Microbial Metabolism

I. Overview of Microbial Metabolism

Microbial metabolism encompasses all the chemical reactions that occur within a microbial cell, enabling growth, energy production, and maintenance. These reactions are classified into catabolic and anabolic processes.

II. Energy Requirements of Chemical Reactions

Chemical reactions in cells require an input of energy to proceed, known as the energy of activation (Ea). Enzymes are biological catalysts that lower the energy of activation, thereby increasing the rate of reaction without being consumed or altered by the reaction.

• Enzyme: Lowers energy of activation, increases reaction rate, remains unchanged by the reaction.

• Active site: Region on the enzyme that binds the substrate and competitive inhibitors.

Enzyme-Catalyzed Reaction Energy Profile

The diagram below illustrates how enzymes lower the activation energy required for a reaction, allowing it to proceed more efficiently and at lower temperatures.

• Substrate: Reactant in an enzyme-catalyzed reaction.

• Inhibitors:

  • Competitive: Binds to the active site, prevents substrate binding. Can be overcome by increasing substrate concentration. Examples: sulfanilamide, penicillin, AZT.

  • Non-competitive: Binds elsewhere on the enzyme, changes enzyme shape, prevents substrate binding regardless of substrate concentration.

Generation of ATP

II. Mechanisms of ATP Generation

ATP (adenosine triphosphate) is the primary energy currency in microbial cells. There are several mechanisms by which cells generate ATP:

A. Substrate Level Phosphorylation (SLP)

• High-energy phosphate group is directly transferred from a phosphorylated compound to ADP to form ATP in a single step chemical reaction.

• Occurs during glycolysis and the Krebs cycle.

B. Oxidative Phosphorylation

• Electrons are transferred from organic compounds to electron carriers (NAD+, FAD), which become reduced (NADH, FADH2).

• Electrons are delivered to the electron transport chain (ETC) in the plasma membrane (prokaryotes) or inner mitochondrial membrane (eukaryotes).

• Movement of electrons through the ETC powers the transport of H+ ions across the membrane, creating a proton gradient.

• ATP synthase uses the energy from H+ diffusion back across the membrane to synthesize ATP.

C. Photophosphorylation (Photosynthesis)

• Similar to oxidative phosphorylation, but electrons come from chlorophyll and are raised to high energy by absorption of solar energy.

• Energy source is solar (radiation), not chemical.

D. Chemoautotrophs

• Manufacture ATP similarly to oxidative phosphorylation, but electrons come from inorganic compounds (e.g., H2S).

Biochemical Pathways

III. Major Pathways in Microbial Metabolism

A. Glycolysis

Glycolysis is the process by which glucose is converted to pyruvic acid, producing ATP and NADH.

• 2 pyruvic acid, 2 ATP (via substrate level phosphorylation), and 2 NADH are produced from the oxidation of glucose.

• In fermentation, pyruvic acid is reduced, and glycolysis continues by oxidation of NADH and reduction of pyruvate or its derivatives. Maximum of 2 ATP produced.

B. Krebs Cycle (Citric Acid Cycle)

• Pyruvate from glycolysis is oxidized to acetyl-CoA, which enters the Krebs cycle.

• Acetyl-CoA is oxidized, producing NADH and FADH2 (reduction products), and some ATP via substrate level phosphorylation.

C. Electron Transport Chain (ETC)

• In respiration (cellular respiration), the ETC is used to generate more ATP from a single glucose molecule than fermentation.

• Aerobic respiration: O2 is the final electron acceptor. Electrons exit ETC and reduce O2 to H2O.

• Anaerobic respiration: Some inorganic molecule other than O2 is the final electron acceptor. Efficiency is variable and generally lower than aerobic respiration.

D. Efficiency of ATP Production

• Glycolysis and fermentation: 2 ATP net per glucose molecule

• Entner-Doudoroff pathway: 1 ATP net

• Aerobic respiration: 36-38 ATP net (eukaryotes and prokaryotes, respectively)

• Anaerobic respiration: <36 ATP net (variable, depends on final electron acceptor)

IV. Major Cell Types in Microbial Metabolism

Microorganisms are classified based on their energy and carbon sources:

V. Key Terms and Concepts

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules, energy releasing.

• Anabolism: Synthesis of molecules, energy requiring.

• Enzyme: Biological catalyst, lowers activation energy.

• Substrate: Reactant in enzyme-catalyzed reaction.

• Competitive inhibitor: Binds active site, blocks substrate.

• Non-competitive inhibitor: Binds elsewhere, changes enzyme shape.

• ATP: Main energy currency of the cell.

• Electron Transport Chain (ETC): Series of proteins that transfer electrons and generate ATP.

• Aerobic respiration: Uses O2 as final electron acceptor.

• Anaerobic respiration: Uses other inorganic molecules as final electron acceptor.

VI. Important Equations

• General equation for aerobic respiration:

• ATP synthesis via substrate level phosphorylation:

• Reduction of NAD+:

Example: During glycolysis, glucose is converted to pyruvate, producing ATP and NADH, which are then used in further metabolic pathways such as fermentation or respiration.

Additional info: The above notes expand on the brief points in the original material, providing definitions, examples, and context for key concepts in microbial metabolism and energy production.