Progress toward improving ethanol production through decreased glycerol generation in Saccharomyces cerevisiae by metabolic and genetic engineering approaches

Bioethanol, a prominent biofuel mostly produced industrially by the yeast Saccharomyces cerevisiae, has been among the main pillars of sustainable development in the transportation sector. However, there exist different factors negatively affecting ethanolic fermentation pathway including i.e., microbial contamination, ethanol stress, and byproducts production (CO₂, biomass, and glycerol). Removal of these barriers are essential to achieve a more efficient and cleaner production of this eco-friendly commodity. Among various solutions, by reducing glycerol production, i.e., through redirecting carbon flux into bioethanol production pathway, yields beyond optimal values could be expected. The present article strives to review and discuss glycerol production in S. cerevisiae including its significance and metabolisms. Subsequently, over two decades of investigation (1997–2018) aimed at improving ethanol production by blocking glycerol production pathway in S. cerevisiae using metabolic engineering approaches have been presented and comprehensively elaborated. Various metabolic engineering strategies put forth to enhance ethanol production at the expense of glycerol production have been inclusively reviewed. More specifically, the effect of manipulation of the genes GPD, GLT, GLN, GDH, DAK, GCY, ADH, PDC, and GAPN invidually or in combination on decreasing glycerol and improving ethanol production have been reviewed. Overall, it could be concluded that glycerol production was hindered by the deletion of the most important genes in glycerol production, i.e., GPD genes, generally resulting in increased ethanol production. However, this strategy is also accompanied with reduced yeast growth rate or stopped growth owing to the crucial roles of glycerol, e.g., osmoregulation and redox balancing. Therefore, other strategies such as expression of foreign genes (Escherichia coli mhpF/Bacillus subtilis GAPN) and/or overexpression of yeast genes (GTL1, GLN1, GDH) should be considered simultaneously to compensate for the unfavorable impacts of GPD manipulations. The findings reviewed and critically discussed herein could shed light on the various aspects of yeast metabolic engineering to improve ethanol production and could be instrumental in directing future research efforts toward a more efficient and eco-friendly production of bioethanol as a cleaner alternative of its fossil-oriented counterpart.