04: TCA Cycle and PDH Complex Overview

TCA Cycle Overview

• TCA Cycle, also known as Krebs Cycle or Citric Acid Cycle,
• Two stages:
  • Acetyl-CoA Production (Stage I)
  • Acetyl-CoA Oxidation (Stage II)
• Major Processes:
• Pyruvate conversion, Oxidation through a series of enzyme-catalyzed reactions.

Section Objectives

• Understand TCA cycle characteristics and stages.
• Identify components of the Pyruvate Dehydrogenase (PDH) Complex.
• Explain PDH mechanism and reactions associated with TCA cycle.
• Identify enzymes, intermediates, and cofactors involved.
• Explain TCA cycle energetics and overall energy from glucose breakdown.
• Calculate ATP yields for pathways.
• Explain regulation factors of the citric acid cycle.
• Describe connections between TCA intermediates and other pathways.
• Understand anaplerotic reactions.

Cellular Respiration

• Process where cells consume O2 and produce CO2.
• Provides more ATP from glucose than glycolysis.
• Origins date back ~2.5 billion years.
• Three major stages:

1. Acetyl CoA production
2. Acetyl CoA oxidation (TCA cycle)
3. Electron transfer and oxidative phosphorylation

Stage I: Acetyl-CoA Production

• Generates ATP, NADH, and FADH2.
• Carbohydrates release 1/3 of CO2 during this stage.

Stage II: Acetyl-CoA Oxidation

• Generates more NADH, FADH2, and one GTP.
• Remaining C atoms from carbohydrates, amino acids, fatty acids released.

Stage III: Oxidative Phosphorylation

• Majority of ATP generated in catabolism of NADH/FADH2.
• Involves electron transfer chain, resulting in ATP synthesis.

Energetics of Glucose Oxidation

• Full oxidation of glucose yields -2840 kJ/mol; only small energy captured in glycolysis (-146 kJ/mol).

PDH Complex

• Consists of three main enzymes:
• E1: Pyruvate Dehydrogenase
• E2: Dihydrolipoyl Transacetylase
• E3: Dihydrolipoyl Dehydrogenase
• 5 Cofactors required:
• TPP (Thiamine pyrophosphate), lipoyl-lysine, FAD, NAD+, CoA-SH.

Decarboxylation of Pyruvate

• Catalyzed by PDH complex via oxidative decarboxylation.
• First carbon oxidation forms Acetyl CoA.

TCA Cycle Steps

1. C-C Bond Formation: Acetyl-CoA + Oxaloacetate → Citrate.

• Catalyzed by citrate synthase; irreversible.

1. Isomerization: Via dehydration/rehydration.
2. Oxidative Decarboxylation: Isocitrate → -Ketoglutarate.
3. Oxidative Decarboxylation: -Ketoglutarate → Succinyl CoA.
4. Substrate-Level Phosphorylation: Succinyl CoA → Succinate (GTP formation).
5. FAD Reduction: Succinate → Fumarate (FADH2 formed).
6. Hydration: Fumarate → L-Malate.
7. Final Decarboxylation: Malate → Oxaloacetate (NADH formed).

Net Result of TCA Cycle

• From one Acetyl-CoA:
• 3 NADH, 1 FADH2, 1 GTP (equivalent to ATP), yielding significant energy.
• Full conversion:
• Acetyl-CoA + 3 NAD⁺ + FAD + GDP + Pi + 2 H₂O → 2 CO₂ + 3 NADH + FADH₂ + GTP + CoA + 3 H⁺.

Regulation of TCA Cycle

• Key enzymes and steps are regulated:
• PDH, Citrate Synthase, IDH, KDH.
• Regulation by substrate availability and product inhibition (NADH, ATP inhibitors; NAD+, AMP activators).
• Feedback inhibition ensures balance in metabolic pathways.

Anaplerotic Reactions

• Replenishment of TCA intermediates is critical for metabolic processes.
• Intermediates serve as precursors in many biosynthetic pathways.

Summary

• The PDH complex converts pyruvate into acetyl-CoA, utilizing several cofactors.
• TCA cycle is a crucial catabolic process that produces energy and also serves anabolic roles.
• Regulation of activity is key to maintaining cellular energy balance, ensuring metabolic pathways function efficiently.