Chapter 19: Oxidative Phosphorylation

Chapter Overview

  • Focuses on oxidative phosphorylation, a critical phase of cellular respiration that generates ATP.

Oxidative Phosphorylation: Key Concepts

  • Burning Questions:

    • What happens to NADH and FADH2 in the cell?

    • What is the connection between NADH, FADH2, and ATP production?

    • Explanation of the P/O ratio: 2.5 ATP per NADH and 1.5 ATP per FADH2.

Location and Process

  • Location:

    • Occurs in the mitochondria, which are specialized organelles within eukaryotic cells.

  • Process Overview:

    • Final stage of cellular respiration.

    • Involves the coupling of oxidation (removal of electrons) of NADH/FADH2 to the reduction of oxygen to form water (H2O).

    • Overall equation: C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O.

Stages of Cellular Respiration Leading to Oxidative Phosphorylation

  1. Stage 1: Acetyl-CoA production

    • Glycolysis converts glucose to pyruvate which is converted to Acetyl-CoA.

    • Pyruvate dehydrogenase complex converts pyruvate to Acetyl-CoA, releasing CO2.

    • Key molecules: NAD+ is reduced to NADH.

  2. Stage 2: Acetyl-CoA oxidation

    • Acetyl-CoA enters the citric acid cycle (Krebs cycle), producing NADH and FADH2 as electron carriers.

  3. Stage 3: Electron transport and oxidative phosphorylation

    • Electron carriers transfer electrons through a series of complexes to ultimately reduce oxygen.

    • Generates a proton (H+) gradient across the inner mitochondrial membrane.

    • The flow of protons back into the matrix powers ATP synthesis via ATP synthase.

Components of Mitochondrion

  • Inner Membrane:

    • Contains respiratory electron carriers and is impermeable to H+, crucial for maintaining the proton gradient.

  • Outer Membrane:

    • Freely permeable to small molecules and ions.

  • Matrix:

    • Holds enzymes for citric acid cycle and beta-oxidation of fatty acids, as well as mitochondrial DNA and ribosomes.

Electron Transport Chain (ETC) Components

  • Major Electron Carriers:

    • Ubiquinone (Coenzyme Q): Lipid-soluble, accepts 1 or 2 electrons, carries protons.

    • Cytochromes: Three classes (a, b, and c) with iron-containing groups facilitating electron transfer.

    • Iron-Sulfur Proteins: Participate in one-electron transfers, cycling between Fe²+ and Fe³+ states.

Major Functions of Complexes in the ETC

  • Complex I: NADH transfers electrons to FMN, pumping H+ into the intermembrane space.

  • Complex II: FADH2 transfers electrons from succinate to CoQ without H+ pumping.

  • Complex III: Transfers electrons from CoQH2 to cytochrome c, pumping H+ into intermembrane space.

  • Complex IV: Transfers electrons from cytochrome c to oxygen, producing water and pumping additional H+.

ATP Synthase Function

  • Utilizes the proton gradient created by the ETC to synthesize ATP from ADP and inorganic phosphate (Pi).

  • Details how ATP production is driven by the proton motive force.

  • Key points: 4 protons are required to synthesize one ATP.

Regulation of Oxidative Phosphorylation

  • Regulated primarily by the availability of ADP and oxygen; high ADP levels increase the rate of ATP synthesis.

  • Coordination between substrate oxidation, oxygen consumption, and ATP production.

Clinical Connections

  • Discussion of mitochondrial diseases which reflect the impact of dysfunctional oxidative phosphorylation.

  • Modern implications of such dysfunction as observed in cancer and other metabolic conditions.

Summary of Key Outputs

  • For every NADH oxidized, approximately 2.5 molecules of ATP are produced, while FADH2 leads to about 1.5 molecules of ATP.

  • Importance of oxidative phosphorylation in overall cellular energy efficiency.

Chapter Overview

Focuses on oxidative phosphorylation, a critical and final phase of cellular respiration that generates ATP, the primary energy currency of the cell. It plays a significant role in energy metabolism and the efficient conversion of metabolic fuel into usable energy.

Oxidative Phosphorylation: Key Concepts

Burning Questions:

  • What happens to NADH and FADH2 in the cell?

  • What is the connection between NADH, FADH2, and ATP production?

  • Explanation of the P/O ratio: For every 2.5 ATP produced per NADH and 1.5 ATP produced per FADH2, highlighting the varying efficiency of these electron carriers in ATP synthesis.

Location and Process

Location:

  • Occurs in the mitochondria, which are specialized organelles within eukaryotic cells that serve as the powerhouse of the cell.

Process Overview:

  • The final stage of cellular respiration involves two major biochemical processes: the electron transport chain (ETC) and chemiosmosis. It couples the oxidation of electron carriers NADH and FADH2 with the reduction of oxygen, producing water (H2O) as a byproduct.

  • Overall equation: C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O, indicating the complete oxidation of glucose.

Stages of Cellular Respiration Leading to Oxidative Phosphorylation

Stage 1: Acetyl-CoA production

  • Glycolysis converts glucose to pyruvate, which is then converted to Acetyl-CoA by the pyruvate dehydrogenase complex, releasing CO2 and producing NADH in the process.

Stage 2: Acetyl-CoA oxidation

  • Acetyl-CoA enters the citric acid cycle (Krebs cycle), resulting in the production of multiple NADH and FADH2 electron carriers, essential for the next stages of cellular respiration.

Stage 3: Electron transport and oxidative phosphorylation

  • Electron carriers transfer electrons through a series of protein complexes within the inner mitochondrial membrane, ultimately reducing oxygen to form water.

  • This electron transfer generates a proton (H+) gradient across the inner mitochondrial membrane, which is crucial for ATP production.

  • The flow of protons back into the mitochondrial matrix drives ATP synthesis via ATP synthase, a complex that utilizes this proton motive force.

Components of Mitochondrion

Inner Membrane:

  • Contains respiratory electron carriers and is impermeable to protons (H+), essential for maintaining the necessary proton gradient for ATP synthesis.

Outer Membrane:

  • Freely permeable to small molecules and ions, facilitating transport across the organelle.

Matrix:

  • Contains enzymes for the citric acid cycle and beta-oxidation of fatty acids, as well as mitochondrial DNA (mtDNA) and ribosomes, which are involved in the production of mitochondrial proteins.

Electron Transport Chain (ETC) Components

Major Electron Carriers:

  • Ubiquinone (Coenzyme Q): A lipid-soluble molecule that accepts one or two electrons and transfers protons across the membrane.

  • Cytochromes: Three classes (a, b, and c), each containing iron groups that facilitate electron transfer, integral to the chain’s function.

  • Iron-Sulfur Proteins: Proteins that participate in one-electron transfers, cycling between Fe²+ and Fe³+ states, critical for maintaining the flow of electrons.

Major Functions of Complexes in the ETC

  • Complex I: NADH transfers electrons to flavin mononucleotide (FMN), pumping protons (H+) into the intermembrane space, contributing to the proton gradient.

  • Complex II: FADH2 transfers electrons from succinate to coenzyme Q without proton pumping, demonstrating its different energy yield compared to Complex I.

  • Complex III: Transports electrons from ubiquinol (CoQH2) to cytochrome c, pumping H+ ions into the intermembrane space, further enhancing the proton gradient.

  • Complex IV: Transfers electrons from cytochrome c to molecular oxygen (O2), producing water and pumping additional H+ ions, a vital step in energy conversion.

ATP Synthase Function

  • ATP synthase utilizes the electrochemical proton gradient established by the ETC to synthesize ATP from ADP and inorganic phosphate (Pi).

  • Key points: Approximately 4 protons are required to synthesize one ATP molecule, quantifying the efficiency of ATP production linked to the proton motive force generated during electron transport.

Regulation of Oxidative Phosphorylation

  • The primary regulation occurs through the availability of ADP and oxygen; elevated levels of ADP stimulate increased ATP synthesis. This balance ensures the coordination between substrate oxidation, atmospheric oxygen consumption, and ATP production, aligning cellular respiration with energy demands.

Clinical Connections

  • Discusses various mitochondrial diseases that impact energy production and reflect the consequences of dysfunctional oxidative phosphorylation, including conditions like Leber's hereditary optic neuropathy and mitochondrial myopathies.

  • Modern implications of such dysfunction are significant in understanding cancer metabolism and other metabolic disorders, highlighting the research direction and clinical significance of metabolic adaptations.

Summary of Key Outputs

  • For every NADH oxidized, approximately 2.5 molecules of ATP are produced, while FADH2 yields about 1.5 ATP molecules, underlining the efficiency of electron carriers in oxidative phosphorylation.

  • This pathway highlights the importance of oxidative phosphorylation in overall cellular energy efficiency and metabolic regulation, critical for maintaining cellular homeostasis.

Answers to Burning Questions

  1. What happens to NADH and FADH2 in the cell?NADH and FADH2 are produced during earlier stages of cellular respiration, specifically during glycolysis and the citric acid cycle. In oxidative phosphorylation, these electron carriers donate their electrons to the electron transport chain (ETC), initiating a series of redox reactions that ultimately lead to ATP synthesis. NADH donates electrons to Complex I of the ETC, while FADH2 donates them to Complex II.

  2. What is the connection between NADH, FADH2, and ATP production?The oxidation of NADH and FADH2 in the electron transport chain is coupled to ATP production. As electrons are passed through the complexes of the ETC, protons (H+) are pumped across the mitochondrial membrane, creating a proton gradient. This gradient drives ATP synthesis through ATP synthase. Specifically, for each NADH, approximately 2.5 ATP molecules are generated, and for each FADH2, about 1.5 ATP molecules are produced.

  3. Explanation of the P/O ratio.The P/O ratio refers to the amount of ATP produced per atom of oxygen reduced during oxidative phosphorylation. The key focus is on the ratios derived from NADH and FADH2: 2.5 ATP are generated per NADH and 1.5 ATP per FADH2. This indicates that NADH is a more efficient electron carrier in terms of ATP yield compared to FADH2.

Objectives as Questions and Answers

  1. What happens to NADH and FADH2 in the cell?NADH and FADH2 are produced during earlier stages of cellular respiration, specifically during glycolysis and the citric acid cycle. In oxidative phosphorylation, these electron carriers donate their electrons to the electron transport chain (ETC), initiating a series of redox reactions that ultimately lead to ATP synthesis. NADH donates electrons to Complex I of the ETC, while FADH2 donates them to Complex II.

  2. What is the connection between NADH, FADH2, and ATP production?The oxidation of NADH and FADH2 in the electron transport chain is coupled to ATP production. As electrons are passed through the complexes of the ETC, protons (H+) are pumped across the mitochondrial membrane, creating a proton gradient. This gradient drives ATP synthesis through ATP synthase. Specifically, for each NADH, approximately 2.5 ATP molecules are generated, and for each FADH2, about 1.5 ATP molecules are produced.

  3. What is the explanation of the P/O ratio?The P/O ratio refers to the amount of ATP produced per atom of oxygen reduced during oxidative phosphorylation. The key focus is on the ratios derived from NADH and FADH2: 2.5 ATP are generated per NADH and 1.5 ATP per FADH2. This indicates that NADH is a more efficient electron carrier in terms of ATP yield compared to FADH2.

Conceptual Questions and Answers on Oxidative Phosphorylation

  1. What is oxidative phosphorylation and why is it important?
    Oxidative phosphorylation is the process that accounts for the majority of ATP synthesized in most organisms. It is crucial for cellular energy production, as ATP serves as the primary energy currency for biological processes.

  2. Where does oxidative phosphorylation occur within the cell?
    Oxidative phosphorylation occurs in the mitochondria, specifically in the inner mitochondrial membrane, which is crucial for its selective permeability that facilitates the electron transport process.

  3. What initiates the process of oxidative phosphorylation?
    The process begins when a substrate transfers its electrons to universal electron acceptors such as nicotinamide nucleotides (NAD+ or NADP+) or flavin nucleotides (FMN or FAD).

  4. How do NAD+ and NADP+ differ from FMN and FAD in their electron acceptance?
    NAD+/NADP+ can accept two electrons at a time, while FMN and FAD can accept either one or two electrons. Additionally, NAD+/NADP+ are soluble but cannot cross the inner mitochondrial membrane, while FMN and FAD are usually protein-bound.

  5. What are the three main types of electron carriers involved in oxidative phosphorylation?
    The three main types are:

  • Hydrophobic quinones (ubiquinone and ubiquinol), which are lipid-soluble and can accept one or two electrons.

  • Cytochromes, which contain iron and can accept one electron per molecule.

  • Iron-sulfur proteins, which participate in electron transfers and can exist in reduced (Fe2+) or oxidized (Fe3+) states.

  1. What should one note about the respiratory chain involving Complexes I-IV?
    Each complex in the respiratory chain has specific inputs and outputs, as well as the ability to pump protons across the membrane, which is important for establishing the proton gradient.

  2. What is the flow of electrons through the respiratory chain?
    The flow involves NADH donating electrons to Complex I, followed by a specific sequence through various carriers including ubiquinone, cytochromes b, c1, c, a, and a3, and ultimately to oxygen (O2). FADH2 enters at Complex II.

  3. How much energy from the electron transfer is stored in the proton gradient?
    Approximately 90% of the energy released during the electron transfer process is stored in the proton gradient.

  4. What is the chemiosmotic model in the context of ATP synthesis?
    The chemiosmotic model explains that as protons flow back through the membrane via ATP synthase, ATP is synthesized. This couples substrate oxidation, oxygen consumption, and ATP synthesis into one process.

  5. How do inhibiting electron transfer or ATP synthase affect each other?
    Inhibiting electron transfer blocks ATP synthesis, while inhibiting ATP synthase also blocks electron transfer. This illustrates a coupling between the processes, which can be disrupted by various methods.

  6. What are the components of ATP synthase and their functions?
    ATP synthase consists of two main parts: Fo, an integral membrane protein that forms the proton pore, and F1, a peripheral component that catalyzes ATP synthesis or hydrolysis.

  7. How does ATP synthase convert the reaction of ADP and inorganic phosphate into ATP?
    ATP synthase transforms the endergonic reaction of ADP + Pi → ATP (ΔG'º = +30.5 kJ/mol) into a reaction with ΔG ≈ 0 by capturing and removing the product ATP, allowing the reaction to proceed spontaneously.

  8. How is the ATP production linked with the proton gradient established by electron transport?
    For every pair of electrons from NADH, approximately 10 protons are pumped across the membrane, while 6 protons are pumped for each pair of electrons from FADH2. It takes 4 protons moving back through the membrane to form one ATP molecule, resulting in 2.5 ATP from NADH and 1.5 ATP from FADH2, defined as the P/O ratios.

  9. What other transport processes are driven by the proton gradient established during oxidative phosphorylation?
    The proton gradient also drives other transport processes including the export of ATP from the mitochondrial matrix, import of ADP into the matrix, and the import of inorganic phosphate into the matrix.

  10. How do cells manage NADH produced in the cytosol during glycolysis?
    NADH formed in the cytosol cannot directly enter the mitochondrial matrix. Instead, its electrons can be transferred to reducing equivalents that can enter via mechanisms like the Malate-Aspartate Shuttle or by converting to FAD in other shuttle processes, affecting ATP yield.

  11. What is “acceptor control” and why is it significant in the regulation of oxidative phosphorylation?
    Acceptor control refers to the regulatory system where the availability of ADP influences the rate of ATP production; higher ADP concentrations speed up ATP synthesis and oxygen consumption, which is crucial for efficient energy production in aerobic cells.

  12. What is the role of various regulatory molecules like ATP, ADP, NADH?
    Many steps leading to ATP production are coordinated and regulated by the concentrations of ATP, ADP, AMP, NADH, and NAD+, which ensure metabolic balance according to cellular energy demands.

  13. What is Brown Adipose Tissue and its relevance in oxidative phosphorylation?
    Brown adipose tissue (BAT) is an example of uncoupling, where instead of producing ATP, heat is generated. It is characterized by a high concentration of mitochondria containing cytochromes, and the presence of UCP1 (Thermogenin) that facilitates this uncoupling process.