Butanol production by Clostridium acetobutylicum is accompanied by coproduction of acetone and ethanol, which reduces the yield of butanol and increases the production cost. Here, we report development of several clostridial aldehyde/alcohol dehydrogenase (AAD) variants showing increased butanol selectivity by a series of design and analysis procedures, including random mutagenesis, substrate specificity feature analysis, and structure-based butanol selectivity design. The butanol/ethanol ratios (B/E ratios) were dramatically increased to 17.47 and 15.91 g butanol/g ethanol for AAD(F716L) and AAD(N655H), respectively, which are 5.8-fold and 5.3fold higher than the ratios obtained with the wild-type AAD. The much-increased B/E ratio obtained was due to the dramatic reduction in ethanol production (0.59 +/- 0.01 g/liter) that resulted from engineering the substrate binding chamber and the active site of AAD. This protein design strategy can be applied generally for engineering enzymes to alter substrate selectivity.
IMPORTANCE Renewable biofuel represents one of the answers to solving the energy crisis and climate change problems. Butanol produced naturally by clostridia has superior liquid fuel characteristics and thus has the potential to replace gasoline. Due to the lack of efficient genetic manipulation tools, however, clostridial strain improvement has been slower than improvement of other microorganisms. Furthermore, fermentation coproducing various by-products requires costly downstream processing for butanol purification. Here, we report the results of enzyme engineering of aldehyde/alcohol dehydrogenase (AAD) to increase butanol selectivity. A metabolically engineered Clostridium acetobutylicum strain expressing the engineered aldehyde/alcohol dehydrogenase gene was capable of producing butanol at a high level of selectivity.
1.Korea Adv Inst Sci & Technol, Syst Metab Engn & Syst Healthcare SMESH Cross Gen, Dept Chem & Biomol Engn,Metab Engn Natl Res Lab, Ctr Syst & Synthet Biotechnol,Inst BioCentury,Plu, Daejeon, South Korea 2.Korea Food Res Inst, Res Div Food Funct, Wanju Gun, Jeollabuk Do, South Korea 3.Korea Adv Inst Sci & Technol, Dept Bio & Brain Engn, Daejeon, South Korea 4.Korea Adv Inst Sci & Technol, BioProc Engn Res Ctr, Daejeon, South Korea 5.Korea Adv Inst Sci & Technol, BioInformat Res Ctr, Daejeon, South Korea 6.Hanwha Chem Res & Dev Inst, Future Technol R&D Ctr, Daejeon, South Korea 7.Gyeongsang Natl Univ, IALS, Dept Agr Chem & Food Sci Technol, Jinju, Gyeongsangnam D, South Korea