Multiphase Bienzyme Reactors Driven by CO2-Containing Bubbles for Catalytic Carbon Fixation

Enzymatic carbon fixation yielding an organic compound is an environmentally benign approach if gaseous CO₂ can be used as a carbon source. In the present work, phosphoenolpyruvate carboxylase (PEPC) was immobilized in polymeric porous beads for the catalytic carboxylation of 3.1–20 mM phosphoenolpyruvate (PEP), yielding oxaloacetate (OAA) at 25 °C and pH = 7.5. The reactor used was an external loop airlift bubble column (ELBC), which was driven by 10 vol % CO₂ bubbles. The maximum carboxylation rate of 262 μM·min^–1 was obtained with immobilized PEPC ([PEPC] = 80 μg·mL^–1). The reaction was accelerated by the addition of bovine carbonic anhydrase (BCA) in its immobilized form through, catalyzing the hydration of CO₂, yielding HCO₃^–. The column height was the critical geometrical characteristic of the reactor affecting the CO₂ transfer. The immobilized enzyme beads were storable, recoverable from a reaction mixture, and reusable practically without a loss of activity for 180 min. Co-immobilized PEPC and BCA beads did not improve the carboxylation efficiency, probably because the spatial arrangement of the two enzymes was inappropriate for their catalytic synergy. The results obtained demonstrated that a stable operation of enzymatic carbon fixation was possible using a single multiphase reactor driven by CO₂-containing bubbles.