Sunday, April 29, 2007

Enzymatic Synthesis of Methanol from CO2 with in situ Cofactor Regeneration

Ping Wang, B. El-Zahab, and F. Suhan Baskaya. Dept. of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, MN 55108

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 polymeric particles (500 nm in diameter), which can be easily recovered and reused, effectively mediated multistep reactions catalyzed by enzymes immobilized in the same way.   This was demonstrated for the production of methanol from CO2, a gasification product of biomass.  Enzymes including formate, formaldehyde, alcohol and glutamate dehydrogenases were coimmobilized onto the same particles as that used for cofactor immobilization.  Reactions were performed by bubbling CO2 in an aqueous solution containing both enzyme- and cofactor-carrying particles.  It appeared that the collision among the particles afforded adequate interactions between the cofactor and enzymes, and thus enabled the sequential reactions for methanol production and the NAD+/NADH regeneration cycle. For a 30-min batch reaction, a productivity of 0.02 μ-mol methanol/h/g-enzyme was achieved.  That was lower than but comparable to the 0.04 μ-mol methanol/h/g-enzyme observed for free enzymes and cofactor at the same reaction conditions.  The immobilized enzymes and cofactor showed fairly good stabilities in reusing.  Over 80% of their original productivity was retained after 11 reusing cycles.  A cumulative methanol yield based on the amount of cofactor applied reached 127% after 11 reusing cycles.  That was a promising enhancement in cofactor utilization as compared to the single-batch yield of 12% observed with free enzymes and cofactor.