Figure 1

Scheme of yeast S. cerevisiae able to ferment D-glucose and D-xylose . D-glucose and D-xylose are transported into the cell via the expressed hexose transporters Hxt1-7. The expressed Hxt36 chimera in the DS68616 lineage is a result of a deletion of a C-terminal part of the HXT3 coding sequence, the HXT3 terminator, the HXT6 promoter, and an N-terminal part of the HXT6 coding sequence. The metabolism of glucose is mediated via the enzymes hexokinase (HK) and glucose 6P isomerase (Pgi1) to yield fructose-6P, which is further converted in the glycolytic pathway into ethanol. The conversion of xylose into xylulose is possible in S. cerevisiae via the introduction of xylose isomerase (XI), followed by the phosphorylation of xylulose into xylulose-5P by xylulose kinase (Xks1). Xylulose-5P enters the pentose phosphate pathway to eventually yield fructose-6P and glyceraldehyde-3P which can be converted into ethanol. The key enzymes of the pentose phosphate pathway (Tal1, Rpe1, Rki1, and Tki1) were overexpressed for an efficient conversion. Since the endogenous Hxt transporters show a higher affinity for D-glucose compared to D-xylose, in mixed sugar fermentation, the D-glucose is used first before the D-xylose can be metabolized. In the transporter engineered D-xylose-fermenting yeast, the specificity of the Hxt36 transporter is changed by mutagenesis, allowing preferred uptake of D-xylose relative to D-glucose.