Particle-tethered NADH for production of methanol from CO₂ catalyzed by coimmobilized enzymes

Efficient cofactor regeneration and reuse are highly desired for many important biotransformation applications. Here we show for the first time that cofactor NAD(H) covalently attached to micro particles, which can be easily recovered and reused, effectively mediated multistep reactions catalyzed by enzymes that were also immobilized with the micro particles. Such an immobilized enzyme-cofactor catalytic system was examined for the production of methanol from CO₂ with in situ cofactor regeneration. Four enzymes including formate, formaldehyde, alcohol, and glutamate dehydrogenases were coimmobilized using the same particles as that used for cofactor immobilization (enzymes and cofactor were immobilized separately). Reactions were performed by bubbling CO₂ in a suspension solution of the particle-attached enzymes and cofactor. It appeared that the collision among the particles afforded sufficient interactions between the cofactor and enzymes, and thus enabled the sequential transformation of CO₂ to methanol along with cofactor regeneration. For a 30-min batch reaction, a productivity of 0.02 μmol methanol/h/g-enzyme was achieved. That was lower than but comparable to the 0.04 μmol methanol/h/g-enzyme observed for free enzymes and cofactor at the same reaction conditions. The immobilized system showed fairly good stabilities in reusing. Over 80% of their original productivity was retained after 11 reusing cycles, with a cumulative methanol yield based on the amount of cofactor reached 127%. That was a promising enhancement in cofactor utilization as compared to the single-batch yield of 12% observed with free enzymes and free cofactor.