Microbial Metabolism

• Chemical elements predominant in the cell

  • C (50% of cell's dry weight)
  • O and H (combined 25% of dry weight)
  • N (13%)
  • P, S, K, Mg, and Se (less than 5% of dry weight)
• Types of nutrients required by microbes

  • Macronutrients (required in large amounts)
  • Micronutrients (required in minute amounts, e.g. trace elements, vitamins, iron)
• Transporting nutrients into the cell

  1. Active transport driven by ATP or proton motive force

  2. Three classes of transport systems: simple, group translocation, and ABC systems

  3. Simple transport reactions driven by proton motive force

  4. Symport and antiport reactions

  5. Group translocation involves chemical modification of transported substance and energy-rich organic compound driving transport

  6. ABC transport systems use ATP-binding cassette proteins
• Energy classes of microorganisms

  • Chemotrophs (conserve energy from chemicals)
  • Chemoorganotrophs (use organic chemicals as electron donors)
  • Chemolithotrophs (use inorganic chemicals)
  • Phototrophic organisms (convert light energy to chemical energy)
  • Heterotrophs (obtain cell carbon from organic compounds)
  • Autotrophs (use CO2 as carbon source)
• Calvin Cycle


  Major biochemical pathway by which phototrophic organisms incorporate CO2 into cell material
• Enzymes


  • Protein catalysts that increase rate of biochemical reactions by activating substrates
  • Highly specific in the reactions they catalyze, based on 3D structure of polypeptides
• Redox


  • Chemical reactions accompanied by changes in energy, expressed as ∆G0
  • Require electron donors and acceptors, with reduction potential (E0') expressing tendency to accept/release electrons
  • Often employ redox coenzymes like NAD+/NADH as electron shuttles
  • Energy released conserved in compounds with energy-rich phosphate or sulfur bonds
• ATP


  Prime energy carrier in the cell, consisting of adenosine with three bonded phosphate molecules
• Catabolism: Fermentation and Respiration

  1. Glycolysis is universal pathway for catabolism of glucose, used by fermentative anaerobes for energy conservation

  2. Citric acid cycle generates CO2 and electrons for electron transport chain in respiration

  3. Glyoxylate cycle necessary for catabolism of two-carbon electron donors

  4. Electron transport chains composed of membrane-associated redox proteins arranged by increasing E0' values, function to carry electrons from primary donor to terminal acceptor O2
• Biosyntheses


  1. Polysaccharides biosynthesized from activated monomers

  2. Gluconeogenesis produces glucose from nonsugar precursors

  3. Amino acids formed from carbon skeletons with added ammonia

  4. Nucleotides biosynthesized using carbon skeletons from various sources

  5. Fatty acids synthesized from malonyl-ACP precursor, attached to glycerol to form lipids