Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a cell, divided into two main categories: catabolism and anabolism. Catabolic reactions release energy by breaking down molecules, while anabolic reactions consume energy to build cellular components.

• Catabolism: Energy-producing reactions; breakdown of macromolecules.

• Anabolism: Biosynthetic (energy-using) reactions; synthesis of macromolecules and cell components.

• Energy sources: Organic and inorganic compounds, light.

• Waste products: Acids, alcohols, gases, reduced electron acceptors (e.g., fermentation products).

Oxidation-Reduction (Redox) Reactions

Principles of Redox Reactions

Redox reactions are fundamental to microbial metabolism, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox reactions are coupled: the electron donor is oxidized, and the electron acceptor is reduced.

Example of Redox Reaction

Glucose oxidation coupled to oxygen reduction:

• Reactants:

• Products:

• Glucose is oxidized to CO2; O2 is reduced to H2O.

Electron Carriers

Role and Types of Electron Carriers

Electron carriers facilitate the transfer of electrons from donors to acceptors in metabolic pathways.

• Membrane-bound: Cytochrome c

• Freely diffusible coenzymes: NAD+/NADH

NAD+/NADH Cycling

NAD+ accepts electrons during catabolic reactions, becoming NADH, which can then donate electrons in other reactions.

• Example: Isocitrate dehydrogenase catalyzes the conversion of isocitrate to α-ketoglutarate, reducing NAD+ to NADH and releasing CO2.

• Glutamate dehydrogenase catalyzes the conversion of α-ketoglutarate and NH3 to glutamate, oxidizing NADH to NAD+.

Energy Release and Storage

Energy from Redox Reactions

Energy released from redox reactions is stored in molecules and transported where needed.

• Short-term energy storage:

  • Adenosine triphosphate (ATP): Contains two high-energy phosphoanhydride bonds; primary energy carrier in the cell.

  • Coenzyme A derivatives: Thioester bonds.

• Long-term energy storage:

  • Glycogen

  • Poly-β-hydroxybutyrate

  • Elemental sulfur

Energy Stores Table

Types of Phosphorylation

Mechanisms of ATP Generation

• Substrate-level phosphorylation: A phosphate group (PO43-) is transferred from a substrate to ADP, forming ATP. Occurs in glycolysis and fermentation.

• Oxidative phosphorylation: Inorganic phosphate (Pi) is added to ADP by ATP synthase, driven by the proton motive force generated by electron transport chains.

Enzyme Classes in Metabolism

Major Enzyme Types and Examples

Glycolysis

Pathway and Key Steps

Glycolysis is the major pathway for glucose metabolism, occurring in the cytoplasm. It generates 2 ATP and 2 pyruvate per glucose molecule.

• Step 6: Requires NAD+ (glyceraldehyde-3-phosphate dehydrogenase)

• Steps 7 and 10: Substrate-level phosphorylation (ATP generation)

• Key inputs: Glucose, 2 NAD+, 2 ADP

• Key outputs: 2 pyruvate, 2 ATP, 2 NADH

Equation:

Fermentation

Principles and Types

Fermentation is an anaerobic process for generating ATP, where organic compounds serve as both electron donors and acceptors. ATP is produced by substrate-level phosphorylation.

• Occurs when no exogenous electron acceptor is available.

• Common fermentable substances: sugars, amino acids, nucleic acids.

• Syntrophy: Cooperative degradation of compounds by multiple microorganisms.

Common Fermentations Table

Alcohol Fermentation

• Occurs in yeast (Saccharomyces cerevisiae).

• Glucose is converted to 2 pyruvate, then to 2 acetaldehyde and 2 CO2, and finally to 2 ethanol.

• Equation:

Homolactic Fermentation

• Performed by Streptococcus and Lactococcus.

• Glucose is converted to 2 lactate.

Heterolactic Fermentation

• Performed by Leuconostoc.

• Produces lactic acid, ethanol, and CO2 from glucose.

Fermentation in Anaerobic Eukaryotes

• Some flagellated protozoa (e.g., Trichomonas) use hydrogenosomes to oxidize pyruvate and generate ATP by substrate-level phosphorylation.

Summary of Fermentation

• ATP is created by substrate-level phosphorylation.

• Starts with glycolysis (glucose to pyruvate, 2 ATP).

• Reduction of pyruvate regenerates NAD+ and releases fermentation products.

Respiration

Overview and ATP Yield

Respiration is a process where electrons from organic or inorganic donors are transferred to an exogenous electron acceptor via an electron transport system, generating a proton motive force for ATP synthesis.

• Complete oxidation of glucose to CO2 yields up to 38 ATP per glucose.

• Glycolysis: 2 pyruvate + 2 ATP

• Citric acid cycle: Oxidation of pyruvate to CO2 (30 ATP)

• NADH and FADH2 donate electrons to the electron transport chain, creating a proton motive force used by ATP synthase for oxidative phosphorylation.

Citric Acid Cycle and Glyoxylate Cycle

Citric Acid Cycle

The citric acid cycle (CAC) is a series of redox reactions that fully oxidize acetyl-CoA to CO2, generating NADH, FADH2, and ATP/GTP.

• Key intermediates: citrate, isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate.

• Each turn produces 3 NADH, 1 FADH2, and 1 GTP/ATP.

Glyoxylate Cycle

Some C2 compounds (e.g., acetate) are catabolized through the glyoxylate cycle, which bypasses decarboxylation steps to conserve carbon for biosynthesis.

Electron Transport Systems

Types and Components

Electron transport systems transfer electrons to generate ATP. The initial electron donor can be organic or inorganic, and the terminal electron acceptor is required from the environment.

Protein Electron Carriers

• NADH dehydrogenases: Bind NADH, accept 2 electrons and 2 protons, release NAD+, donate electrons to flavoproteins.

• Flavoproteins (FMN, FAD): Receive electrons and protons from NADH dehydrogenase, donate electrons to next carrier.

Additional info: These notes cover core concepts from microbial metabolism, including glycolysis, fermentation, respiration, energy storage, and electron transport, suitable for college-level microbiology students.