Microbial Metabolism: Enzymes, Energy, and Catabolic Pathways

Microbial Metabolism

Introduction to Metabolism

Metabolism refers to the sum of all chemical reactions that occur within a living organism. These reactions are divided into two main categories: catabolism (breakdown of molecules to release energy) and anabolism (synthesis of complex molecules from simpler ones).

• Catabolic reactions release energy, often stored as ATP.

• Anabolic reactions use energy to build cellular components.

• Energy is stored in adenosine triphosphate (ATP).

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

• Electron Carriers: Molecules such as NAD+, NADH, FAD, and FADH2 that transport electrons during metabolic reactions.

• Enzyme: Biological catalyst that speeds up chemical reactions without being consumed.

• Substrate: The molecule upon which an enzyme acts.

• Competitive Inhibitor: A molecule that competes with the substrate for binding to the active site of an enzyme.

• Noncompetitive Inhibitor: A molecule that binds to an enzyme at a site other than the active site, altering enzyme function.

• Oxidation-Reduction (Redox) Reaction: Chemical reactions involving the transfer of electrons from one molecule to another.

Enzymes and Their Function

Structure and Activity

Enzymes are proteins that catalyze biochemical reactions. They have specific structures, including an active site where substrates bind and reactions occur.

• Enzymes lower the activation energy required for reactions.

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

• Denaturation occurs when environmental conditions disrupt enzyme structure, leading to loss of function.

Enzyme Inhibition

Enzyme inhibitors are molecules that decrease or prevent enzyme activity.

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

• Noncompetitive inhibitors bind elsewhere, changing enzyme shape and function.

• Examples: Penicillin (competitive), Mercury (noncompetitive).

Table: Types of Enzyme Inhibition

Type	Binding Site	Effect
Competitive	Active site	Blocks substrate binding
Noncompetitive	Allosteric site	Alters enzyme shape, reduces activity

Energy Production and Storage

ATP and Phosphorylation

ATP is produced by adding a phosphate group to ADP, a process called phosphorylation. There are three main types of phosphorylation in cells:

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP from a substrate.

• Oxidative phosphorylation: ATP synthesis using energy from electron transport chain (ETC).

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

Oxidation-Reduction (Redox) Reactions

Electron Carriers and Redox

Redox reactions involve the transfer of electrons between molecules. Electron carriers such as NAD+, NADH, FAD, and FADH2 play a crucial role in these processes.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron carriers shuttle electrons to the ETC for ATP production.

Carbohydrate Catabolism

Overview

Most organisms use carbohydrates as their primary energy source. The two main pathways for carbohydrate catabolism are cellular respiration and fermentation.

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

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

Stages of Cellular Respiration

• Glycolysis: Glucose is converted to pyruvate, producing ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Pyruvate is further oxidized, generating NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through a series of proteins, creating a proton gradient used to synthesize ATP.

Table: Summary of Glucose Catabolism

Pathway	Main Products	ATP Yield
Glycolysis	Pyruvate, NADH, ATP	2 ATP
Krebs Cycle	CO2, NADH, FADH2, ATP	2 ATP
ETC (Oxidative Phosphorylation)	ATP, H2O	~34 ATP
Fermentation	Organic acids/alcohols, NAD+	2 ATP

Fermentation

Fermentation is an anaerobic process that regenerates NAD+ for glycolysis by transferring electrons to organic molecules. It produces less ATP than respiration and results in products such as lactic acid or ethanol.

• Occurs when ETC is not available or oxygen is absent.

• Essential for cells to continue glycolysis in anaerobic conditions.

Integration and Regulation of Metabolism

Regulation

Cells regulate metabolism by controlling gene expression and enzyme activity. Catabolic and anabolic pathways are coordinated to meet cellular needs and respond to environmental changes.

• Catabolic regulation: Genes encoding enzymes for breakdown are expressed as needed.

• Anabolic regulation: Synthesis of molecules occurs only when they are not available from the environment.

Review Questions and Applications

• What is the energy currency of the cell? (ATP)

• List the main electron carriers in cells. (NAD+, NADH, FAD, FADH2)

• Compare types of phosphorylation: substrate-level, oxidative, photophosphorylation.

• Describe the effects of temperature, pH, and substrate concentration on enzyme activity.

• Explain competitive and noncompetitive enzyme inhibition.

• Summarize the stages and ATP yield of aerobic respiration and fermentation.

Key Equations

• ATP hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

• General redox reaction: $\text{A} + \text{B} \rightarrow \text{A}^+ + \text{B}^-$

• Glycolysis summary: $\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP}$

Additional info: Some content inferred and expanded for clarity, including definitions, tables, and equations. The notes are structured to cover all major topics from the provided materials and to serve as a comprehensive study guide for college-level microbiology students.