Tuesday, May 1, 2012
Napoleon Ballroom C-D, 3rd fl (Sheraton New Orleans)
Lignocellulosic hydrolysates contain abundant xylose which consists of up to 35% of the total carbohydrate from cellulosic hydrolysates. Thus efficient fermentation of xylose has to be ensured for economically feasible bioethanol production from cellulosic biomass. Extensive research efforts have been made to develop xylose-utilizing strains of S. cerevisiae through metabolic engineering. However, the XR/XDH pathway has a redox imbalance problem because of the different cofactor preference by XR and XDH enzymes. Xylitol formation could be prevented by providing sufficient oxygen, but aeration has prohibitively high cost and also aerobic respiration may reduce ethanol yield significantly by competing with the fermentation pathway. Glycerol production is a way that yeast employ to re-oxidize NADH during anaerobic metabolism, but this occurs at the expense of channeling pentose carbon to glycerol formation and thus lowers ethanol yield. Considering the role of oxygen as an external electron sink, providing appropriate electron acceptors other than oxygen may also alleviate redox imbalance by regenerating NAD+ without sacrificing carbon to respiration. After constructing a recombinant S. cerevisiae strain that can grow on and ferment xylose alone under anaerobic conditions, we investigated the quantitative relationship between NAD+ regeneration and improvement of anaerobic xylose fermentation (e.g. ethanol yield and byproduct formation) by using acetoin as an external electron acceptor. Then, we developed engineered strains capable of fermenting xylose with co-consumption of acetic acid even under strict anaerobic conditions. This in situ detoxification of acetate in cellulosic hydrolyzates via co-consumption with xylose will enable more efficient biofuel production from cellulosic biomass.