Effect of carbonic anhydrase on enzymatic conversion of CO₂ to formic acid and optimization of reaction conditions

Enzymatic reduction of CO₂ to formic acid promises the production of high-value chemicals from greenhouse gases in the way of high selectivity and low energy consumption. However, slow hydration process of CO₂ in reaction solution leads to low production rate of formic acid. Carbonic anhydrase (CA), a vigorous biocatalyst for CO₂ hydration, has attracted much attention for its potential applications in CO₂ capture and sequestration in recent years, while its use in biosynthesis of chemicals and fuels based on CO₂ has hardly been reported. Herein we report the feasibility of CA in facilitating biosynthesis of formic acid from CO₂ with the catalysis of formate dehydrogenase (FDH), and various reaction conditions were optimized for the first time. With the addition of CA, the substrate of FDH might transform from CO₂ to more soluble HCO₃^-, together with the increase in the hydration rate of CO₂, resulting in that the production rate of formic acid was increased by a factor of 4.2-fold. Due to the decrease in pH value induced by CA with the reaction proceeded, the ratio of two enzymes was important to coordinate the reaction rate and avoid the accumulation of hydrogen ions, at the same time, buffer system was critical to ensure the synergize effect of CA. It appeared to us that the introduction of CA represents a new bioprocessing strategy for efficient biotansformation of CO₂ to formic acid and its further products.