Effects of Furfural on the Respiratory Metabolism of Saccharomyces cerevisiae in Glucose-Limited Chemostats

Abstract

Effects of furfural on the aerobic metabolism of the yeast Saccharomyces cerevisiae were studied by performing chemostat experiments, and the kinetics of furfural conversion was analyzed by performing dynamic experiments. Furfural, an important inhibitor present in lignocellulosic hydrolysates, was shown to have an inhibitory effect on yeast cells growing respiratively which was much greater than the inhibitory effect previously observed for anaerobically growing yeast cells. The residual furfural concentration in the bioreactor was close to zero at all steady states obtained, and it was found that furfural was exclusively converted to furoic acid during respiratory growth. A metabolic flux analysis showed that furfural affected fluxes involved in energy metabolism. There was a 50% increase in the specific respiratory activity at the highest steady-state furfural conversion rate. Higher furfural conversion rates, obtained during pulse additions of furfural, resulted in respirofermentative metabolism, a decrease in the biomass yield, and formation of furfuryl alcohol in addition to furoic acid. Under anaerobic conditions, reduction of furfural partially replaced glycerol formation as a way to regenerate NAD ؉ . At concentrations above the inlet concentration of furfural, which resulted in complete replacement of glycerol formation by furfuryl alcohol production, washout occurred. Similarly, when the maximum rate of oxidative conversion of furfural to furoic acid was exceeded aerobically, washout occurred. Thus, during both aerobic growth and anaerobic growth, the ability to tolerate furfural appears to be directly coupled to the ability to convert furfural to less inhibitory compounds.An important problem in fermentative conversion of lignocellulose to ethanol is the severe inhibitory effects often exerted by lignocellulosic hydrolysates (17,21). Furfural is a characteristic compound present in dilute acid hydrolysates, particularly in hydrolysates from deciduous woods, in which the furfural concentration can be about 2 to 3 g/liter (37). Furfural has been found to severely inhibit metabolism in the yeast Saccharomyces cerevisiae under anaerobic conditions both during batch cultivation (5,7,27) and in glucose-limited chemostats (9,12,35). Furfural has been reported to have inhibitory effects on the specific growth rate, as well as on the fermentation rate of yeasts (4, 28). However, only few studies have described the effects of furfural under aerobic conditions. These effects are important, since inhibition caused by furfural during respiratory growth has a great impact on yeast propagation in an ethanol production plant based on a lignocellulosic feedstock. Taherzadeh et al. (39) compared the levels of conversion of furfural in anaerobic and aerobic batch cultures of S. cerevisiae growing on glucose. It was found that furfural was mainly converted to furfuryl alcohol by exponentially growing cells under both conditions and that the specific conversion rate was 0.6 g/g ⅐ h. Almost the same value for the maximum...

Inhibitor tolerance and flocculation of a yeast strain suitable for second generation bioethanol production

“…The physiology of CBS8066 has been studied extensively (see e.g. Verduyn et al 1990;Sárvári Horváth et al 2003;Talebnia and Taherzadeh, 2007) making it a good reference strain.…”

Flocculation Causes Inhibitor Tolerance in Saccharomyces cerevisiae for Second-Generation Bioethanol Production

“…The glycerol yields were significantly lower in the two furfural-containing media. Furfural is known to act as a redox sink and can replace glycerol for reoxidation of excess NADH formed in, e.g., biosynthetic reactions or due to acid formation (47).…”

Bioconversion of lignocellulose: inhibitors and detoxification

“…Furfural and HMF inhibit the growth of yeast and decrease ethanol yield and productivity [48,55,56]. Under anaerobic conditions, S. cerevisiae can convert furfural to furfuryl alcohol [57,58] and HMF to 2,5-bis-hydroxymethylfuran [59]. Reduction of furfural has been linked to the co-factor NADH, while reduction of HMF has been found to be associated with consumption of NADPH [60].…”