CO2(aq) concentration–dependent CO2 fixation via carboxylation by decarboxylase

Abstract

Enzymatic carbon fixation is one of the most interesting processes in CO₂ sequestration. A number of decarboxylases can catalyze the reversible decarboxylation reaction in vivo, while can strengthen the carboxylation reaction in vitro. The Bio-Kolbe–Schmitt reaction is the carboxylation of phenols and CO₂ by hydroxybenzoic acid decarboxylases to generate hydroxybenzoic acids under a mild condition, which requires an alkaline environment. It remains unclear how the carbon fixation via carboxylation can be improved. In this study, the major issues regarding the switching from decarboxylase to carboxylase through thermodynamic calculation and determination of enzyme characteristics, CO₂ concentration, and other related species in the reaction system such as HCO₃⁻ are described. We identified that the concentration of CO₂ in aqueous solution was the main factor for obtaining high CO₂ fixation yield. In order to obtain sufficient CO₂(aq) to promote the carboxylation by decarboxylase under an alkaline environment, a high concentration of HCO₃⁻ is required. The highest CO₂ fixation yield of 30% was achieved from the reaction under 0.2 MPa CO₂ in 2.7 M KHCO₃ buffer for 15 min. Under the optimal reaction conditions, the carboxylation of catechol by 2,3-DHBD_Ao (2,3-dihydroxybenzoic acid decarboxylase from Aspergillus oryzae) achieved a good “turnover frequency” (TOF) of 47 min⁻¹. This study discloses that CO₂(aq) concentration is a crucial parameter for increasing CO₂ fixation yield in a homogenous reaction under an alkaline condition.