Metabolic engineering strategies for optimizing acetate reduction, ethanol yield and osmotolerance in S accharomyces cerevisiae

Glycerol, whose formation contributes to cellular redox balancing and osmoregulation in , is an important by-product of yeast-based bioethanol production. Replacing the glycerol pathway by an engineered pathway for NAD -dependent acetate reduction has been shown to improve ethanol yields and contribute to detoxification of acetate-containing media. However, the osmosensitivity of glycerol non-producing strains limits their applicability in high-osmolarity industrial processes. This study explores engineering strategies for minimizing glycerol production by acetate-reducing strains, while retaining osmotolerance. encodes one of two isoenzymes of NAD -dependent glycerol-3-phosphate dehydrogenase (G3PDH). Its deletion in an acetate-reducing strain yielded a fourfold lower glycerol production in anaerobic, low-osmolarity cultures but hardly affected glycerol production at high osmolarity. Replacement of both native G3PDHs by an archaeal NADP -preferring enzyme, combined with deletion of , yielded an acetate-reducing strain the phenotype of which resembled that of a glycerol-negative strain in low-osmolarity cultures. This strain grew anaerobically at high osmolarity (1 mol L glucose), while consuming acetate and producing virtually no extracellular glycerol. Its ethanol yield in high-osmolarity cultures was 13% higher than that of an acetate-reducing strain expressing the native glycerol pathway. Deletion of provides an attractive strategy for improving product yields of acetate-reducing strains in low, but not in high-osmolarity media. Replacement of the native yeast G3PDHs by a heterologous NADP -preferring enzyme, combined with deletion of , virtually eliminated glycerol production in high-osmolarity cultures while enabling efficient reduction of acetate to ethanol. After further optimization of growth kinetics, this strategy for uncoupling the roles of glycerol formation in redox homeostasis and osmotolerance can be applicable for improving performance of industrial strains in high-gravity acetate-containing processes.