Engineering the xylose metabolism of Saccharomyces cerevisiae for ethanol and single cell protein bioconversion

Xylose isomerase (XI) pathway has been widely employed to enable Saccharomyces cerevisiae to convert xylose and glucose into commercially feasible lignocellulosic ethanol products. Nevertheless, studies about the effect of different promoters to the expression of xylA are lacking. Therefore, five strains with ADH1, GAPDH, PDC1, PGK, TEF1 promoters were constructed. Among them, S. cerevisiae INVSc1/pHM368-P-ADH1-xylA generated with xylA driven by ADH1 promoter displayed the highest xylose utilization rate (approximately 19.98 %) using xylose as the only carbon source. With 4 g/L glucose and 10 g/L xylose as the carbon sources, the xylose utilization rate was 60.04 %. Moreover, the utilization rate increased to 64.04 % with fermentation temperature elevated from 28 degrees C to 30 degrees C and reached 83.09 % with peptone and yeast extract as the nitrogen sources. The ethanol titer reached 1.74 g/L with a yield of 0.38 g/g sugar under this condition. In Comparison with direct fermentation, the single cell protein (SCP) was 1.27-fold higher during aerobic fed-batch fermentation. Furthermore, INVSc1/pHM368-P-ADH1-xylA attains high ethanol productivities and yields by converting glucose and xylose from non-detoxified bagasse hydrolysates as carbon sources. The results extend our understanding of the xylose metabolism of S. cerevisiae and provide a platform for biomass conversion to ethanol and SCP, hence paving the way for the development of a more economical and sustainable approach to co-fermentation performance and capabilities for future engineering.