Improved ethanol production by glycerol-3-phosphate dehydrogenase mutants of Saccharomyces cerevisiae.

The anaerobic performance of gpd1 delta and gpd2 delta mutants of Saccharomyces cerevisiae was characterized and compared to that of a wild-type strain under well-controlled conditions by using a high-performance bioreactor. There was a 40% reduction in glycerol level in the gpd2 delta mutant compared to the wild-type. Also the gpd1 delta mutant showed a slight decrease in glycerol formation but to a much lesser degree. As a consequence, ethanol formation in the gpd2 delta mutant was elevated by 13%. In terms of growth, the gpd1 delta mutant and the wild-type were indistinguishable. The gpd2 delta mutant, on the other hand, displayed an extended lag phase as well as a reduced growth rate under the exponential phase. Even though glycerol-3-phosphate dehydrogenase 2 (GPD2) is the important enzyme under anaerobic conditions it can, at least in part, be substituted by GPD1. This was indicated by the higher expression level of GPD1 in the gpd2 delta mutant compared to the wild type. These results also show that the cells are able to cope and maintain redox balance under anaerobic conditions even if glycerol formation is substantially reduced, as observed in the gpd2 delta mutant. One obvious way of solving the redox problem would be to make a biomass containing less protein, since most of the excess NADH originates from amino acid biosynthesis. However, the gpd2 delta mutant did not show any decrease in the protein content of the biomass.