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.