Carbon Fixation in Phototrophs

  1. Photoautotrophy vs photoheterotrophy
    1. Carbon Fixation can occur in heterotrophic organisms, and many carboxylation reactions are known that are not the basis for autotrophy.
    2. The hallmark of autotrophy is the ability to live with CO2 as the only carbon source.
    3. Photoheterotrophy in Heliobacteria
    4. Facultative photoheterotrophy in other taxa
  2. Calvin Cycle
    1. Used by cyanobacteria and proteobacteria (including lithoautotrophs).
    2. Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase)
      1. Most of the calvin cycle is present in many organisms; rubisco is one of the defining enzymes of the calvin cycle
      2. Photorespiration
        1. oxygen competes with CO2 at active site
      3. Enzyme cannot self assemble. GroEL (AKA "HSP60" and "rubisco binding protein") acts as a chaperonin
      4. Rubisco enzyme activity is also affected by carbamylation (activation)
      5. Form I rubisco
        1. L8S8
        2. 550 kd in cyanobacteria, large subunit is about 55 kd, small subunit about 14 kd
        3. Arranged (L2)4(S4)2 Relatively low oxygen sensitivity chan
        4. Relatively slow carbon fixation
        5. Tabita's classification of form I rubiscos
          1. Ia - Proteobacteria, some cyanobacteria
          2. Ib - Cyanobacteria and plastids of glaucocystophytes and greens
          3. Ic - Proteobacteria
          4. Id - Plastids of red algae & their secondary kin
      6. Form II rubisco
      7. Cyanobacteria only have form I rubisco
      8. Proteobacteria may have form I, form II, or both (Hydrogenovibrio has two form I genes, and a form II gene)
      9. Archaeal rubisco
        1. Rubisco genes have been identified in at least two archaeal genomes (Archaeoglobus and Methanococcus) by genome sequencing projects.
        2. Based on sequence analysis, this rubisco is highly divergent from both bacterial forms of the enzyme
        3. Rubisco activity had been reported from archaea prior to the genome work, but these reports were not widely heeded.
    3. Phosphoribulokinase is another critical enzyme, unique to the calvin cycle
    4. Despite the presence of rubisco, the complete calvin cycle does not seem to be present in archaea
  3. Reductive (reverse) TCA pathway
    1. Tricarboxylic acid cycle, also known as citric acid cycle or Krebs cycle
    2. Many bacteria have the tricarboxylic acid cycle as an oxidative mechanism
    3. Anaerobes can also use essentially the same reactions as a reductive mechanism
    4. Carbon fixation pathway for green sulfur bacteria (Chlorobium)
  4. Reductive Acetyl-CoA pathway
    1. Carbon fixation in green nonsulfur bacteria (flexibacteria) remains controversial.
    2. Green nonsulfur bacteria often live photoheterotrophically in nature, but are capable of autotrophy
    3. Apparently use of a cyclic reductive acetyl-CoA pathway.
      1. Acetyl-CoA is carboxylated to malonyl-CoA, which is subsequently converted to 3-Hydroxypropionate
      2. Succinate and 3-Hydroxypropionate are excreted by Chloroflexus during late phase autotrophic growth.
      3. Net product is glyoxylate
  5. Other carbon fixation pathways
  6. Significance of carbon fixation pathyway
    1. Contribution to global carbon cycle
    2. Isotope fractionation
      1. Calvin cycle discriminates against 13C moderately strongly (delta 13C ~ -26 ppt)
      2. Reductive TCA discriminates much less strongly than Calvin cycle (delta 13C ~ -10 ppt)
      3. Reductive acetyl-CoA discriminates even more strongly than Calvin cycle (delta 13C ~ -40 ppt)
      4. This complicates interpretation of isotope ratios in paleontology and photosynthesis research.
      5. Isotope fractionation can be an economically important application of photosynthetic microorganisms

Required Reading: (M&C pp. 104-120); browse chapters 6 and 8.

Supplementary Reading:

Fuchs, G. 1989. Alternative pathways of CO2 fixation. In H.G. Schlegel and B. Bowien. Autotrophic Bacteria, Science & Technology Press.

Sirevag, R. 1995. Carbon metabolism in green bacteria. Pp 871-883 in R.E. Blankenship, M.T. Madigan, and C.E. Bauer (eds): Anoxygenic Photosynthetic Bacteria. Kluwer, Amsterdam.

Tabita, F.R. 1995. The biochemistry and metabolic regulation of carbon metabolism and CO2 fixation in purple bacteria. Pp 885-914 in R.E. Blankenship, M.T. Madigan, and C.E. Bauer (eds): Anoxygenic Photosynthetic Bacteria. Kluwer, Amsterdam.

[Note: Anoxygenic Photosynthetic Bacteria was requested for library reserve, but has not yet been received].