Microbial Metabolism I & II: Nutrients, Pathways, and Bioenergetics

Microbial Metabolism: Overview

Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within a microbial cell, enabling growth, energy production, and maintenance. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes.

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

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

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy; highly specific for substrates.

Microbial Nutritional Requirements

Macronutrients and Micronutrients

Microbes require a variety of nutrients for growth and cellular function. These are classified based on the quantity needed:

• Macronutrients: Required in large amounts; include carbon (C), nitrogen (N), oxygen (O), hydrogen (H), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg).

• Micronutrients: Required in small amounts; include iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), vitamins, and trace metals.

Cellular Composition

Microbial cells are composed of various macromolecules and water:

• Water: ~70% of cell mass

• Chemicals: ~30% of cell mass

• Macromolecular composition (by dry weight):

  • Protein: ~55%

  • RNA: ~20.5%

  • Lipid: ~9.1%

  • Polysaccharide: ~5.0%

  • Lipopolysaccharide: ~3.4%

  • DNA: ~3.1%

HTML Table: Major Nutrients and Their Functions

Macronutrient	Function
Carbon	Structural backbone of organic molecules
Nitrogen	Component of amino acids, nucleic acids
Phosphorus	Component of nucleic acids, ATP, phospholipids
Sulfur	Component of some amino acids, vitamins
Potassium, Magnesium, Calcium	Enzyme cofactors, membrane stability
Iron	Electron transport, enzyme cofactor

Classification of Microbes by Metabolic Type

Energy and Carbon Sources

Microorganisms are classified based on their sources of energy and carbon:

• Energy Sources:

  • Chemotrophs: Obtain energy from chemicals.

  • Phototrophs: Obtain energy from light.

  • Chemoorganotrophs: Use organic chemicals for energy.

  • Chemolithotrophs: Use inorganic chemicals for energy (specific to Bacteria and Archaea).

• Carbon Sources:

  • Autotrophs: Use CO2 as carbon source.

  • Heterotrophs: Use organic carbon sources.

  • Mixotrophs: Can utilize both inorganic and organic sources.

HTML Table: Microbial Metabolic Types

Type	Energy Source	Carbon Source	Example
Chemoorganotroph	Organic chemicals	Organic compounds	Escherichia coli
Chemolithotroph	Inorganic chemicals	CO2 or organic compounds	Thiobacillus
Phototroph	Light	CO2 or organic compounds	Cyanobacteria

Bioenergetics and Energy Conservation

High-Energy Bonds and ATP

Microbes store energy in high-energy bonds, primarily in adenosine triphosphate (ATP). The hydrolysis of these bonds releases energy for cellular work.

• High-energy bonds: Phosphoanhydride bonds in ATP, thioester bonds in acetyl-CoA.

• ATP: Universal energy currency; generated via substrate-level phosphorylation, oxidative phosphorylation, or photophosphorylation.

Mechanisms of Energy Conservation

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

• Oxidative phosphorylation: Electron transport chain generates a proton motive force, driving ATP synthesis.

• Photophosphorylation: Light energy drives electron transport and ATP synthesis in phototrophs.

Redox Reactions and Electron Transport

Redox Potential and Electron Flow

Redox reactions involve the transfer of electrons between molecules, with energy released or required depending on the direction of electron flow.

• Redox potential (E0'): Tendency of a substance to gain or lose electrons; measured in volts.

• Electron donors: Substances with negative redox potential; lose electrons.

• Electron acceptors: Substances with positive redox potential; gain electrons.

Electron Tower Concept

The electron tower ranks redox couples by their reduction potential. Electrons flow from donors (higher energy, more negative E0') to acceptors (lower energy, more positive E0'), releasing energy.

Electron Transport Chain (ETC)

The ETC is a series of membrane-bound carriers that transfer electrons, generating a proton gradient (proton motive force) used by ATP synthase to produce ATP.

• Location: Cytoplasmic membrane in prokaryotes.

• Components: NADH, FADH2, cytochromes, quinones.

• Terminal electron acceptor: Oxygen (aerobic respiration) or other molecules (anaerobic respiration).

Glycolysis and Fermentation

Glycolysis (Embden-Meyerhof-Parnas Pathway)

Glycolysis is the central pathway for glucose catabolism, converting glucose to pyruvate and generating ATP and NADH.

• Inputs: Glucose, 2 NAD+, 2 ADP, 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP

Equation:

$ext{Glucose} + 2 ext{NAD}^+ + 2 ext{ADP} + 2 ext{P}_i ightarrow 2 ext{Pyruvate} + 2 ext{NADH} + 2 ext{ATP} + 2 ext{H}_2 ext{O}$

Fermentation

Fermentation is an anaerobic process that generates ATP by substrate-level phosphorylation, using organic molecules as electron acceptors.

• Does not require oxygen

• Does not use the Krebs cycle or ETC

• Produces small amounts of ATP

• Types: Lactic acid fermentation, alcoholic fermentation, mixed acid fermentation

HTML Table: Fermentation Types and Products

Type	Microbe Example	End Products
Lactic acid fermentation	Streptococcus	Lactic acid
Alcoholic fermentation	Saccharomyces (yeast)	Ethanol, CO2
Mixed acid fermentation	Escherichia coli	Acetic acid, succinic acid, ethanol, CO2, H2

Respiration and ATP Synthesis

Respiration

Respiration involves the complete oxidation of substrates, using an electron transport chain to generate a proton motive force and synthesize ATP via ATP synthase.

• Aerobic respiration: Oxygen is the terminal electron acceptor.

• Anaerobic respiration: Other molecules (e.g., nitrate, sulfate) serve as terminal electron acceptors.

ATP Synthase Mechanism

ATP synthase is a reversible enzyme that uses the proton motive force to convert ADP and inorganic phosphate into ATP.

$ext{ADP} + ext{P}_i ightarrow ext{ATP}$

• Protons flow through ATP synthase, driving the phosphorylation of ADP.

• Can also hydrolyze ATP to pump protons under certain conditions.

Additional info:

• Some details on the electron tower and redox couples were inferred for completeness.

• Specific examples of metabolic types and fermentation products were added for academic context.