MODULATION OF GLYCEROL AND ETHANOL YIELDS DURING ALCOHOLIC FERMENTATION IN SACCHAROMYCES-CEREVISIAE STRAINS OVEREXPRESSED OR DISRUPTED FOR GPD1 ENCODING GLYCEROL 3-PHOSPHATE DEHYDROGENASE

The possibility of the diversion of carbon flux from ethanol towards g lycerol in Saccharomyces cerevisiae during alcoholic fermentation was investigated. Variations in the glycerol 3-phosphate dehydrogenase (GP DH) level and similar trends for alcohol dehydrogenase (ADH), pyruvate decarboxylase and glycerol-3-phosphatase were found when low and high glycerol-forming wine yeast strains were compared. GPDH is thus a lim iting enzyme for glycerol production. Wine yeast strains with modulate d GPD1 (encoding one of the two GPDH isoenzymes) expression were const ructed and characterized during fermentation on glucose-rich medium. E ngineered strains fermented glucose with a strongly modified [glycerol ]:[ethanol] ratio. gpd/Delta mutants exhibited a 50% decrease in glyce rol production and increased ethanol yield. Overexpression of GPD1 on synthetic must (200 g/l glucose) resulted in a substantial increase in glycerol production (x4) at the expense of ethanol. Acetaldehyde accu mulated through the competitive regeneration of NADH via GPDH. Accumul ation of by-products such as pyruvate, acetate, acetoin, 2,3 butane-di ol and succinate was observed, with a marked increase in acetoin produ ction. (C) 1997 by John Wiley & Sons, Ltd.