Microbial Metabolism

Introduction to Metabolism

Microbial metabolism encompasses all chemical reactions that occur within microorganisms, enabling them to grow, reproduce, and respond to their environment. These reactions are broadly classified into catabolic (breakdown) and anabolic (biosynthetic) processes.

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.

• Energy Currency: ATP (adenosine triphosphate) is the primary energy carrier in cells.

Oxidation-Reduction (Redox) Reactions

Redox reactions are central to energy production in cells, involving the transfer of electrons from one molecule (donor) to another (acceptor).

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron Carriers: NAD+, NADH, FAD, FADH2 are key molecules that shuttle electrons during metabolic reactions.

Equation:

ATP Production and Energy Storage Reactions

ATP is generated by three main phosphorylation mechanisms:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Oxidative phosphorylation: ATP synthesis powered by the electron transport chain (ETC) and chemiosmosis.

• Photophosphorylation: ATP production using light energy (in photosynthetic organisms).

Equation:

Enzymes and Their Roles in Metabolism

Enzymes are biological catalysts that accelerate chemical reactions by lowering activation energy. They are essential for metabolic pathways.

• Structure: Most enzymes are proteins with a specific active site for substrate binding.

• Function: Catalysis, regulation, transport, defense.

• Sensitivity: Enzyme activity is affected by temperature, pH, and substrate concentration.

• Denaturation: Extreme conditions can permanently inactivate enzymes.

Enzyme Inhibitors

• Competitive inhibitors: Bind to the active site, blocking substrate access.

• Noncompetitive inhibitors: Bind elsewhere, altering enzyme shape and function.

• Examples: Penicillin, aspirin, lipitor.

Carbohydrate Catabolism

Most microbes use carbohydrates, especially glucose, as their primary energy source. Catabolic pathways include:

• Cellular respiration: Complete oxidation of glucose to CO2 and H2O, producing ATP.

• Fermentation: Partial oxidation of glucose, producing less ATP and organic waste products.

Major Pathways of Glucose Catabolism

• Glycolysis: Converts glucose to pyruvate, generating ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Oxidizes acetyl-CoA to CO2, producing NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Uses electrons from NADH and FADH2 to generate ATP via oxidative phosphorylation.

Summary Table: Comparison of Aerobic Respiration, Anaerobic Respiration, and Fermentation

Electron Transport Chain (ETC) and Chemiosmosis

The ETC is a series of protein complexes that transfer electrons, creating a proton gradient across a membrane. This gradient powers ATP synthesis via chemiosmosis.

• Location: Cytoplasmic membrane (prokaryotes), mitochondrial inner membrane (eukaryotes).

• Oxidative phosphorylation: ATP synthesis driven by the ETC and proton gradient.

Fermentation

Fermentation is an anaerobic process that regenerates NAD+ for glycolysis by transferring electrons to organic molecules. It produces less ATP than respiration and yields organic waste products.

• Purpose: Regenerate NAD+ for continued glycolysis.

• Products: Lactic acid, ethanol, CO2, other organic acids.

• Commercial Applications: Food production (bread, yogurt, alcohol).

Integration and Regulation of Metabolism

Metabolic pathways are tightly regulated to ensure efficient energy use and synthesis of necessary molecules.

• Catabolic reactions: Genes encoding catabolic enzymes are regulated based on substrate availability.

• Anabolic reactions: Synthesis of molecules is regulated; if a molecule is available, synthesis is suppressed.

Key Terms and Definitions

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones.

• ATP: Main energy currency of the cell.

• Electron carriers: NAD+, NADH, FAD, FADH2.

• Enzyme: Protein catalyst that speeds up chemical reactions.

• Competitive inhibitor: Molecule that blocks the active site of an enzyme.

• Noncompetitive inhibitor: Molecule that binds elsewhere on the enzyme, altering its function.

• Cellular respiration: Complete oxidation of glucose to CO2 and H2O.

• Fermentation: Partial oxidation of glucose, producing organic waste products.

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

Example: Substrate-Level Phosphorylation

• Occurs during glycolysis and Krebs cycle.

• Direct transfer of phosphate group to ADP.

• Produces a small amount of ATP compared to oxidative phosphorylation.

Example: Competitive Inhibition

• Sulfanilamide: Antibiotic that inhibits an enzyme by mimicking the substrate.

• Prevents substrate from binding, stopping the reaction.

Summary of Glucose Catabolism (Fig. 5.13)

• Glycolysis: Glucose → Pyruvate + ATP + NADH

• Krebs Cycle: Pyruvate → Acetyl-CoA → CO2 + NADH + FADH2 + ATP

• ETC: NADH/FADH2 → ATP (via oxidative phosphorylation)

Additional info:

• Some microbes can use alternative electron acceptors (e.g., nitrate, sulfate) in anaerobic respiration.

• Fermentation is essential in environments lacking oxygen and is exploited in food and industrial microbiology.