A 3-Hydroxypropionate/4-Hydroxybutyrate Autotrophic Carbon Dioxide Assimilation Pathway in Archaea

The assimilation of carbon dioxide (CO 2 ) into organic material is quantitatively the most important biosynthetic process. We discovered that an autotrophic member of the archaeal order Sulfolobales, Metallosphaera sedula , fixed CO 2 with acetyl–coenzyme A (acetyl-CoA)/propionyl-CoA carboxylase as the key carboxylating enzyme. In this system, one acetyl-CoA and two bicarbonate molecules were reductively converted via 3-hydroxypropionate to succinyl-CoA. This intermediate was reduced to 4-hydroxybutyrate and converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase. The key genes of this pathway were found not only in Metallosphaera but also in Sulfolobus, Archaeoglobus , and Cenarchaeum species. Moreover, the Global Ocean Sampling database contains half as many 4-hydroxybutyryl-CoA dehydratase sequences as compared with those found for another key photosynthetic CO 2 -fixing enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase. This indicates the importance of this enzyme in global carbon cycling.