Besides d-glucose, d-xylose is the second most abundant sugar present in lignocellulosic biomass that is regarded as a renewable feedstock for biotechnological production of fuels and chemicals. Simultaneous consumption of both sugars—a prerequisite for economically feasible bioconversions—has primarily been prevented by one impediment: all known d-xylose transporters are competitively inhibited by d-glucose. Using a growth-based screening platform, we could identify two positions in yeast hexose transporters Hxt7 and Gal2 that can be mutated to yield glucose-insensitive xylose transporters. One of the positions is conserved among the hexose transporter family members, which opens the possibility to transfer our findings to other biotechnologically relevant organisms. This work will also significantly contribute to the understanding of sugar-transport mechanisms.
All known d-xylose transporters are competitively inhibited by d-glucose, which is one of the major reasons hampering simultaneous fermentation of d-glucose and d-xylose, two primary sugars present in lignocellulosic biomass. We have set up a yeast growth-based screening system for mutant d-xylose transporters that are insensitive to the presence of d-glucose. All of the identified variants had a mutation at either a conserved asparagine residue in transmembrane helix 8 or a threonine residue in transmembrane helix 5. According to a homology model of the yeast hexose transporter Gal2 deduced from the crystal structure of the d-xylose transporter XylE from Escherichia coli, both residues are found in the same region of the protein and are positioned slightly to the extracellular side of the central sugar-binding pocket. Therefore, it is likely that alterations sterically prevent d-glucose but not d-xylose from entering the pocket. In contrast, changing amino acids that are supposed to directly interact with the C6 hydroxymethyl group of d-glucose negatively affected transport of both d-glucose and d-xylose. Determination of kinetic properties of the mutant transporters revealed that Gal2-N376F had the highest affinity for d-xylose, along with a moderate transport velocity, and had completely lost the ability to transport hexoses. These transporter versions should prove valuable for glucose–xylose cofermentation in lignocellulosic hydrolysates by Saccharomyces cerevisiae and other biotechnologically relevant organisms. Moreover, our data contribute to the mechanistic understanding of sugar transport because the decisive role of the conserved asparagine residue for determining sugar specificity has not been recognized before.
Source: Proceedings of the National Academy of Sciences of the USA (PNAS), press release, 2014-03-19.
Author: Alexander Farwick, Stefan Bruder, Virginia Schadeweg, Mislav Oreb, and Eckhard Boles