5-2-_Energetics_and_Redox_reactions

5-2: Energetics, Redox Reactions, and Enzymes Lecture Overview

• Overview of energy acquisition and storage in microbes, covering:

  • Anabolism and catabolism with the role of ATP

  • Free energy of reactions and nutrient requirements

• Introduction to redox reactions and their importance in energy provision:

  • Explanation of electron transfer and electron carrier molecules

• Reference: Textbook Chapter 3

Importance of Microbial Metabolism

• Microbes as the premier biochemists:

  • Essential role in organic elements cycling (e.g., carbon, nitrogen)

  • Production of essential molecules such as Vitamin B12

• Contribution to animal gut health through microbial metabolism.

  • Example: Vitamin B12's chemical structure (Gruber et al., 2011)

Metabolic Requirements for Life

1. Liquid Water

2. Nutrients: Sources of key elements such as carbon and nitrogen

3. Energy Source: Needed to perform biological work

4. Source of Electrons: Fundamental for biochemical reactions

Nutritional Needs of Cells

• Cells require basic atoms/molecules for constructing vital materials (e.g., DNA, RNA, proteins).

• Key elements needed include carbon (C), nitrogen (N), phosphorus (P), and sulfur (S).

• Essential trace elements like iron (Fe) and magnesium (Mg) are also required.

• Some microbes have lost the ability to synthesize certain essential nutrients, therefore relying on their environment.

Classifying Microbes by Carbon Source

• Autotrophs:

  • Utilize CO2 (inorganic carbon) to synthesize cellular materials.

  • Require significant energy, includes chemolithotrophs and phototrophs.

  • Function as primary producers, generating organic molecules for heterotrophs.

• Heterotrophs:

  • Derive carbon from organic compounds.

  • Most are chemoorganotrophs.

Energy Sources for Microbes

• Regardless of the source, ATP is the primary energy storage compound in cells.

ATP: Energy Currency of the Cell

• Hydrolysis of ATP to ADP drives the synthesis of biological structures (proteins, membranes, etc.) and cellular functions (e.g., transport).

• Many ATPase enzymes link ATP hydrolysis to various biochemical reactions.

• Alternative energy-rich compounds also exist (e.g., PEP).

Metabolism: Encompassing Biochemical Reactions

• Catabolism: Break down complex molecules to generate energy.

• Anabolism: Use energy to synthesize cellular components.

Gibbs Free Energy and Reactions

• Standard Gibbs Free Energy (ΔG0’):

  • Free energy change under standard conditions (25°C, pH 7, 1 M concentration).

  • Calculable based on the nature of reactants/products:

    • Negative ΔG: Exergonic (releases energy) - spontaneous.

    • Positive ΔG: Endergonic (requires energy) - non-spontaneous.

• Reaction example: A + B ➞ C + D

Actual Gibbs Free Energy (ΔG)

• Influenced by:

  • Temperature, concentration of substrates/products

• Higher product concentration increases ΔG; less favorable reactions occur as a result.

• Formula: ΔG = ΔG0 + RT ln(K), where K = [products]/[substrates].

Energy from Aerobic Respiration of Glucose

• Releases 2895 kJ of energy per mole.

• ATP production requires about 31.8 kJ/mole, theoretically producing up to 91 moles of ATP per mole of glucose.

• Actual yield typically around 38 moles due to inefficiencies and non-standard conditions.

Redox Reactions and Source of Electrons

• Redox reactions involve electron transfer from high to low energy states, crucial for cell metabolism.

• Both chemotrophs and phototrophic bacteria rely on redox reactions to generate energy.

Fundamentals of Redox Reactions

• Consist of two half-reactions:

  • Glucose oxidized to CO2 and O2 reduced to H2O.

  • Key Terms:

    • Glucose: electron donor (oxidized)

    • O2: electron acceptor (reduced)

Reducing Power of Electrons

• Glucose as a high-energy electron source; oxygen serves as an electron sink, facilitating energy capture through redox reactions.

Redox Couples and the Redox Tower

• Redox couples indicate possible reactions:

  • Example: Glucose (reduced) and CO2 (oxidized).

  • Arrangement in a redox tower showcases which couple acts as donor/acceptor and quantifies energy from reactions.

Redox Tower Insights

• E0’ values determine electron donor/acceptor tendencies:

  • More negative values indicate a preference for oxidation (electron donation).

  • More positive values indicate a tendency for reduction (electron acceptance).

• Glucose and O2 serve as prime examples of donor and acceptor.

Calculating Free Energy from Redox Tower

• Formula: ΔGo’ = -nFΔEo’

  • n = number of electrons transferred

  • F = Faraday's constant.

Energetics of Specific Redox Reactions

• Example calculation for different redox couples, determine which acts as donor or acceptor based on potential differences in energetic favorability.

• Larger differences in E0' lead to greater energy production.

Electron Carriers in Metabolism

• Essential for transferring electrons from donors to ultimate acceptors via various biochemical pathways.

• Key carriers include NAD+/NADH, functioning as oxidizing and reducing agents.

• Soluble carriers are vital for redox reactions across the cell.

Summary of Reactant Dynamics in Redox Reactions

• Electron Donor (e.g., Glucose):

  • Oxidized, classified as a reducing agent, loses electrons.

• Electron Acceptor (e.g., O2):

  • Reduced, classified as an oxidizing agent, gains electrons.

• Mnemonic: "LEO the lion says GER" (Lose Electrons = Oxidized, Gain Electrons = Reduced).