electron transfer

• Metabolism


  Breaking down food to harvest electrons
• Key Electron Carriers


  • NADH and FADH₂ are molecules that carry high-energy electrons due to their electronegative redox potential
• Sources of Electrons


  • Proteins
  • Carbohydrates
  • Fatty Acids
• Electron-Transport Chain (ETC)


  Central pathway linking food intake to ATP synthesis
• Complete Metabolic Pathway


  Glucose+10NAD++2Q+2ADP+2GDP+4Pi+2H2O→10NADH+10H++2QH2+2ATP+2GTP+6CO2
• Hydride Ion (H⁻)


  A negatively charged ion consisting of a proton and two electrons
• Role of Hydride Ion in Biology

  Transfers high-energy electrons during metabolic reactions
• NADH/NAD⁺


  • NADH (Reduced Form): Main carrier of high-energy electrons
  • NAD⁺ (Oxidized Form): Accepts electrons to become NADH
• Electron Transfer Mechanism


  Hydride ion (H⁻) transfer involves two electrons moving from one molecule to another
• • Proton: ~1.7 x 10⁻²⁷ g
  • Electron: ~1 x 10⁻³⁰ g (2000 times lighter than a proton)
  • NAD⁺: ~1 x 10⁻²¹ g (1 billion times heavier than an electron)
• Main Types of Electron Transfer in Biology

  • Hydride Ion Transfer (NADH)
  • Concomitant Transfer of an Electron and a Proton (FADH₂)
  • Direct Electron Transfer (Cytochromes)
• Glycolysis and Citric Acid Cycle


  Among the oldest metabolic pathways, present in archae bacteria
• Iron–Sulfur World Theory


  Suggests early life formed on iron sulfide minerals, with primitive biochemistry predating proteins and genes
• Electric Bacteria


  • Shewanella and Geobacter: Bacteria that use electrons from rocks and metals, demonstrating early forms of electron-based metabolism
• Eukaryotic cells retain bacterial biochemistry to produce NADH through glycolysis, pyruvate oxidation, and the citric acid cycle
• Citric Acid Cycle


  • Objective: Load electrons into highly electronegative carriers (NADH, FADH₂)
  • Process: Involves 8 key reactions transforming acetyl-CoA into CO₂, NADH, FADH₂, and GTP/ATP
• Reaction 1: Citrate Synthase

  1. Action: Joins acetyl-CoA to oxaloacetate to form citrate

  2. Carbon Skeleton: 4C (oxaloacetate) + 2C (acetyl-CoA) = 6C (citrate)

  3. Details: Initiates the cycle by forming a 6-carbon molecule
• Reaction 2: Aconitase


  1. Action: Isomerizes citrate to isocitrate

  2. Purpose: Makes the target carbon more accessible for subsequent reactions

  3. Details: Converts citrate to its isomer isocitrate
• Reaction 3: Isocitrate Dehydrogenase


  1. Action: Isocitrate donates electrons to NAD⁺, forming NADH and releasing CO₂

  2. Carbon Skeleton: 6C → 5C

  3. Details: The first NADH is produced, and the first CO₂ is released
• Reaction 4: Alpha-Ketoglutarate Dehydrogenase


  1. Action: Alpha-ketoglutarate donates electrons to NAD⁺, forming NADH and releasing CO₂

  2. Produces: High-energy intermediate succinyl-CoA

  3. Carbon Skeleton: 5C → 4C

  4. Details: The second NADH is produced, and the second CO₂ is released
• Reaction 5: Succinyl-CoA Synthetase

  1. Action: Cleaves succinyl-CoA to form succinate and transfers energy to GDP, forming GTP

  2. Process: Substrate-level phosphorylation

  3. Details: Produces GTP (an ATP equivalent)
• Reaction 6: Succinate Dehydrogenase


  1. Action: Succinate donates electrons to FAD, forming FADH₂

  2. Enzyme Location: Bound to the inner mitochondrial membrane, marking mitochondria during subcellular fractionation

  3. Details: The only citric acid cycle enzyme that is membrane-bound
• Reaction 7: Fumarase


  1. Action: Hydrates fumarate to malate

  2. Result: Addition of water

  3. Details: Converts fumarate to malate by adding water
• Reaction 8: Malate Dehydrogenase


  1. Action: Malate donates electrons to NAD⁺, forming NADH and regenerating oxaloacetate

  2. Carbon Skeleton: 4C oxaloacetate is recycled

  3. Details: Completes the cycle by regenerating oxaloacetate and producing the third NADH
• Key Products of the Citric Acid Cycle

  • Per Turn of the Cycle:
  • 2 CO₂
  • 3 NADH
  • 1 FADH₂
  • 1 GTP (or ATP)
• Understanding the citric acid cycle is fundamental to grasping cellular respiration and metabolism
• Electrons harvested from food drive ATP synthesis, highlighting the link between diet and energy production