Wk 7&8 Lecture 4: Pyruvate Dehydrogenase Complex and the Citric Acid Cycle

Overview of Pyruvate Metabolism

• Sunni's training run illustrates how glycolysis leads to ATP production through substrate-level phosphorylation.

• Focus on mitochondrial oxidative metabolism that converts pyruvate to a large amount of ATP via the citric acid cycle (CAC).

Pyruvate Dehydrogenase Complex (PDH)

• Pyruvate Conversion: Pyruvate enters the mitochondria and is converted to acetyl CoA by PDH, releasing CO2

• Importance of Oxygen: This process occurs in the presence of oxygen and is critical for ATP production.

• Substrate Link: PDH serves as a vital link between glycolysis and CAC.

Reaction Overview

• The PDH reaction:

  • Converts 3-carbon pyruvate to 2-carbon acetyl CoA.

  • Releases one CO2 and produces NADH.

  • Key Energy Production: The reaction has a large negative free energy change (-33 kJ/mol), signaling its favoritism.

Enzymatic Components

• E1 (Pyruvate Dehydrogenase): Catalyzes decarboxylation, reducing TPP (Thiamine Pyrophosphate).

• E2 (Dihydrolipoate Transacetylase): Oxidizes TPP-bound acetyl group to form acetyl CoA.

• E3 (Dihydrolipoate Dehydrogenase): Regenerates FAD and reduces NAD+ to NADH.

Coenzymes Required

• Key Coenzymes: NAD+, FAD, Coenzyme A, Thiamine Pyrophosphate, Lipoamide.

• Importance of for enzyme function and electron transport in ATP synthesis.

Structure of Mitochondria

• Two Membranes:

  • Outer membrane: Porous, allowing metabolite passage.

  • Inner membrane: Contains proteins for electron transport and ATP synthesis, highly invaginated as cristae for increased surface area.

• Matrix: Contains PDH and CAC enzymes, gel-like fluid, site for metabolic processes.

Citric Acid Cycle (CAC)

• Overview of the Cycle: Generates high energy electron carriers from the oxidation of acetyl CoA, while releasing CO2.

• Key Inputs: Acetyl CoA from PDH and other sources like fatty acids and amino acids.

Citric Acid Cycle Steps

• Stage One:

  1. Citrate Formation: acetyl CoA combines with oxaloacetate to form citrate (catalyzed by citrate synthase).

  2. Decarboxylations: Two oxidative decarboxylations lead to the generation of NADH and CO2 (from isocitrate to alpha-ketoglutarate and the latter to succinyl CoA).

• Stage Two:

  1. Succinyl CoA to Succinate: Generates GTP via substrate-level phosphorylation and NADH.

  2. Oxaloacetate Regeneration: Recycles oxaloacetate, allowing continuous cycle operation. Generates more NADH and FADH2.

Energy Production

• Low ATP Yield: Cycle directly produces modest ATP (mostly GTP but predominantly NADH and FADH2).

• Inputs to Electron Transport Chain: NADH and FADH2 provide electrons for the mitochondrial inner membrane's electron transport chain, leading to oxidative phosphorylation and ATP synthesis.

Regulatory Mechanisms

• Control of PDH Complex: Regulated through covalent modification (phosphorylation by PDH kinase and dephosphorylation by PDH phosphatase).

• Allosteric Regulation: Active/inactive forms of PDH depend on substrate and product concentrations (e.g., acetyl CoA activates kinase, while pyruvate stimulates phosphatase).

• Feedback Inhibition: Rising levels of acetyl CoA and NADH inhibit PDH directly, maintaining metabolic balance.

Conclusion

• Integration of Metabolic Pathways: Understanding PDH and CAC regulatory mechanisms is critical as they connect multiple metabolic pathways, impacting energy production during high demand, such as in endurance running.

• Importance of Fuel Sources: Besides glucose, fatty acids and amino acids can also produce acetyl CoA, showcasing metabolic flexibility.