NM

Respiration-Electron Transport Systems

Overview of Electron Transport Systems

  • Focus on the critical processes in aerobic respiration, particularly electron transport systems (ETS) as explained in Prescott's Principles of Microbiology.

Page 1: Introduction

  • Title: Prescott's Principles of Microbiology

  • Authors: Joanne Willey, Kathleen Sandman

  • Section covered: Aerobic Respiration: Electron Transport Systems

Page 2: Key Components of Electron Transport Systems

  • Topics covered in this section include:

    • The respiratory ETS

    • E. coli ETS

    • Mitochondrial ETS

    • The proton motive force (PMF)

    • ATP synthase

Page 3: Overview of Glucose Oxidation

  • Complete oxidation process of glucose includes:

    • Glycolysis → 2 Pyruvate → 2 Acetyl-CoA → TCA cycle → Electron Transfer System (ETS)

  • Net energy production:

    • 2 ATP from glycolysis

    • 34 ATP from oxidative phosphorylation

    • Overall reaction: ( 6O_2 + 24H^+ + 24e^- \to 12H_2O )

Page 4: NADH and FADH2 Contribution

  • Each glucose results in:

    • 10 NADH and 2 FADH2

    • Breakdown:

      • Glycolysis: 2 NADH

      • Transition step: 2 NADH

      • TCA cycle: 6 NADH and 2 FADH2

Page 5: Electron Transport Chains

  • Definition: A series of electron carriers that transfer electrons from NADH and FADH2 to terminal electron acceptor (O2).

  • Carriers function by moving electrons from more negative to more positive reduction potentials.

Page 6: Redox Pairs in the ETC

  • Each carrier undergoes reduction and reoxidation, recycling energy.

  • Large difference between NADH and O2 reduction potentials releases significant energy during the process.

Page 7: Bacterial and Archaeal ETCs

  • Distinctive features:

    • Located in the plasma membrane.

    • Variability in carriers and terminal oxidases.

    • Some resemble mitochondrial ETC; generally shorter and may have branched pathways.

Page 8: The Big Picture of Energy Transfer

  1. Electrons transfer from reduced molecules to carriers.

  2. Carriers pass electrons to membrane proteins.

  3. Electrons finally reach oxygen or oxidized minerals.

  • Generation of proton motive force (PMF) is crucial for ATP synthesis.

Page 9: Classes of Metabolism Using ETS

  • Types of Metabolism:

    • Organotrophy: organic electron donors

    • Lithotrophy: inorganic electron donors

    • Phototrophy: light-driven electron excitation

  • Example: Rhodopseudomonas palustris utilizes all three electron sources.

Page 10: The Respiratory ETS in Microbes

  • ETS is mainly located in bacterial cell membranes.

  • Uses various terminal electron acceptors (O2 for aerobic, metals for anaerobic reactions).

  • Example: Salmonella Typhimurium utilizes tetrathionate as a terminal electron acceptor.

Page 11: Paracoccus denitrificans

  • Characteristics:

    • Gram-negative bacterium in soil.

    • Capable of both aerobic and anaerobic respiration.

    • Can utilize methanol as an electron donor.

Page 12: Components of a Respiratory ETS

  • Functionality includes three main components:

    1. Oxidoreductase (dehydrogenase)

    2. Mobile electron carrier

    3. Terminal oxidase

Page 13: E. coli Electron Transport System

  • Process overview:

    • NADH donates electrons to NADH dehydrogenase.

    • Electrons transferred through quinone (Q) to oxygen (O2) to form water (H2O).

Page 14 - Page 19: Summary of Electron Transport Process

  1. Electrons from NADH are donated to substrates.

  2. Electrons are transferred to Q, which picks up H+.

  3. The pumping of protons across the membrane generates PMF:

    • Up to 10 H+ per NADH and 6 H+ per FADH2 are pumped across the membrane.

Page 20: Chemiosmotic Hypothesis

  • Description of how protons move outward during electron transport creating electrochemical gradients.

  • Important for ATP synthesis via ATP synthase.

Page 21: The Central Role of Proton Motive Force

  • Proton motive force (PMF) derived from the movement of electrons drives various cellular activities, including:

    • ATP synthesis

    • Flagella rotation

    • Active transport mechanisms

Page 22: Generation of ATP from PMF

  • Process of converting ADP to ATP through ATP synthase driven by proton motive force.

  • Developed the chemiosmotic theory by Peter Mitchell (1978 Nobel Prize).

Page 23: F1Fo ATP Synthase Structure

  • Structure of ATP synthase:

    • Fo: embedded in the membrane, pumping protons

    • F1: projects into the cytoplasm, generating ATP.

Page 24: Yields of ATP from NADH and FADH2

  • ATP synthesis rates:

    • 1 NADH produces 3 ATP and transfers 10 protons.

    • 1 FADH2 produces 2 ATP and transfers 6 protons.

Page 25: ATP Synthase Functionality

  • Mechanism of ATP production through the rotation of protein complexes in ATP synthase.

Page 26: Theoretical vs. Actual ATP Yield

  • Variability in ATP production due to:

    • Length of bacterial ETCs

    • Environmental conditions

    • PMF utilization for functions other than ATP production.

Page 27: Summary of ATP Yield from Glucose

  • Total from substrate-level phosphorylation and oxidative phosphorylation:

    • Substrate-level: 4 ATP (net 2 from glycolysis + 2 from TCA).

    • Oxidative phosphorylation: 34 ATP.

    • Total yield: 38 ATP (theoretical max).

  • Eukaryotic cells typically yield 36 ATP due to mitochondrial transport costs.

Page 28: Summary of Electron Transport Systems

  • ETS details:

    • Comprised of membrane-embedded proteins for electron transfer.

    • Generates proton motive force for ATP synthesis and other cell functions.

    • Includes substrate dehydrogenase, mobile electron carrier, and terminal oxidase.