Microbial Metabolism: Enzymes, Pathways, and Energy Production

Microbial Metabolism

Definitions: Metabolism, Catabolism, and Anabolism

Metabolism encompasses all chemical reactions occurring within a cell or organism. These reactions are divided into two main categories:

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

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

Example: Glycolysis is a catabolic pathway, while protein synthesis is anabolic.

Catabolic Processes and Their Products

• Catabolic processes break down carbohydrates, lipids, and proteins to produce energy.

• Possible products: ATP, NADH, FADH2, CO2, water, and metabolic intermediates.

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons between molecules, crucial for energy production in cells.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

Role of NAD+ and FAD in Metabolism

• NAD+ (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide) are electron carriers.

• They accept electrons during catabolic reactions and transfer them to the electron transport chain, facilitating ATP production.

Enzymes and Their Function

Structure of an Enzyme

• Enzymes are biological catalysts, typically proteins, with a specific three-dimensional structure.

• They have an active site where substrates bind and reactions occur.

Substrate, Cofactor, and Coenzyme

• Substrate: The specific reactant acted upon by an enzyme.

• Cofactor: A non-protein component (often a metal ion) required for enzyme activity.

• Coenzyme: An organic cofactor, often derived from vitamins (e.g., NAD+, FAD).

Function of Enzymes

• Enzymes lower the activation energy of biochemical reactions, increasing reaction rates without being consumed.

Factors Affecting Enzyme Activity

• Temperature: High temperatures can denature enzymes, reducing activity.

• pH: Each enzyme has an optimal pH; deviations can reduce activity or denature the enzyme.

• Substrate concentration: Increasing substrate increases activity up to a saturation point.

Example: Human enzymes typically function best at 37°C and neutral pH.

Enzyme Inhibition

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

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

Feedback Inhibition

• End product of a metabolic pathway inhibits an earlier enzyme, regulating pathway activity.

Cellular Respiration and Fermentation

Cellular Respiration vs. Fermentation

• Cellular respiration: Complete oxidation of glucose to CO2 and H2O, using oxygen (aerobic) or other electron acceptors (anaerobic).

• Fermentation: Partial oxidation of glucose without an external electron acceptor; produces less ATP.

Glycolysis: Key Events and Products

• Glycolysis converts glucose to pyruvate, producing ATP and NADH.

• Products: 2 ATP, 2 NADH, 2 pyruvate per glucose molecule.

Pentose Phosphate Pathway vs. Entner-Doudoroff Pathway vs. Glycolysis

Pathway	ATP Yield	Other Products
Glycolysis	2 ATP	2 NADH, 2 pyruvate
Pentose Phosphate	1 ATP (variable)	NADPH, ribose-5-phosphate
Entner-Doudoroff	1 ATP	1 NADH, 1 NADPH, 2 pyruvate

Aerobic and Anaerobic Respiration

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Uses oxygen as the final electron acceptor.

• Anaerobic respiration: Uses other molecules (e.g., nitrate, sulfate, CO2) as final electron acceptors.

Electron Acceptors in Anaerobic Respiration

• Examples: Nitrate (NO3-), sulfate (SO42-), CO2

Microbes Capable of Anaerobic Respiration

• Many bacteria and archaea can use anaerobic respiration, especially in oxygen-poor environments.

Fermentation

Process of Fermentation

• Fermentation allows cells to regenerate NAD+ from NADH by transferring electrons to organic molecules.

• Fate of NADH and pyruvic acid: NADH reduces pyruvate or its derivatives, producing various end products.

End Products of Fermentation

• Lactic acid (e.g., in muscles, yogurt)

• Ethanol and CO2 (e.g., in yeast, alcoholic beverages)

• Other products: Acetic acid, acetone, isopropanol, propionic acid, cheese, bread, beer, kimchi

Integration of Catabolism and Anabolism

Connection Between Catabolism and Anabolism

• Catabolic pathways provide energy and building blocks for anabolic reactions.

• Many intermediates are shared between the two processes, allowing efficient resource use.