Lecture Outline

Bacterial Metabolism

1. Fueling reaction products needed by all organism irrespective of their growth requirements:

  • reducing power (source of electrons/protons)- electrons/protons can not exist alone but must be associated with certain carrier molecules like NAD+, NADP+,FAD+. See Fig. 4.8 p. 114
  • Metabolic energy- in the form of chemical (i.e., ATP or PEP) and/or electrochemical (proton motive force) energy. Fig. 4.10 p. 116
  • 12 essential precursor molecules- it is from these molecules that a larger array of building blocks are made which are used to make the macromolecules of the cell. Why must these precursors be made by cells versus being provided in the medium?
  • C-1 units.

2. Types of metabolism carried out by chemoorganotrophic heterotrophs (E.coli a facultative anaerobe).

  • grown anaerobically:
    • absence of an external electron acceptor- fermentation.
    • presence of an external electron acceptor (N03- in the specific case of E. coli)- anaerobic respiration.
  • grown aerobically: aerobic respiration.

3. Aerobic Respiration.

  • Central pathways of metabolism:
    • Glycolytic pathway (Embden-Meyerhof (Parnas) Pathway). Fig 4.12 p. 119
      • starting substrate and final product (glucose, pyruvic acid).
      • fueling reaction products made (ATP, NADH, and 6 of 12 essential precursor molecules).
      • chemical energy generated by substrate level phosphorylation (SLP).
      • pathway used to metabolize other hexoses and trioses.
    • TCA Cycle (Krebs Citric Acid Cycle). Fig. 4.20 p. 127
      • Starting substrate and final products (acetylCoA, C02/Oxalacetate).
      • fueling reaction products made (ATP by SLP, NADH and FADH, 3 of 12 essential precursor molecules.
    • Oxidative Pentose Phosphate Pathway (cycle)-
      • starting substrate (glucose, pentoses and/or tetroses).
      • fueling reaction products (NADPH, C02, glyceraldehyde-3-phosphate, and 2 of 12 essential precursor molecules).
    • Where is the final essential precursor made?
  • re-oxidation of NADH/FADH: For continued metabolism of glucose need to re-oxidize the reduced forms of NADH and FADH generated by metabolism.
    • overview of oxidation reduction reactions, reduction potential of chemicals and transfer of electrons in biological systems (the electron tower fig 4.7 p. 113).
    • electron transport chain-
      • carriers.
      • terminal electron acceptor (02).
      • establishment of electrochemical gradient across cytoplasmic membrane. Fig. 4.18 p. 125
      • generation of ATP (chemiosmotic hypothesis; oxidative phosphorylation). fig. 4.19 p. 126

4. Anaerobic respiration.

  • Central pathways of metabolism similar, but because cells are growing anaerobically certain enzyme of the TCA are not made and this compromises the amount of reducing power generated.
  • re-oxidation of NADH/FADH:
    • Electron transport chain (ETC)-
      • carriers similar.
      • terminal electron acceptor - nitrate (N03- ) in case of E. coli.. In the case of other bacteria that carry out anaerobic respiration other inorganic molecules are used (Fe+3, S04-2, C03-2, Fig 4.22 p. 129). In case of E. coli product, nitrite, is further reduced to ammonia which bacterium can use as its inorganic nitrogen source. Under aerobic conditions nitrite can not be used by E. coli..
      • less ATP made because of O-R potential of N03- compared to 02.

5. Fermentation.

  • Central pathways of metabolism similar to those carried out under anaerobic respiration.
  • re-oxidation of NADH: accomplished by an internally balanced O-R reaction(s) using an organic molecule as the terminal electron acceptor.
    • examples- the lactic acid bacteria and alcohol fermentation (yeast).
  • Energy: All energy, ATP, generated by SLP (no ETC).
  • Products are species specific and can be used to identify bacteria.

6. Glyoxylate Cycle (Fig 15.61 p. 629, an anapleurotic pathway).

  • Anapleurotic reactions/pathways: an enzymatic reaction or set of chemical reactions that link metabolic pathways which allow bypass of certain parts of the pathway or allow the reversal of carbon flow.
  • Example: the glyxolate cycle which allows by-pass of part of the TCA cycle. Why is this pathway essential to the metabolism of acetate?
    • key enzymes are isocitrate lyase an malate synthase.

7. Fueling Reactions of Chemolithotrophic Autotrophs.

  • Hydrogen oxidizing bacteria. Fig 15.25 p. 595
  • nitrifying bacteria (Nitrosomas, oxidizes ammonia to nitrite, and Nitrobacter, oxidizes nitrite to nitrate). Fig. 13.10 p. 462. How is reducing power generated?

8. Assimilation of inorganic nitrogen (Fig 4.26 p. 132).

  • Two mechanisms:
    • glutamic dehydrogenase (GH).
    • L-gluamine synthetase (GS) and glutamate synthetase (GOGAT enzyme).
  • Why is it advantages for E. coli to have two systems?

9. Synthesis of other nitrogen-containing compounds (transamination reactions).