second reaction reversible or irreversible?

Reversible

What is E3’s role?

To reset E2 and E3 to their active state

How is E2 reset?

Oxidation-reduction reactions to regenerate oxidized E2-lipoamide

How is E3 reset?

Re-oxidation of FADH² into FAD⁺ by NAD⁺

First advantage of multienzyme complexes

Minimal distance between active sites=large pools of intermediates

2nd advantage of multienzyme complexes

Channeling = less side reactions + protection from unstable intermediates

3rd advantage of multienzyme complexes

Control = easy to effectively shut it down

What is the first level of regulation of the PDC?

Product inhibition of E2 and E3 by, respectively, acetyl-CoA and NADH

What happens to the reactions when E2 and E3 are inhibited?

They run backwards!

How does E2-E3 inhibition eventually inhibit E1?

Bc TPP won’t be regenerated

What is the second layer of PDC regulation?

Covalent modification of E1 through phosphorylation

What enzyme inactivates E1?

PDase kinase or PDK

What enzyme activates E1?

PDase phosphatase or PDP

Allosteric activators of PDK

Acetyl-CoA and NADH

Allosteric inhibitors of PDK (4)

Pyruvate, ADP, Ca²⁺ and K⁺

Allosteric activators of PDP

Ca²⁺ and Mg²⁺

How does arsenic poisoning work?

Shuts down E2 = no acetyl-CoA = toxic to many organs

How is the PDC interrupted by colon cancer?

Overexpressed PDK3 → No actyl-CoA → pyruvate gets converted to lactate via LDH → Anaerobic glycolysis