Cofermentation of Glucose, Xylose, and Cellobiose by the Beetle-Associated Yeast Spathaspora passalidarum

Abstract

dFermentation of cellulosic and hemicellulosic sugars from biomass could resolve food-versus-fuel conflicts inherent in the bioconversion of grains. However, the inability to coferment glucose and xylose is a major challenge to the economical use of lignocellulose as a feedstock. Simultaneous cofermentation of glucose, xylose, and cellobiose is problematic for most microbes because glucose represses utilization of the other saccharides. Surprisingly, the ascomycetous, beetle-associated yeast Spathaspora passalidarum, which ferments xylose and cellobiose natively, can also coferment these two sugars in the presence of 30 g/liter glucose. S. passalidarum simultaneously assimilates glucose and xylose aerobically, it simultaneously coferments glucose, cellobiose, and xylose with an ethanol yield of 0.42 g/g, and it has a specific ethanol production rate on xylose more than 3 times that of the corresponding rate on glucose. Moreover, an adapted strain of S. passalidarum produced 39 g/liter ethanol with a yield of 0.37 g/g sugars from a hardwood hydrolysate. Metabolome analysis of S. passalidarum before onset and during the fermentations of glucose and xylose showed that the flux of glycolytic intermediates is significantly higher on xylose than on glucose. The high affinity of its xylose reductase activities for NADH and xylose combined with allosteric activation of glycolysis probably accounts in part for its unusual capacities. These features make S. passalidarum very attractive for studying regulatory mechanisms enabling bioconversion of lignocellulosic materials by yeasts.

Activating and Elucidating Metabolism of Complex Sugars in Yarrowia lipolytica

“…For instance, a higher-level CCR effect causes diauxic growth (5); a milder effect allows simultaneous sugar utilization but often makes the specific uptake rate of one sugar higher than that of others (6). For biotechnological application, it is highly desirable to engineer microorganisms as microbial cell factories that can efficiently convert complex biomass-derived sugars to desirable chemicals with minimal CCR effect (7,8). Fig.…”

Combining C6 and C5 sugar metabolism for enhancing microbial bioconversion

“…Few microorganisms can naturally ferment these mixed sugars sequentially, and even then, only with low yields and productivities [6,7,8]. This sequential utilization of mixed sugars leads to inefficient, incomplete, and slow fermentation of non-glucose sugars, and prevents implementation of a continuous process [9,10,11].…”

Remaining Challenges in the Metabolic Engineering of Yeasts for Biofuels

“…169 This yeast could potentially serve as a host for lignocellulosic ethanol production, because it has been recently shown to be tolerant to toxins and can conferment several components of lignocellulosic derivatives. 170,171 It is unclear whether these yeasts will be better hosts for advanced biofuels production from lignocellulosic biomass over the model organism of S. cerevisiae. The lack of metabolic engineering tools for these yeasts prevents replicating approaches taken in S. cerevisiae to compensate for its shortcomings.…”