Lecture 2

Oxidation-Reduction Reactions

• Basic Concepts

  • Movement of electrons plays a crucial role in biochemistry, serving as a major source of free energy for living organisms.

  • Biological pathways utilize relatively reduced compounds to reduce O₂ (e.g., glucose is oxidized in multiple steps and pathways eventually reduce O2 to make water).

Components of Biological Electron Flow

• Key Elements

  • Reduced substrate: The initial state prior to oxidation.

  • Electron carrier chain: Sequence of molecules that transfer electrons during reactions.

  • Coupling mechanisms: Processes that enable energy conversion between different forms (e.g., chemical energy to mechanical work).

  • Types of Work:

    • Chemical (substrate accumulation)

    • Mechanical (motion)

    • Osmotic work

Oxidation-Reduction Mechanics

• Reactions Explained

  • Oxidation: Loss of electrons by a molecule.

  • Reduction: Gain of electrons by a molecule.

  • Reducing agent or reductant: The molecule that donates electrons.

  • Oxidizing agent or oxidant: The molecule that accepts electrons.

  • Common examples in reactions (e.g., Fe³⁺ + Cu⁺ ↔ Fe²⁺ + Cu²⁺).

Thermodynamics in Oxidation

• Favorable Reactions

  • Reduced organic compounds serve as fuels for oxidation; this process is stepwise and controlled in biochemistry.

  • Thermodynamic favorability does not equate to kinetic rapidity.

Electron Transfer Mechanisms

• Methods of Electron Transfer

  • Direct transfer as single electrons

  • Transfer as Hydrogen atoms (H⁺ + e⁻)

  • Transfer as hydride ions (:H-)

  • Direct combination with O₂ (hydrocarbon is electron donor, O atom is acceptor).

Nernst Equation & Reduction Potential

• Equation Components

  • n: Number of electrons transferred per molecule

  • F: Faraday's constant (96.5 kJ/mol)

  • Gibbs free energy and relationship to potential changes in electron flow.

Enzyme Interactions with Coenzymes

• Key Coenzymes

  • Main electron carriers include: NADP⁺, NADH, FMN, FAD, quinones, iron-sulfur clusters, and cytochromes.

  • Each enzyme associated with a coenzyme has a specific reduction potential that affects its activity.

Differences in NAD⁺ and NADP⁺

• Roles in Biochemical Pathways

  • NAD⁺: Generally involved in oxidations (catabolism).

  • NADP⁺: Generally involved in reductions (anabolism).

  • Both enzymes are recycled without changes in concentration.

Glycolysis Overview

• Definition and Importance

  • Breakdown of glucose through a series of enzyme-catalyzed reactions producing 2 molecules of pyruvate, ATP, and NADH.

  • Central player in glucose metabolism and significant energy source for cells.

Detailed Glycolysis Phases

Preparatory Phase

• Steps Involved

  • Initial phosphorylation of glucose by hexokinase, conversion to glucose 6-phosphate, and isomerization reactions (e.g., fructose-6-phosphate to fructose-1,6-bisphosphate).

Payoff Phase

• Energy Harvesting

  • Oxidative conversion of glyceraldehyde 3-phosphate to pyruvate producing NADH and ATP through substrate-level phosphorylation.

Fate of Pyruvate

• Different Pathways

  • Under hypoxic conditions: Fermentation to ethanol (in yeast) or lactate (in muscles).

  • Under aerobic conditions: Further oxidation in the citric acid cycle into acetyl-CoA.

Historical Significance of Glycolysis

• Evolutionary Insight

  • Glycolysis is likely among the earliest metabolic pathways, predating oxygenic photosynthesis in an anaerobic atmosphere, highlighting importance in energy extraction from glucose.

Summary of Key Concepts

• Glycolysis is crucial in cellular metabolism, allowing limited energy extraction under anaerobic conditions and serving as a regulatory pathway in different organisms.