In-Class Activity: Introduction to Energy Metabolism

Learning Objectives

• Compare and contrast anabolic and catabolic pathways.
• Explain how unfavorable reactions can become favorable and why it's necessary in metabolic pathways.
• Discuss how the same enzyme can catalyze reactions in opposite directions in different cells or compartments.

Key Features of Anabolic and Catabolic Pathways

• Anabolic Pathways: Build larger molecules from smaller ones, requiring energy input (e.g., ATP, NADPH).
• Catabolic Pathways: Break down larger molecules into smaller ones, releasing energy (e.g., producing ATP, heat).

Activity 1: Anabolic and Catabolic Pathways and Energetics

Oxidation and Synthesis of Glucose

• Glucose Oxidation:
  • ΔG = -2800 kJ/mol (energetically favorable).
  • Coupled to ATP and heat production.
• Glucose Synthesis:
  • ΔG = +2800 kJ/mol (energetically unfavorable).
  • Coupled to ATP and NADPH consumption.

Activity 2: Specific Coupled Reactions in Metabolic Pathways

Citric Acid Cycle (CAC)

• Reaction: Isocitrate + NAD⁺ → α-Ketoglutarate + NADH + CO₂.
• Redox Reaction:
  • Carbon 1 in IC is oxidized (loses electrons).
  • NAD⁺ is reduced (gains electrons).
• Pathway Type: Catabolic (breaking down isocitrate to release energy).
• Equilibrium: Decreased isocitrate shifts equilibrium to conserve IC rather than produce NADH.

Calvin Cycle

• Reaction: 1,3-bisphosphoglycerate + NADPH + H⁺ → Glyceraldehyde-3-phosphate + Pi.
• Redox Reaction:
  • Carbon 1 in BPG is reduced (gains electrons).
  • NADPH is oxidized (donates electrons).
• Pathway Type: Anabolic (storing energy as sugars).
• Requirement for NADPH: Essential for producing sugars from the Calvin cycle.

Activity 3: Effect of Substrate and Product Concentrations on Reactions

Lactate Dehydrogenase Reaction

• Reaction (Left to Right): Pyruvate + NADH + H⁺ → NAD⁺ + Lactate.
• Enzyme: Lactate Dehydrogenase (catalyzes reversible reactions).
• Energetics in Muscle and Liver Cells:
  • Both directions are energetically favorable (ΔG < 0).
  • Muscle (low oxygen): Pyruvate to lactate more likely.
  • Liver: Lactate to pyruvate more likely.
• Le Chatelier’s Principle:
  • Conversion of pyruvate to lactate increases NADH and H⁺.
  • Muscle cells maintain equilibrium by balancing reactant/product concentrations.

Glycolysis (Optional Practice)

• Reaction: G3P + NAD⁺ + Pi → NADH + H⁺ + BPG.
• Energetics: Reaction is energetically favorable (ΔG = -1.3 kJ/mol).
• Concentration Effects: Higher NAD⁺ and lower NADH make the reaction more favorable.

Key Concepts

• Metabolic Pathways:
  • Catabolic pathways require high NAD⁺ and low NADH.
  • Anabolic pathways require high ATP and low ADP.
• Cellular Respiration: Eating and breathing supply glucose and oxygen for energy production.
• Photosynthesis in Plants: CO₂ and water are converted to sugars, requiring gas exchange and material absorption.