Microbial Metabolism Overview

Microbial Metabolism

• Microbial metabolism is the series of biochemical reactions needed to sustain life.
• These reactions include catabolism and anabolism.
• Catabolism breaks down large molecules into smaller ones, releasing energy (exergonic).
• Anabolism uses energy to build larger molecules from smaller ones (endergonic).
• Key components for microbial life include water, a source of energy, a source of electrons, and a source of carbon.

Metabolic Classifications of Microbes

• Chemotrophs: Obtain energy from chemical reactions.

  • Organotrophs: Use organic molecules as electron sources.
    • Glycolysis (oxidizes glucose to pyruvate)
    • Fermentation (anaerobic process where organic compounds donate and accept electrons)
    • Respiration (aerobic and anaerobic processes, an electron donor is oxidized using an external electron acceptor)
  • Lithotrophs: Use inorganic molecules as electron sources.

• Phototrophs: Obtain energy from light.

• Autotrophs: Use carbon dioxide as their carbon source

• Heterotrophs: Use organic compounds as their carbon source

Electron Transport Reactions

• Redox reactions (electron transfer) are critical for all life.
• Electrons are transferred from a donor (reduced) to an acceptor (oxidized).
• Reduction potential (E°) measures the tendency of a substance to gain electrons.
• A larger difference in reduction potential between electron donor and acceptor = more free energy released

The Redox Tower

• The redox tower lists different redox couples in order of increasing reduction potentials.
• Strongest electron donors are at the top of the negative end (more likely to lose electrons and be oxidized)
• Strongest electron acceptors are at the bottom of the positive end (more likely to gain electrons and be reduced).
• The larger the difference between each pair, the greater the energy released.

Respiration

• Aerobic respiration uses oxygen as a terminal electron acceptor.
• Anaerobic respiration uses other components as terminal electron acceptors (nitrate, sulfate, or fumarate)
• Electron transport chain (ETC) during respiration uses the energy released during electron transport to pump protons (ions) across a membrane. This creates an electrochemical gradient.
• Protons flow through ATP synthase which produce ATP.
• Different terminal electron acceptors result in various forms of anaerobic respiration, and different yields of ATP.

Fermentation

• Anaerobic catabolism in which organic compounds donate and accept electrons.
• Redox balance achieved without external electron acceptors.
• Substrate-level phosphorylation also occurs in fermentation, producing ATP from high-energy molecules.
• Example: lactic acid fermentation, alcoholic fermentation

Chemolithotrophy and Phototrophy

• Chemolithotrophy: use inorganic compounds as a source of energy and electrons
• Phototrophy: use light as energy source.
  • Oxygenic (water oxidzed, oxygen produced)
  • Anoxygenic (different electron donor, no oxygen produced).

Nitrogen Fixation

• Bacteria and Archaea perform nitrogen fixation: converting atmospheric nitrogen (N2) to ammonia (NH3).
• The enzyme complex nitrogenase catalyzes this process, requiring ATP and special conditions to avoid oxygen inactivation of the enzyme.
• Azotobacter vinelandii produces slime to protect cells from oxygen.

Calvin Cycle

• A series of biosynthetic reactions used by some photosynthetic and chemolithotrophic organisms to convert carbon dioxide (CO2) into organic compounds.
• The key enzyme is Rubisco, which fixes CO2 to RuBP, forming an unstable 6-carbon molecule.
• The resulting 3-phosphoglycerate molecules are then used for biosynthesis