Utilization of glucose and galactose for production of ethanol by HXK2-deleted Saccharomyces cerevisiae

Galactose is one of the major sugar components of red seaweeds along with glucose. In Saccharomyces cerevisiae, the synthesis of galactose-metabolizing enzymes (the Leloir proteins) is under tight and complex regulation in the presence of glucose by with a mechanism called catabolite repression. As a result, when both sugars are concurrently present in the medium under oxygen-limited conditions, the yeast cannot utilize galactose after glucose consumption. The previous researches on glucose/galactose co-fermentation, therefore, were conducted in presence oxygen, thereby resulting in relatively low ethanol yield from galactose. In this study, oxygen-limited glucose/galactose co-fermentation was firstly achieved by deleting the HXK2 gene encoding a repressor involved in catabolite repression of S. cerevisiae. Remarkably, the S. cerevisiae D452-2Δhxk2 strain consumed galactose after glucose consumption without a diauxic lag period, whereas the parental strain could not use galactose under oxygen-limited conditions. The Δhxk2 strain fermented galactose at the consumption rate of 3.02±0.10 g/L∙h and produced ethanol with yield of 0.44±0.01 g ethanol/g galactose. The galactose consumption rates were inversely related to the mRNA levels of the HXK2 gene. The transcriptional patterns of the GAL genes were dramatically elevated by deleting the HXK2 gene, whereas the expressions of GLK1 and HXK1 genes were not significantly affected. In conclusion, the engineered strain will provide not only a basis for efficient bioethanol production from marine biomass but also pinpointing information for mixed sugar fermentation by S. cerevisiae.