Overview of Electron Transport Chain and Oxidative Phosphorylation

Net ATP yield per glucose molecule:

• Glycolysis: Produces 2 ATP through substrate-level phosphorylation, where glucose is broken down into two molecules of pyruvate.

• Citric acid cycle: Produces 2 ATP, along with 6 NADH and 2 FADH2, which are crucial for the subsequent steps in ATP production.

The human body requires approximately $2 imes 10^{26}$ ATP molecules daily, indicating the immense energy demand for cellular functions and the need for stellar efficiencies in energy extraction from glucose beyond just the 4 ATP produced directly.

High-Energy Products of Glucose Oxidation

Overview of Pathways:

• Glycolysis: This initial pathway occurs in the cytoplasm, and it produces 2 molecules of pyruvate and 2 molecules of NADH per glucose molecule, facilitating energy extraction in the first steps.

• Citric Acid Cycle: Taking place in the mitochondria, this cycle further processes the products of glycolysis, yielding 2 ATP, 6 NADH, and 2 FADH2 that play significant roles in powering the electron transport chain.

• Pyruvate Dehydrogenase (PDH): This critical enzyme converts 2 pyruvates into 2 molecules of Acetyl-CoA, while also generating 2 NADH. This step is essential for linking glycolysis to the citric acid cycle and maximizing energy yield.

The overall reaction contributes to the high-energy outputs required for various cellular processes, including biosynthesis, muscle contraction, and maintaining ion gradients.

Electron Transport Chain (ETC)

Role of ETC:

• The ETC facilitates the transfer of electrons from NADH and FADH2 to molecular oxygen ($O_2$), a fundamental process that consumes oxygen and produces water as a byproduct.

• It is responsible for creating a proton gradient across the mitochondrial inner membrane, a critical requirement for ATP synthesis via oxidative phosphorylation.

• ATP Yield: Approximately 34 ATP can be produced from one glucose molecule during oxidative phosphorylation, highlighting the efficiency of the mitochondrial processes.

Mechanism of Electron Transport

2. Oxidation of NADH:

• Reaction:

NADH + H^+ \rightarrow NAD^+ + 2H^+ + 2e^-

4. Reduction of Oxygen:

• Reaction:

\frac{1}{2}O2 + 2e^- + 2H^+ \rightarrow H2O

6. Process:

• Electrons are transferred through a series of complex IV, I, III, and their associated cofactors, necessitating the efficient functioning of these molecules in facilitating energy conversion.

Structure of Mitochondria

Double-Membrane Structure:

• Outer Membrane:

  • This membrane is permeable to small molecules and ions, allowing the passage of metabolites into the mitochondria.

• Intermembrane Space:

  • A fluid-filled space between the inner and outer membranes where protons accumulate during electron transport.

• Inner Membrane:

  • Contains key proteins for the electron transport chain and ATP synthase.

• Matrix:

  • The inner compartment containing enzymes for the citric acid cycle and other metabolic pathways, important for energy production.

Protein Complexes in ETC

Key Complexes:

• I: NADH-ubiquinone oxidoreductase

• II: Succinate-ubiquinone oxidoreductase (linked to the citric acid cycle)

• III: Ubiquinol-cytochrome c oxidoreductase

• IV: Cytochrome c oxidase

These complexes facilitate the transport of electrons, and their sequential activation contributes to the generation of a proton gradient.

Redox Reactions in the ETC

Electron Flow:

• Electrons from NADH and FADH2 travel through complexes I, III, and IV, ultimately leading to the reduction of oxygen to water.

• NADH:

  • Oxidation: NADH \rightarrow NAD^+ + H^+ + 2e^-

• Oxygen Reduction:

  • \frac{1}{2}O2 + 2H^+ + 2e^- \rightarrow H2O

This process elucidates the electron transport dynamics and importance of redox reactions in cellular respiration with distinct energy levels at each step.

Proton Pumps and Proton Motive Force

Function of Complexes I, III, and IV:

• These complexes act as electron-driven proton pumps, moving $H^+$ ions (protons) into the intermembrane space, thereby establishing a proton motive force.

Result:

• Increased H+ concentration in the intermembrane space creates a chemical gradient that is critical for ATP synthesis and other mitochondrial functions.

ATP Synthase Function

Structure:

• Transmembrane protein complex with F0 and F1 subunits.

• F0: Proton channel that spans the membrane, allowing protons to flow back into the matrix.

• F1: Contains the catalytic site for ATP synthesis, linking the proton flow to ATP production.

Process:

• Protons flow back through ATP synthase, driving the conversion of ADP + Pi into ATP, a pivotal reaction that generates energy for cellular activities.

• Roughly 3 H+ ions result in the synthesis of 1 ATP, demonstrating the efficiency of ATP production through this mechanism.

Analogy and Energy Capture

• ATP synthesis is akin to a hydroelectric dam, where potential energy stored in the proton gradient leads to energy capture as protons flow back into the matrix, powering the ATP production process.

Role of Cofactors in the ETC

Key Cofactors:

• NAD+: A coenzyme involved in glycolysis and the citric acid cycle, enhancing the oxidation reactions necessary for energy extraction.

• FAD: Participates in the citric acid cycle, specifically linked to complex II (succinate dehydrogenase), acting as an electron carrier and facilitating energy production in the mitochondria.

Summary

• The electron transport chain and oxidative phosphorylation are integral to ATP production. Their efficient functioning enables robust cellular activity and energy metabolism, making them essential areas of study in biochemistry and physiology.