Electron Transport Chain Overview

Electron Transport and Oxidative Phosphorylation

• This process is crucial for ATP production in cells.
• The TCA cycle is the final oxidative step in carbohydrate, fatty acid, and amino acid catabolism.
• The TCA cycle provides a flow of simple carbon compounds into anabolic processes.
• It generates energy compounds: 1 ATP by substrate-level phosphorylation, 3 NADH (7.5 ATP), and 1 FADH₂ (1.5 ATP) per cycle turn.
• Oxidative phosphorylation is distinct from substrate-level phosphorylation, which occurs in glycolysis.
• Oxidation is the loss of electrons; reduction is the gain of electrons.
• In the electron transport chain, NADH is oxidized (loses electrons), and these electrons are passed to oxygen, reducing it to water.

Substrate-Level Phosphorylation

• Takes place in glycolysis.
• If the glycerol-3-phosphate shuttle is used, FADH₂ is generated, which leads to 3 ATP production (not 5).

Substrate-Level Phosphorylation and NADH/FADH2

• NADH generates 2.5 ATP.
• FADH₂ generates 1.5 ATP.

Cellular Respiration

• Electron Transport Chain (ETC) and oxidative phosphorylation produce ATP.
• Includes 4 protein complexes within or on the inner mitochondrial membrane.

Nature of Electron Transport Chain

• Located within the inner mitochondrial membrane.
• Organized into complexes.
• Electrons flow through these complexes.
• Mobile components (coenzyme Q and cytochrome c) shuttle electrons between complexes.
• Complexes I (NADH dehydrogenase), II (succinate dehydrogenase), III (ubiquinone-cytochrome c oxidoreductase), and IV (cytochrome c oxidase).
• They do not make ATP.

Electron Flow and Work

• The tendency of NADH to lose electrons and oxygen to gain them defines the work that can be carried out.
• Reduction/oxidation (redox) potential(Eº') is measured in volts;
• Strong oxidizing agents exhibit a positive redox potential.

Electron Transport is Exergonic

• The difference in redox potentials reflects the work that can be obtained (Eº' net).
• The amount of work is given by ΔG°' = -nF Eº'net, where:
  • n = number of electrons transferred
  • F = Faraday's constant (96 kJ/mol)
  • Eº'net = net potential difference

Oxidation of NADH by O2

• Reaction 1: NAD⁺ + H⁺ + 2e⁻ → NADH
• Reaction 2: ½O₂ + 2H⁺ + 2e⁻ → H₂O
• The greater standard reduction potential is assigned to Reaction 2.

Location of Glycolysis, TCA Cycle, and Oxidative Phosphorylation in a Eukaryotic Cell

• Glycolysis occurs in the cytosol.
• The TCA cycle and oxidative phosphorylation occur in the mitochondrial matrix and inner mitochondrial membrane, respectively.
• The inner mitochondrial membrane is impermeable to H⁺.

ETC and ATP Production in Mitochondria

• Electron transport chain and ATP synthesis are coupled.
• H⁺ gradient formation drives the synthesis of ATP from ADP + Pi.
• Energy generated by electron flow is used to pump H⁺ ions across the inner mitochondrial membrane into the intermembrane space, creating a concentration gradient.

ATP Synthase

• Located in the inner mitochondrial membrane.
• The stalk (F1) is the catalytic region.
• The FO region is associated with the membrane.

How is Electron Flow Coupled to ATP Synthesis?

• Coupled by chemiosmosis.
• Proposed by biochemist Peter Mitchell.
• Nobel prize for Chemistry in 1978.
• The phenomenon is crucial for ATP synthesis.

Principles of Chemiosmosis

• ETC and ATP synthase have particular orientation in space.
• The inner mitochondrial membrane does not allow H⁺ to pass through.
• H⁺ movement across the membrane is the energy-conserving step.

Generating the Proton Gradient

• Complexes I, III, IV of the electron transport chain pump protons from the matrix to the intermembrane space.
• This process is essential when electrons are passed from NADH to oxygen.

Using the Proton Gradient

• Flow of H⁺ down the gradient into the matrix drives ATP synthesis by ATP synthase.
• The process is highly energetically efficient.
• Protein conformation changes associated with ATP synthase allow ATP to be released..

Net Yield of ATP from Oxidation of One Glucose Molecule

• One NADH molecule produces 2.5 ATP.
• One FADH₂ molecule produces 1.5 ATP.
• 10 protons are pumped out of the matrix when NADH is oxidized.
• 6 protons are pumped out of the matrix when FADH₂ is oxidized.
• 4 protons need to flow back into the matrix to generate one ATP molecule.

What Is the Evidence for Chemiosmosis?

• Detecting protons as they are pumped out of the inner membrane
• Creating a pH gradient leads to ATP production.
• Vectorial organization of the electron transport chain: ATP synthase is on the inner face (matrix side) of the membrane, while cytochrome c is on the outside.
• A closed compartment is needed for this synthesis.
• Experiments using bacteriorhodopsin and ATP synthase, in artificial membranes, support this theory.
• Uncoupling agents disrupt the process.

Reconstitution Experiments

• Using bacteriorhodopsin and ATP synthase from cow heart in artificial membranes allows researchers to assess component contribution.

Action of Uncoupling Agents

• Artificial substances (e.g., DNP) that reversibly bind H⁺ and return them through the membrane.
• They break the H⁺ gradient.

Natural Uncoupling Agent: Thermogenin

• Brown adipose tissue contains this specialized protein.
• Thermogenin acts as an uncoupling agent; it uses a proton gradient to generate heat.

Other Inhibitors of Chemiosmosis

• Cyanide: Inhibits complex IV.
• Rotenone: Inhibits complex I.
• Mitochondrial diseases can decrease ATP production.
• Some disorders (e.g., LHON, MELAS) affect complex I.

Other Uses of the Proton Gradient

• ATP production
• Heat generation
• Movement (e.g., bacterial flagella rotation)
• Transport of ions across membranes
• Photosynthesis

Summary of ATP Production

• Aerobic conditions maximize ATP production (32 ATP from one glucose molecule).
• Anaerobic conditions result in significantly less ATP (2 ATP from one glucose molecule).
• Oxidative phosphorylation is more efficient.

In Summary

• Glucose breakdown releases electrons.
• Electron flow drives proton pumps, creating a gradient.
• The return of protons drives ATP production.

MCQ Quiz for Lecture 21: ETC and Oxidative Phosphorylation

• This is part of a learning module for biochemistry; questions will likely cover the topics discussed.
• Answers for the questions will be provided.

Question 1

• Evidence supporting chemiosmosis:
  • Detection of protons during their pumping.
  • ATP synthesis when a pH gradient is created.
  • Evidence of vectorial organization of the ETC.
  • A closed compartment is essential.

Question 2 (Which statement is wrong, related to False Statements from Previous Pages)

• Answers are from pages 50 and 51, providing multiple-choice questions.

The Oxygen Consumed During Cellular Respiration

• Oxygen is directly involved in accepting electrons at the end of the electron transport chain.

To Correctly Predict the Maximum Amount of ATP

• Knowing the tissue where glycolysis is occurring is crucial for calculating the maximum ATP yield. (True)

This quiz delves into the essential aspects of the electron transport chain, covering topics like the roles of NADH and FADH2, the calculation of standard reduction potential, and the location and function of specific complexes. Test your understanding of how the electron transport chain contributes to cellular respiration and energy production.