Chapter 18 - Prep for Citric Acid Cycle

Is the conversion of pyruvate to acetyl CoA reversible?

No, it is irreversible; once pyruvate has been converted to acetyl CoA by the pyruvate dehydrogenase complex, acetyl CoA must either by oxidized or stored as fat

• animals don’t have the enzymes to convert acetyl CoA → pyruvate

What complex accomplishes the conversion oxidative decarboxlation of pyruvate to acetyl CoA?

The pyruvate dehydrogenase complex, which is in the mitochondrial matrix

• PDC produces CO₂ and captures high-transfer potential electrons as NADH

• PDC has 3 enzymes and 5 coenzymes

• E2 enzyme makes up the core of PDC

What are the three enzymes of the PDC?

• Pyruvate dehydrogenase (E1) → uses TPP; oxidative decarboxylation of pyruvate

• Dihydrolipoyl transacetylase (E2) → uses lipoamide and CoA; transfer of acetyl group to CoA

• Dihydrolipoyl dehydrogenase (E3) → uses FAD and NAD+; regeneration of oxidized lipoamide

What is the structure of acetyl CoA and where does the energy reside?

Acetyl group connected to CoA via a thioester bond; the energy comes from the thioester bond, which is favorably hydrolyzed

• Thioester bond is high-energy because it offers less resonance stabilization with the C=O group

What are the five coenzymes of the PDC?

• Catalytic coenzymes → TPP, lipoamide, FAD

• Stoichiometric conenzymes (function as substrates) → CoA, NAD+

All are derivatives of vitamin B, except lipoamide

What’s the overall reaction for the conversion of pyruvate to acetyl CoA?

Pyruvate + CoA + NAD+ → acteyl CoA + CO₂ + NADH

What links glycolysis to TCA cycle?

The pyruvate dehydrogenase complex, which converts pyruvate to acetyl CoA

• this is where the first CO₂ is lost in cellular respiration

What are the three steps in the synthesis of acetyl CoA from pyruvate?

decarboxylation, oxidation, and transfer to CoA

Why would a patient with vitamin B deficiency experience lactic acidosis?

Because vitamin B is precursor for 4 of 5 coenzymes of PDC, without them PDC cannot function so pyruvate builds up and is converted to lactate by LDH

What is the first step in the conversion of pyruvate to acetyl CoA?

Decarboxylation → pyruvate combines with ionized carbanion of TPP and is decarboxylated

• CO₂ lost

• catalyzed by E1 pyruvate dehydrogenase

• this is the rate-limiting step and main regulatory point

What is the second step in the conversion of pyruvate to acetyl CoA?

Oxidation → the hydroxyethyl group attached to TPP is oxidized to form an acetyl group that is transferred to a lipoamide, creating an energy rich thioester bond

• also catalyzed by E1 pyruvate dehydrogenase

What is the structure of the lipoamide in PDC?

It is covalently attached to lysine residue in E2, and it acts as an arm that swings from E1 → E2 → E3, allowing for substrate channeling

• the disulfide bond in the ring is the reactive site

What is the third step in the conversion of pyruvate to acetyl CoA?

Formation of acetyl CoA → the acetyl group from acetyl-lipoamide is transferred to CoA to form acetyl CoA

• catalyzed by E2 dihydrolipoyl transacetylase

What is the fourth step in the conversion of pyruvate to acetyl CoA?

Dihydrolipoamide produced in the formation of acetyl CoA must be reoxidized to lipoamide in order for the PDC to complete another reaction

• catalyzed by E3 dihydrolipoyl dehydrogenase

• two electrons from dihydrolipoamide are transferred to FAD prosthetic group and then to NAD+ to produce NADH

What is the NADH producing step in PDC?

The fourth step, the regeneration of lipoamide from dihydrolipoamide by E3 dihydrolipoamide dehydrogenase

How are the E2 and E3 components of PDC regulated?

• Dihydrolipoyl transacetylase E2 → inhibited by acetyl CoA, its product

• Dihydrolipoyl dehydroganse E3 → inhibited by NADH, its product

How is PDC mainly regulated via pyruvate dehydrogenase E1?

Allosteric

• ATP, acetyl CoA, and NADH inhibit PDH

• ADP and pyruvate stimulate PDH

Covalent modification

• Pyruvate dehydrogenase kinase (PDK) phosphorylates PDH, inactivating it; ATP, NADH, and acetyl CoA stimulate PDK

• Pyruvate dehydrogenase phosphatase (PDP) dephosphorylates PDH, activating it; Ca²⁺ stimulates PDP (muscle contraction)

What factors stimulate PDH and thus formation of acetyl CoA?

• PDP dephosphorylates PDH, activating it; Ca²⁺ stimulates PDP

• ADP and pyruvate stimulate PDH allosterically

What factors inhibit PDH and thus formation of acetyl CoA?

• PDK phosphorylates PDH, inactivating it; ATP, NADH, and acetyl CoA stimulate PDK

• ATP, NADH, and acteyl CoA also allosterically inhbit PDH

Where do PDP and PDK reside in the PDC?

They are attached to the pyruvate dehydroganse E1 part of the complex and the phosphorylate/dephosphorylate a serine residue on E1