Chapter 3: Microbial Metabolism

I. Fundamentals of Metabolism

Microbial metabolism encompasses all chemical reactions that occur within microorganisms, enabling them to grow, reproduce, and respond to their environment. Understanding metabolism is essential for grasping how microbes obtain energy and nutrients.

• Defining the Requirements for Life: Microbes require energy, carbon, and electrons to sustain life and drive cellular processes.

• Electron Transfer Reactions: Central to metabolism, these reactions involve the movement of electrons from donors to acceptors, facilitating energy conservation.

• Calculating Changes in Free Energy: The change in Gibbs free energy () determines whether a reaction can occur spontaneously.

• Cellular Energy Conservation: Microbes conserve energy through substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.

• Catalysis and Enzymes: Enzymes are biological catalysts that accelerate metabolic reactions by lowering activation energy.

II. Classification of Metabolic Types Based on Energy Sources

Microorganisms are classified by their energy and electron sources, which determines their metabolic strategies.

• Phototrophs: Use light as an energy source.

• Chemotrophs: Obtain energy from chemical compounds.

• Organotrophs: Use organic molecules as electron donors.

• Lithotrophs: Use inorganic molecules as electron donors.

Table: Classification of Metabolic Types

III. Metabolic Coupling: Catabolism and Anabolism

Metabolism is divided into two main processes: catabolism (breakdown of molecules to release energy) and anabolism (biosynthesis of cellular components).

• Catabolism: Degradative reactions that generate ATP and reducing power.

• Anabolism: Synthetic reactions that consume ATP and reducing power to build cellular structures.

• Energy Coupling: ATP links catabolic and anabolic reactions, transferring energy between them.

IV. Oxidation-Reduction (Redox) Reactions

Redox reactions are fundamental to microbial metabolism, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Example Reaction: Aerobic respiration of glucose:

V. Principles of Bioenergetics

Bioenergetics studies how organisms manage energy resources. The Gibbs free energy change () predicts the direction and spontaneity of reactions.

• Energy Units: Kilojoules (kJ) of heat energy.

• Gibbs Free Energy (): The energy available to do work.

• Standard Free Energy Change (): Calculated under standard conditions (pH 7, 25°C, 1 atm).

• Equation:

Table: Free Energy of Formation (, kJ/mol) for Common Substances

VI. Metabolic Diversity and Relationship to Oxygen

Microbes exhibit diverse metabolic strategies depending on the presence or absence of oxygen.

• Aerobic Respiration: Uses oxygen as the terminal electron acceptor.

• Anaerobic Respiration: Uses alternative electron acceptors (e.g., nitrate, sulfate).

• Fermentation: Organic molecules serve as both electron donors and acceptors.

Table: Examples of Metabolic Diversity

VII. Glycolysis (Embden-Meyerhof-Parnas Pathway)

Glycolysis is a central metabolic pathway that converts glucose to pyruvate, generating ATP and NADH.

• Key Steps: Glucose is phosphorylated, split into two 3-carbon molecules, and oxidized to produce pyruvate.

• ATP Yield: Net gain of 2 ATP per glucose molecule.

• NADH Production: 2 NADH per glucose.

VIII. The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP/GTP.

• Key Steps: Acetyl-CoA combines with oxaloacetate, forming citrate and proceeding through a series of reactions.

• Energy Yield: 3 NADH, 1 FADH2, and 1 ATP (or GTP) per cycle.

IX. NAD+/NADH Cycling and Redox Coenzymes

NAD+ and NADP+ are essential electron carriers in metabolism, cycling between oxidized and reduced forms.

• NAD+: Accepts electrons during catabolic reactions, becoming NADH.

• NADP+: Functions mainly in anabolic reactions.

• Structure: Both contain nicotinamide and adenine moieties.

X. ATP: The Energy Currency

ATP (adenosine triphosphate) stores and transfers energy within the cell.

• Formation: ATP is generated by substrate-level and oxidative phosphorylation.

• Hydrolysis: Releases energy for cellular work.

• Equation:

XI. Electron Transport Chain and Proton Motive Force

The electron transport chain (ETC) transfers electrons from NADH and FADH2 to terminal electron acceptors, generating a proton gradient used to synthesize ATP.

• Complexes: Series of membrane-bound proteins and cofactors.

• Proton Motive Force (PMF): Drives ATP synthesis via ATP synthase.

• Equation:

XII. Redox Tower

The redox tower ranks electron donors and acceptors by their reduction potential (), predicting the direction of electron flow and energy yield.

• High on Tower: Strong electron donors (e.g., H2, glucose).

• Low on Tower: Strong electron acceptors (e.g., O2, nitrate).

• Energy Yield: Greater difference in yields more energy.

XIII. Metabolic Diversity in Microbes

Microbes utilize a wide range of metabolic strategies, including aerobic and anaerobic respiration, fermentation, and chemolithotrophy.

• Escherichia coli: Capable of both aerobic and nitrate-based anaerobic respiration.

• Ralstonia eutropha: Chemolithotroph that oxidizes hydrogen for energy.

XIV. Cytochromes and Their Structure

Cytochromes are heme-containing proteins involved in electron transport. Their structure enables efficient electron transfer within the ETC.

• Heme Group: Central iron atom that cycles between reduced and oxidized states.

• Role: Facilitate electron transfer to terminal acceptors.

Example: Calculating Free Energy Change

To calculate for a reaction:

• Use standard free energy of formation values () for reactants and products.

• Apply the equation:

• Example: For aerobic oxidation of acetate: Calculate using tabulated values.

Additional info:

• These notes expand on slide content with definitions, equations, and context for key metabolic pathways and bioenergetics in microorganisms.

• Tables and figures referenced are reconstructed for clarity and completeness.