Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain (ETC) & Oxidative Phosphorylation

Lecturer Information

  • Dr. Balakrishnan. S, PhD.
    Senior Lecturer of Biochemistry

Objectives

  • Understand the concepts of Biological Oxidation and Respiratory Chain (ETC).
  • Appreciate the importance of the ETC.
  • Describe the Structure of the ETC.
  • Describe the functioning of the ETC.
  • Understand the concepts of Oxidative Phosphorylation.
  • Appreciate the importance of Oxidative Phosphorylation.
  • Describe the coupling of ETC and Oxidative Phosphorylation.
  • Explain the Regulation of ETC and oxidative phosphorylation.
  • Identify the Inhibitors of ETC.
  • Explain the deficiencies of ETC and oxidative phosphorylation.

Biological Oxidation

  • Definition: The process in which substances such as carbohydrates, lipids, and amino acids are oxidized in living organisms.

Pathways Linking the Breakdown of Organic Nutrients to ATP Generation

  • Carbohydrates
    • Glycolysis
  • Proteins
    • Oxidation of amino acids
  • Lipids
    • Beta-oxidation
Mechanism of ATP Generation
  • Substrate Level Phosphorylation:
    • Phosphorylation of ADP or GDP to ATP or GTP coupled to the dehydrogenation of an organic substrate.
    • Example: Conversion of succinyl-CoA to succinate by Succinyl-CoA Synthetase in TCA cycle.

Role of the Respiratory Chain of Mitochondria

  • Converts food energy into ATP.
  • Oxidation of major foodstuffs leads to the generation of reducing equivalents (2H) that are collected by the respiratory chain for oxidation and coupled generation of ATP.

Nature of Biological Oxidation

  1. Occurs at 37℃, pH 7.4, as an enzymatic reaction.
  2. Energy released gradually.
  3. Formation of H₂O.
  4. Formation of CO₂ by decarboxylation.

Enzymes Involved in Biological Oxidation

  • Crucial enzymes belong to the group oxidoreductases, classified into:
  1. Oxidases:
    • Cytochrome oxidase, Flavoproteins
  2. Dehydrogenases:
    • Cytochromes, NAD⁺/NADP⁺ dependent dehydrogenases
  3. Hydroperoxidases:
    • Peroxidase, catalase
  4. Oxygenases:
    • Monooxygenases, Dioxygenases
Function of Enzymes
  • Enzymes catalyze the removal of hydrogen from a substrate and transfer it to oxygen to form water.
Types of Enzymes
  • Oxidases:
    • Form H₂O or H₂O₂ during oxidation of a metabolite.
  • Dehydrogenases:
    • Transfer hydrogen from one substrate to another in coupled redox reactions.
    • Include NAD⁺/NADP⁺ dependent dehydrogenases.

Mechanism of Oxidation and Reduction of Nicotinamide Coenzymes

  • Specific dehydrogenases interact with substrates delivering electrons through the enzyme and coenzyme complexes.

Hydroperoxidases

  • Peroxidase: H₂O₂ + A → 2H₂O + A
  • Catalase: 2H₂O₂ → 2H₂O + O₂

Oxygenases

  • Dioxygenases: Incorporate both oxygen atoms into substrate.
  • Monooxygenases (Mixed-Function Oxidases, Hydroxylases): Incorporate one oxygen atom into substrate, forming hydroxylated products.

Cytochromes P450

  • Monooxygenases essential for detoxifying drugs and hydroxylating steroids.

Respiratory Chain / Electron Transport Chain (ETC)

  • A series of complexes in mitochondria serving as redox carriers for transferring hydrogens from substrates to oxygen leading to water formation.
Structure of Electron Transport Chain
  • Located within the inner mitochondrial membrane with various subunits, including FMN, NADH, Cu, and FeS.

Components of the Mitochondrial Electron-Transport Chain

  • Oxidant or Reductant Enzyme Complex:
    • Mass (kDa), Subunits, Prosthetic groups involved with electron transport.

Overview of Electron Flow through the Respiratory Chain

  • Electrons flow from substrates like succinate, through complexes I-IV, ultimately reducing oxygen at complex IV to form water.

Nicotinamide Coenzymes

  • NAD⁺/NADH and NADP⁺/NADPH, derived from vitamin B3, play critical roles in redox reactions.

Flavin Prosthetic Groups

  • Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) help in electron transport and are derived from vitamin B2.

Iron-Sulfur Centers

  • Involved as prosthetic groups, capable of electron transfer, central to protein functionalities in the electron transport chain.

Coenzyme Q (Ubiquinone)

  • A hydrophobic molecule functioning as a mobile electron carrier within the mitochondrial inner membrane, assisting in electron transport.

Cytochromes

  • Proteins with heme prosthetic groups capable of undergoing oxidation and reduction reactions.
  • Cytochromes absorb light at specific wavelengths and facilitate electron transfer in the ETC.

Organisation of the Electron Transport Chain

  • Order of Respiratory Chain Complexes involves complexes I through IV with various substrates and pathways for electron transport.

Oxidative Phosphorylation

  • Definition: The phosphorylation of ADP to ATP is coupled to the transfer of electrons from substrates to molecular oxygen.
  • Main localization in mitochondria and serves as the major ATP production method in aerobic organisms.

Mechanism of ATP Production

  • Electrons pass through the respiratory chain, pumping protons from the matrix to the intermembrane space, establishing a proton gradient that drives ATP synthesis.

Functional Stages in the Respiratory Chain

  1. H₂ is removed from NADH+H⁺ and FADH₂.
  2. Electrons are passed down through complexes I–IV to O₂.
  3. Complexes I, III, and IV pump protons across the inner mitochondrial membrane.
  4. Protons reenter through ATP synthase to synthesize ATP.

Importance of Oxidative Phosphorylation

  • Necessary for energy production, recycling spent ATP is essential as daily ATP needs far exceed physical ATP reserves.

Chemiosmotic Hypothesis

  • Proposed by Peter Mitchell, states that a proton gradient is an energy reservoir driving ATP formation.

Regulation of ETC & Oxidative Phosphorylation

Factors Affecting ETC
Inhibitors of ETC
  • Examples include Rotenone, Antimycin A, and various inhibitors affecting different complexes of the respiratory chain.
Uncoupling Agents
  • Compounds that disrupt the coupling between electron transport and ATP synthesis, e.g., 2,4-Dinitrophenol (DNP).
Physiological Uncouplers
  • Endogenous uncouplers like Thermogenin which generate heat.
Inhibitors of Oxidative Phosphorylation
  • Compounds that prevent electron transport and ATP phosphorylation, e.g., Oligomycin.
The Role of ADP and Thyroxine in Oxidative Phosphorylation
  • Increased ADP levels stimulate oxidative phosphorylation whereas thyroxine influences ATP catabolism and energy production.

Deficiencies of Electron Transport

  • Impaired conditions can lead to diseases; mutations in nuclear or mitochondrial DNA affecting oxidative phosphorylation capacity leading to conditions like Alzheimer's, Parkinson's diseases, and MELAS syndrome.
Case Study: Luft Syndrome
  • Mitigated by mitochondrial defects in electron transport leading to inefficiency in ATP synthesis, resulting in significant physiological and metabolic effects on individuals.
Transport Systems in Mitochondria
  • Various transporters (e.g., ANT for adenine nucleotides) facilitate movement across mitochondrial membranes, essential for maintaining proper redox balances.

Mitochondrial Disorders

  • Certain mitochondrial diseases result from dysfunction in mitochondrial respiration impacting energy metabolism and leading to pathological states.

Closure

  • Understanding the mechanisms and regulation of the electron transport chain and oxidative phosphorylation is crucial for insights into energy metabolism and associated diseases.