CH 18: PDH Complex

Acetyl CoA Formation from Pyruvate

  • Under aerobic conditions, pyruvate enters mitochondria and converts to Acetyl CoA.

  • Two principal fates of Acetyl CoA: TCA Cycle or lipogenesis.

  • Formation of Acetyl CoA from pyruvate is irreversible in animals.

  • Acetyl CoA acts as a fuel for the citric acid cycle, generating CO2 and high-energy electron carriers for ATP synthesis.

Pyruvate Dehydrogenase Complex

  • Important Role: Catalyzes oxidative decarboxylation of pyruvate to form Acetyl CoA.

  • Located in the mitochondrial matrix, highlighting its importance in metabolism.

Enzymes in the Pyruvate Dehydrogenase Complex

  • Complex requires three distinct enzymes to synthesize Acetyl CoA from pyruvate.

  • These enzymes aggregate into a supramolecular complex for efficiency.

Required Coenzymes for Pyruvate Dehydrogenase Reaction

  • Five coenzymes are essential for the enzymatic reactions:

    • Thiamine pyrophosphate (TPP) - derived from thiamine (Vitamin B1).

    • Lipoic acid - involved in the oxidation step.

    • Coenzyme A (CoA) - accepts the acetyl group.

    • NAD+ - acts as an electron carrier.

    • FAD - serves as a prosthetic group for one of the enzymes.

Steps of Acetyl CoA Synthesis

  1. Decarboxylation:

    • Catalyzed by Pyruvate dehydrogenase (E1).

    • Pyruvate + TPP forms an intermediate through decarboxylation.

  2. Oxidation:

    • The hydroxyethyl group attached to TPP is oxidized to an acetyl group.

    • Transferred to lipoamide (attached to E2 enzyme) resulting in acetyl-lipoamide.

  3. Acetyl CoA Formation:

    • E2 catalyzes the transfer from acetyl-lipoamide to CoA, forming Acetyl CoA.

    • This step forms a high-energy thioester bond.

  4. Reoxidation of Dihydrolipoamide:

    • Catalyzed by dihydrolipoamide dehydrogenase (E3).

    • FADH2 is produced and NAD+ later accepts electrons.

Regulation of Pyruvate Dehydrogenase Complex

  • Allosteric Regulation:

    • Inhibition by products; Acetyl CoA inhibits E2, NADH inhibits E3.

  • Covalent Modification:

    • E1 can be phosphorylated (inactivation) or dephosphorylated (activation) by associated kinases and phosphatases, respectively.

    • ADP, NAD+, CoA, and pyruvate can stimulate activity by promoting dephosphorylation.

Clinical Insights

  • Defective Regulation

    • Lead to lactic acidosis if pyruvate dehydrogenase remains inactive due to phosphorylation, often treated with a ketogenic diet.

  • Thiamine Deficiency

    • Insufficient pyruvate dehydrogenase activity due to lack of thiamine (B1), causes conditions like beriberi, affecting metabolism and neuromuscular function.

Importance of Organizing into Complexes

  • The organization of enzymes into complexes enhances efficiency, reduces substrate diffusion time, and minimizes side reactions during the formation of Acetyl CoA.

Acetyl CoA Formation from Pyruvate

  • Under aerobic conditions, pyruvate enters mitochondria and converts to Acetyl CoA.

  • Two principal fates of Acetyl CoA: TCA Cycle or lipogenesis.

  • Formation of Acetyl CoA from pyruvate is irreversible in animals.

  • Acetyl CoA acts as a fuel for the citric acid cycle, generating CO2 and high-energy electron carriers for ATP synthesis.

Pyruvate Dehydrogenase Complex

  • Important Role: Catalyzes oxidative decarboxylation of pyruvate to form Acetyl CoA.

  • Located in the mitochondrial matrix, highlighting its importance in metabolism.

Enzymes in the Pyruvate Dehydrogenase Complex

  • Three distinct enzymes required to synthesize Acetyl CoA from pyruvate:

    1. E1 (Pyruvate Dehydrogenase): Catalyzes decarboxylation of pyruvate, forming an intermediate.

    2. E2 (Dihydrolipoamide Acetyltransferase): Catalyzes the transfer of the acetyl group to CoA.

    3. E3 (Dihydrolipoamide Dehydrogenase): Regenerates the oxidized form of lipoamide by transferring electrons to NAD+.

  • These enzymes aggregate into a supramolecular complex for efficiency.

Required Coenzymes for Pyruvate Dehydrogenase Reaction

  • Five coenzymes essential for the enzymatic reactions:

    • Thiamine pyrophosphate (TPP) - derived from thiamine (Vitamin B1).

    • Lipoic acid - involved in the oxidation step.

    • Coenzyme A (CoA) - accepts the acetyl group.

    • NAD+ - acts as an electron carrier.

    • FAD - serves as a prosthetic group for E3.

Steps of Acetyl CoA Synthesis

  1. Decarboxylation:

    • Catalyzed by E1.

    • Pyruvate + TPP forms an intermediate through decarboxylation.

  2. Oxidation:

    • The hydroxyethyl group attached to TPP is oxidized to an acetyl group.

    • Transferred to lipoamide (attached to E2 enzyme), resulting in acetyl-lipoamide.

  3. Acetyl CoA Formation:

    • E2 catalyzes the transfer from acetyl-lipoamide to CoA, forming Acetyl CoA with a high-energy thioester bond.

Regulation of Pyruvate Dehydrogenase Complex

  • Allosteric Regulation:

    • Inhibition by products; Acetyl CoA inhibits E2, NADH inhibits E3.

  • Covalent Modification:

    • E1 can be phosphorylated (inactivation) or dephosphorylated (activation) by associated kinases and phosphatases.

    • ADP, NAD+, CoA, and pyruvate can stimulate activity by promoting dephosphorylation.

Clinical Insights

  • Defective Regulation:

    • Can lead to lactic acidosis if pyruvate dehydrogenase remains inactive due to phosphorylation.

    • Potential treatments include a ketogenic diet to reduce pyruvate.

  • Thiamine Deficiency:

    • Insufficient pyruvate dehydrogenase activity due to lack of thiamine (B1) can cause conditions like beriberi, affecting metabolism and neuromuscular function.

    • Treatment often involves thiamine supplementation.

Importance of Organizing into Complexes

  • The organization of enzymes into complexes enhances efficiency, reduces substrate diffusion time, and minimizes side reactions during the formation of Acetyl CoA.