Carbon dioxide fixation is the most important biosynthetic process on earth, and was converted to carbon form through a variety of known pathways.In present study, key enzymes of the autotrophic CO2 fixation pathways in autotrophic and thermophilic archaea species were examined.In the 3-Hydroxypropionate/4-Hydroxybutyrate (3-HP/4-HB) cycle of Sulfolobaceae, two carboxylation reactions extend acetyl-Coenzyme A (CoA) by two carbons to form succinyl-CoA.Succinyl-CoA is further reduced and rearranged to form acetoacetyi-CoA, which is cleaved to regenerate the starting substrate and produce an additional molecule of acetyl-CoA.The concerted biochemical capacity of the two enzymes in the 3-HP/4-HB cycle responsible for the conversion of (S)-Methylmalonyl-CoA to Succinyl-CoA was examined, with an eye towards their functional properties in pathways designed for CO2 fixation.