Autotrophic Archaea of the family Sulfolobaceae (Cre-narchaeota) use a modified 3-hydroxypropionate cycle for carbon dioxide assimilation. In this cycle the ATP-depend-ent carboxylations of acetyl-CoA and propionyl-CoA to malonyl-CoA and methylmalonyl-CoA,respectively,rep-resent the key CO2fixation reactions. These reactions were studied in the thermophilic and acidophilicMetallosphaera sedula and are shown to be catalyzed by one single large enzyme,which acts equally well on acetyl-CoA and pro-pionyl-CoA
