Microbial Metabolism: Study Notes for MIC 205

Microbial Metabolism

Introduction to Metabolism

Metabolism encompasses all chemical reactions within a cell that are necessary for energy exchange, growth, and reproduction. In microbiology, understanding metabolism is crucial for grasping how microbes obtain energy and nutrients from their environment.

• Definition: Metabolism is the sum of all chemical reactions that lead to energy exchanges required for growth and reproduction.

• Key Processes: Involves both catabolic (breaking down molecules to release energy) and anabolic (building up molecules using energy) reactions.

• Examples: Catabolism of glucose, synthesis of proteins.

• Application: Metabolic byproducts are often used for microbial classification.

Cellular Energy Needs

Cells require energy for various functions, including movement, synthesis of macromolecules, and maintaining homeostasis. Microbes have evolved diverse strategies to obtain energy from their environment.

• Energy Sources: Organic and inorganic molecules, light.

• Energy Utilization: Energy is used to drive cellular processes and is stored in chemical bonds.

• Example: Bacteria can use sugars, amino acids, or sunlight as energy sources.

Harvesting Energy and Energy Molecules

Harvesting Energy from Substrates

Microbes harvest energy and raw materials from controlled breakdown of organic substrates such as carbohydrates, proteins, and lipids.

• High-Energy Bonds: Adenosine triphosphate (ATP) stores energy in phosphate bonds.

• ATP Synthesis: Energy is released when phosphate bonds are broken.

• Pathways: Substrate-level phosphorylation, oxidative phosphorylation, photophosphorylation.

The Energy Molecules

ATP is the primary energy currency of the cell. Energy is transferred via activated carriers, such as ATP, NADH, and FADH2.

• ATP: Adenosine triphosphate, stores energy in high-energy phosphate bonds.

• Energy Transfer: Hydrolysis of ATP releases energy for cellular work.

• Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

• Example: Cells spend ATP like cash to power cellular activities.

Metabolic Pathways and Reactions

Catabolic and Anabolic Reactions

Metabolic pathways are divided into two major classes: catabolic (energy-releasing) and anabolic (energy-consuming) reactions.

• Catabolic Reactions: Break down complex molecules into simpler ones, releasing energy.

• Anabolic Reactions: Build complex molecules from simpler ones, requiring energy input.

• Example: Glycolysis (catabolic), protein synthesis (anabolic).

Reduction & Oxidation Reactions (REDOX)

Cells generate ATP by transferring electrons from organic and inorganic compounds. These reactions are always coupled and involve electron carriers.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron Carriers: Molecules like NAD+, FAD, and NADP+ temporarily store and transfer electrons.

• Equation:

$\text{A} + \text{B} \rightarrow \text{A}^+ + \text{B}^-$

• Example: NAD+ + 2e- + 2H+ → NADH + H+

Electron Carrier Molecules

Electron carriers are essential for the temporary storage and transfer of energy during metabolic reactions.

• Types: NAD+, FAD, NADP+

• Function: Shuttle electrons between metabolic pathways.

• Source: Derived from B vitamins.

Proteins and Enzymes in Metabolism

Protein Structure

Proteins are chains of amino acids that perform a variety of essential cellular functions, including catalyzing metabolic reactions as enzymes.

• Levels of Structure: Primary, secondary, tertiary, and quaternary.

• Peptide Bonds: Covalent bonds between amino acids.

• Example: Enzymes are proteins with specific three-dimensional structures.

Enzyme Function and Activity

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy. Their activity depends on several factors.

• Active Site: Region where substrate binds and reaction occurs.

• Specificity: Enzymes are specific to their substrates.

• Factors Affecting Activity: Temperature, pH, substrate concentration, presence of inhibitors.

• Graph: Enzyme activity typically increases with temperature up to an optimum, then decreases.

Enzyme Inhibitors

Inhibitors are substances that alter or block enzyme activity, affecting metabolic pathways.

• Types: Competitive (bind to active site), non-competitive (bind elsewhere).

• Feedback Inhibition: End product of a pathway inhibits an earlier step.

• Example: Antibiotics can act as enzyme inhibitors in microbes.

Carbohydrate Catabolism

Overview of Carbohydrate Catabolism

Carbohydrate catabolism is the breakdown of sugars to release energy. The most common pathway is glycolysis, followed by fermentation or respiration.

• Pathways: Glycolysis, fermentation, aerobic respiration.

• Products: ATP, NADH, pyruvate.

• Example: Glucose breakdown via glycolysis yields ATP and pyruvate.

EMP Pathway (Glycolysis)

The Embden-Meyerhof-Parnas (EMP) pathway, or glycolysis, is the primary route for glucose catabolism in most cells.

• Steps: Glucose is converted to pyruvate through a series of enzyme-catalyzed reactions.

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

• NADH Production: 2 NADH per glucose.

• Equation:

$\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

• Example: Glycolysis occurs in the cytoplasm of both prokaryotic and eukaryotic cells.

Pentose Phosphate Pathway

This alternative pathway generates NADPH and pentoses for biosynthesis.

• Products: NADPH, ribose-5-phosphate.

• Function: Provides reducing power and precursors for nucleotide synthesis.

Aerobic and Anaerobic Respiration

After glycolysis, pyruvate can be further oxidized via aerobic or anaerobic respiration, depending on the presence of oxygen.

• Aerobic Respiration: Complete oxidation of pyruvate to CO2 and H2O, yielding maximum ATP.

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

• Equation (Aerobic):

$\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{NADH} + \text{CO}_2$

Summary Table: Key Metabolic Pathways

Pathway	Main Substrate	Products	ATP Yield	Electron Carriers
Glycolysis (EMP)	Glucose	Pyruvate, ATP, NADH	2 ATP (net)	2 NADH
Pentose Phosphate Pathway	Glucose-6-phosphate	Ribose-5-phosphate, NADPH	Variable	NADPH
Aerobic Respiration	Pyruvate	CO2, H2O, ATP	~38 ATP (per glucose)	NADH, FADH2
Anaerobic Respiration	Pyruvate	Varies (e.g., lactate, ethanol)	Less than aerobic	NADH

Additional info:

• Some context and terminology were expanded for clarity and completeness.

• Diagrams referenced in the slides were described in text form.