Carbon Fixation in Phototrophs
- Photoautotrophy vs photoheterotrophy
- Carbon Fixation can occur in heterotrophic organisms, and many carboxylation
reactions are known that are not the basis for autotrophy.
- The hallmark of autotrophy is the ability to live with CO2
as the only carbon source.
- Photoheterotrophy in Heliobacteria
- Facultative photoheterotrophy in other taxa
- Calvin Cycle
- Used by cyanobacteria and proteobacteria (including lithoautotrophs).
- Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase)
- Most of the calvin cycle is present in many organisms; rubisco is
one of the defining enzymes of the calvin cycle
- oxygen competes with CO2 at active site
- Enzyme cannot self assemble. GroEL (AKA "HSP60" and "rubisco
binding protein") acts as a chaperonin
- Rubisco enzyme activity is also affected by carbamylation (activation)
- Form I rubisco
- 550 kd in cyanobacteria, large subunit is about 55 kd, small
subunit about 14 kd
- Arranged (L2)4(S4)2
Relatively low oxygen sensitivity chan
- Relatively slow carbon fixation
- Tabita's classification of form I rubiscos
- Ia - Proteobacteria, some cyanobacteria
- Ib - Cyanobacteria and plastids of glaucocystophytes and greens
- Ic - Proteobacteria
- Id - Plastids of red algae & their secondary kin
- Form II rubisco
- Cyanobacteria only have form I rubisco
- Proteobacteria may have form I, form II, or both (Hydrogenovibrio
has two form I genes, and a form II gene)
- Archaeal rubisco
- Rubisco genes have been identified in at least two archaeal
genomes (Archaeoglobus and Methanococcus) by genome
- Based on sequence analysis, this rubisco is highly divergent
from both bacterial forms of the enzyme
- Rubisco activity had been reported from archaea prior to the
genome work, but these reports were not widely heeded.
- Phosphoribulokinase is another critical enzyme, unique to the
- Despite the presence of rubisco, the complete calvin cycle does not
seem to be present in archaea
- Reductive (reverse) TCA pathway
- Tricarboxylic acid cycle, also known as citric acid cycle or
- Many bacteria have the tricarboxylic acid cycle as an oxidative mechanism
- Anaerobes can also use essentially the same reactions as a reductive
- Carbon fixation pathway for green sulfur bacteria (Chlorobium)
- Reductive Acetyl-CoA pathway
- Carbon fixation in green nonsulfur bacteria (flexibacteria) remains
- Green nonsulfur bacteria often live photoheterotrophically in nature,
but are capable of autotrophy
- Apparently use of a cyclic reductive acetyl-CoA pathway.
- Acetyl-CoA is carboxylated to malonyl-CoA, which is subsequently
converted to 3-Hydroxypropionate
- Succinate and 3-Hydroxypropionate are excreted by Chloroflexus
during late phase autotrophic growth.
- Net product is glyoxylate
- Other carbon fixation pathways
- Significance of carbon fixation pathyway
- Contribution to global carbon cycle
- Isotope fractionation
- Calvin cycle discriminates against 13C moderately strongly
(delta 13C ~ -26 ppt)
- Reductive TCA discriminates much less strongly than Calvin cycle
(delta 13C ~ -10 ppt)
- Reductive acetyl-CoA discriminates even more strongly than Calvin
cycle (delta 13C ~ -40 ppt)
- This complicates interpretation of isotope ratios in paleontology
and photosynthesis research.
- Isotope fractionation can be an economically important application
of photosynthetic microorganisms
Required Reading: (M&C pp. 104-120); browse chapters 6 and 8.
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.
[Note: Anoxygenic Photosynthetic Bacteria was requested for library
reserve, but has not yet been received].