Bacterial CO2 Fixation

  1. What are the CO2 fixation pathways?
    • Calvin cycle
    • reverse citric acid cycle
    • hydroxypropionate cycle
    • reductive acetyl-coA pathway
  2. discovery of the Calvin cycle
    • use radiolabeled 14CO2 and lollipop device to follow incorporation of radiolabel into specific compounds
    • contained suspension of algae, queched organisms by dropping into alcohol
    • recent discovery of paper chromatography allowed them to produce audioradiograms showing labeled compounds, tried to identify spots by looking at properties
  3. the Calvin/CBB cycle
    • Steps:
    • 1. carboxylation
    • 2. reduction
    • 3. regeneration

    • Requires:
    • 12 NAD(P)H
    • 18 ATP

    • Unique enzymes:
    • rubisco
    • PRK

    1 CO2 entry point
  4. Rubisco
    • performs carboxylation and oxygenation
    • carboxylation 3x faster than oxygenation
    • but, so much more oxygen in the atmosphere -> get considerable photorespiration
    • as T increases, oxygenase activity is more favored kinetically and aqueous solubility of CO2 is decreased more than O2
  5. regulation of the Calvin/CBB cycle
    • expression tightly coordinated and regulated, controlled by CO2, light and T
    • - CO2 readily dissolves through membranes, concentrated by carboxysomes which are so proteinaceous CO2 can't dissolve through like other membranes and biochemical concentrating mechanisms

    modulation of enzyme activity also key to regulation
  6. CO2 concentrating mechanisms
    • inducible high affinity import of CO2 and HCO3-
    • accumulation of HCO3, diffuses into carboxysome
    • converted to CO2 by carbonic anhydrase
    • CO2 can't diffuse out of carboxysome
    • CO2 used by rubisco, O2 not as readily available
  7. reverse citric acid cycle
    5 ATP and reduced ferredoxin from light

    • Unique enzymes:
    • ATP citrate lyase
    • oxoglutarate FD oxidoreductase
    • fumarate reductase

    multiple CO2 entry points
  8. hydroxypropionate pathway
    • green nonsulfur phototrophic bacteria and several hyperthermophilic, nonphototrophic archae
    • -nature's first attempt at autotrophy?
    • limited distribution, enzymes not that unique


  9. acetyl-CoA pathway
    not a cycle - linear

    no net ATP required

    • key enzyme:
    • CO dehydrogenase
Card Set
Bacterial CO2 Fixation
general microbiology midterm2