Photocatalytic CO₂ Reduction Using CO₂-Binding Enzymes.

Area of Science:

• Biocatalysis
• Green Chemistry
• Enzyme Engineering

Background:

• Achieving a circular carbon economy necessitates novel carbon dioxide (CO2) utilization strategies.
• Efficient catalytic systems, particularly in aqueous solutions, are crucial for CO2 conversion.
• Existing catalytic systems for CO2 reduction in water are limited.

Purpose of the Study:

• To develop a general strategy for identifying enzymes capable of CO2 reduction.
• To engineer enzymes for enhanced performance in aqueous photocatalytic CO2 reduction.
• To investigate the mechanism of CO2 reduction by enzymes.

Main Methods:

• Structural analysis to identify potential CO2 binding sites in enzymes.
• Site-directed mutagenesis to engineer enzyme variants.
• Aqueous photocatalytic CO2 reduction assays using ruthenium photosensitizers and sodium ascorbate.
• Kinetic studies to determine electron transfer mechanisms.

Main Results:

• Phenolic acid decarboxylase from Bacillus subtilis (BsPAD) was identified as an enzyme capable of aqueous CO2 reduction to carbon monoxide (CO).
• Engineered BsPAD variants achieved high turnover numbers (TONs) up to 978 and selectivities up to 93% for CO over H2.
• Mutations in the active site improved CO generation, revealing that electron transfer is rate-limiting and occurs via multistep tunneling.
• The approach was validated with eight other enzymes, demonstrating broad applicability for photocatalytic CO2 reduction.

Conclusions:

• A generalizable strategy for enzyme discovery and engineering for CO2 reduction has been established.
• Enzymes can be effectively engineered to catalyze aqueous CO2 reduction to CO with high efficiency and selectivity.
• This work provides a foundation for utilizing biocatalysis in circular carbon economy initiatives.