Modification of butyrate and butanol production in Clostridium acetobutylicum by introduction of a ferredoxin oxidoreductase

Until development of more economical petrochemical processes in the 1950s, Clostridium acetobutylicum fermentation on an industrial scale was used for Acetone-Butanol-Ethanol (ABE) fermentation.  Today, environmental and global availability concerns regarding fossil fuels necessitate the development of alternative renewable fuel sources.  Butanol has better properties as a biofuel than ethanol and can be used as the primary component in fuel mixtures in unmodified gasoline engines and infrastructure.  Butanol and butyrate are also valuable intermediates in chemical synthesis, and increasing the ratio of longer-chain products during ABE fermentation would improve the economic value of this process. 

In this work, we set out to increase butanol yield relative to other products from C. acetobutylicum fermentation by genetically engineering redox balance.  Significant reducing power in the form of NAD(P)H is required for extension of acetyl-CoA into butyryl-CoA, and reducing power in the form of reduced ferredoxin is “wasted” by production of hydrogen gas.  NAD(P)H in C. acetobutylicum is mainly generated by action of ferredoxin-NAD(P) reductases (FNR).  In this work, we introduce additional exogenous FNR to increase NAD(P)H generation at the expense of ferredoxin, and measure the effect on product formation in C. acetobutylicum strains.  In wild-type ATCC824 and high solvent producing buk mutant strains, the ratio of butanol to shorter chain solvents is increased with additional FNR activity.  Similarly, in the non-solventogenic M5 strain, the ratio of butyrate to acetate produced during fermentation is increased.  This strategy may be further refined and will likely be applicable in other ABE organisms.