Microbial Metabolism: Energy, Electron Flow, and Metabolic Diversity

Chapter 3 - Microbial Metabolism

Overview of Metabolism

Microbial metabolism encompasses all chemical reactions that occur within a microbial cell, enabling growth, maintenance, and reproduction. These reactions are broadly classified into two categories:

• Catabolism: Energy-releasing metabolic reactions that break down complex molecules into simpler ones.

• Anabolism: Energy-requiring metabolic reactions that build complex molecules from simpler ones.

Both catabolic and anabolic processes are essential for cellular function and survival.

Cellular Requirements for Metabolism

Microbial cells require several key components to sustain metabolism:

• Water: Essential solvent and participant in biochemical reactions.

• Sources of carbon and other nutrients: Needed for biosynthesis and energy production.

• Free energy: The energy available to do work within the cell.

• Reducing power: Source of electrons (e-) for redox reactions.

Metabolic reactions are classified by their energy changes:

• Exergonic reactions: Release free energy.

• Endergonic reactions: Require free energy input.

The change in free energy is expressed as ΔG0' (standard free energy change).

Energy Classes of Microorganisms

Classification by Energy Source

Microorganisms are classified based on their energy sources, which determine their metabolic strategies:

• Chemotrophs: Obtain energy from chemicals.

  • Chemoorganotrophs: Use organic chemicals (e.g., glucose, acetate).

  • Chemolithotrophs: Use inorganic chemicals (e.g., H2, H2S, Fe2+, NH4+).

• Phototrophs: Obtain energy from light (phototrophy).

Each class uses different metabolic pathways to generate ATP:

• Chemoorganotrophs: Escherichia coli (uses glucose as energy source).

• Chemolithotrophs: Thiobacillus thiooxidans (oxidizes sulfur compounds).

• Phototrophs: Rhodobacter capsulatus (uses CO2 as carbon source, light as energy).

Electron Donors and Acceptors

Redox Reactions in Microbial Metabolism

Energy in microbial cells is often derived from oxidation-reduction (redox) reactions, which involve the transfer of electrons:

• Redox reactions occur in pairs (two half-reactions).

• Electron donor: The substance that is oxidized (loses electrons).

• Electron acceptor: The substance that is reduced (gains electrons).

Example reaction:

• Glucose oxidation: $ext{C}_6 ext{H}_{12} ext{O}_6 + 6 ext{O}_2 ightarrow 6 ext{CO}_2 + 6 ext{H}_2 ext{O}$

• Glucose acts as the electron donor, O2 as the electron acceptor.

Redox Couples and Reduction Potential

Substances can act as electron donors or acceptors depending on the context, forming redox couples. The tendency to donate electrons is measured as the reduction potential (E0'), expressed in volts (V):

• A more negative E0' indicates a stronger electron donor.

• A more positive E0' indicates a stronger electron acceptor.

In a redox couple, the reduced substance with a more negative E0' donates electrons to the oxidized substance with a more positive E0'.

Electron Tower Table

The following table summarizes standard reduction potentials for common electron donors and acceptors:

Electron Acceptor	Electron Donor	E0' (mV)
CO2 + 4H+ + 4e-	CH3OH + H2O	-430
2H+ + 2e-	H2	-420
NAD+ + 2H+ + 2e-	NADH + H+	-320
S0 + 2H+ + 2e-	H2S	-280
FAD + 2H+ + 2e-	FADH2	-219
NO3- + 2H+ + 2e-	NO2- + H2O	+430
O2 + 4H+ + 4e-	2H2O	+820

Additional info: Table entries inferred from standard electron tower data for microbial metabolism.

Comparing Electron Donors and Acceptors

To determine which molecule is a better electron donor or acceptor, compare their E0' values:

• FADH2 vs. NADH: NADH is a stronger electron donor because its E0' is more negative.

• H2 vs. NADH: H2 is a better donor due to its more negative E0'.

• NAD+ vs. H+: NAD+ is a better acceptor because it has a more positive E0'.

Electron flow in metabolism is always from the donor with a more negative E0' to the acceptor with a more positive E0'.

Key Terms and Concepts

• Metabolism: The sum total of all chemical reactions in a cell.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of molecules requiring energy input.

• Redox Reaction: Chemical reaction involving electron transfer.

• Electron Donor: Substance that loses electrons (is oxidized).

• Electron Acceptor: Substance that gains electrons (is reduced).

• Reduction Potential (E0'): Measure of a molecule's tendency to accept or donate electrons.

Example Application

In aerobic respiration, Escherichia coli uses glucose as an electron donor and oxygen as the terminal electron acceptor, generating ATP through a series of redox reactions.

Additional info: Expanded explanations and table content inferred from standard microbiology curriculum and textbook data.