Journal of Bacteriology, 1999
The pathway of autotrophic CO2 fixation was studied in the phototrophic bacterium Chloroflexus aurantiacus
and in the aerobic thermoacidophilic archaeon Metallosphaera sedula. In both organisms, none of the key
enzymes of the reductive pentose phosphate cycle, the reductive citric acid cycle, and the reductive acetyl
coenzyme A (acetyl-CoA) pathway were detectable. However, cells contained the biotin-dependent acetyl-CoA
carboxylase and propionyl-CoA carboxylase as well as phosphoenolpyruvate carboxylase. The specific enzyme
activities of the carboxylases were high enough to explain the autotrophic growth rate via the 3-hydroxypropionate
cycle. Extracts catalyzed the CO2-, MgATP-, and NADPH-dependent conversion of acetyl-CoA to
3-hydroxypropionate via malonyl-CoA and the conversion of this intermediate to succinate via propionyl-CoA.
The labelled intermediates were detected in vitro with either 14CO2 or [14C]acetyl-CoA as precursor. These
reactions are part of the 3-hydroxypropionate cycle, the autotrophic pathway proposed for C. aurantiacus. The
investigation was extended to the autotrophic archaea Sulfolobus metallicus and Acidianus infernus, which
showed acetyl-CoA and propionyl-CoA carboxylase activities in extracts of autotrophically grown cells. Acetyl-
CoA carboxylase activity is unexpected in archaea since they do not contain fatty acids in their membranes.
These aerobic archaea, as well as C. aurantiacus, were screened for biotin-containing proteins by the avidinperoxidase
test. They contained large amounts of a small biotin-carrying protein, which is most likely part of
the acetyl-CoA and propionyl-CoA carboxylases. Other archaea reported to use one of the other known
autotrophic pathways lacked such small biotin-containing proteins. These findings suggest that the aerobic
autotrophic archaea M. sedula, S. metallicus, and A. infernus use a yet-to-be-defined 3-hydroxypropionate cycle
for their autotrophic growth. Acetyl-CoA carboxylase and propionyl-CoA carboxylase are proposed to be the
main CO2 fixation enzymes, and phosphoenolpyruvate carboxylase may have an anaplerotic function. The
results also provide further support for the occurrence of the 3-hydroxypropionate cycle in C. aurantiacus.
NOTE: the article text supplied here is for educational purposes only.
*Don't have Adobe Reader? Get the latest version
NOTE: Some versions of Adobe Reader have problems with Google Chrome. Either resize the browser to view the paper or enable
the Chrome internal PDF viewer by entering chrome://plugins in your address bar and clicking enable for the Chrome PDF Viewer plugin.