Glycerol export and glycerol-3-phosphate dehydrogenase, but not glycerol phosphatase, are rate limiting for glycerol production in Saccharomyces cerevisiae.
Glycerol, one of the most important by-products of alcoholic fermentation, has positive effects on the sensory properties of fermented beverages. It was recently shown that the most direct approach for increasing glycerol formation is to overexpress GPD1, which encodes the glycerol-3-phosphate dehydrogenase (GPDH) isoform Gpd1p. We aimed to identify other steps in glycerol synthesis or transport that limit glycerol flux during glucose fermentation. We showed that the overexpression of GPD2, encoding the other isoform of glycerol-3-phosphate dehydrogenase (Gpd2p), is equally as effective as the overexpression of GPD1 in increasing glycerol production (3.3-fold increase compared to the wild-type strain) and has similar effects on yeast metabolism. In contrast, overexpression of GPP1, encoding glycerol 3-phosphatase (Gpp1p), did not enhance glycerol production. Strains that simultaneously overexpress GPD1 and GPP1 did not produce higher amounts of glycerol than a GPD1-overexpressing strain. These results demonstrate that GPDH, but not the glycerol 3-phosphatase, is rate-limiting for glycerol production. The channel protein Fps1p mediates glycerol export. It has recently been shown that mutants lacking a region in the N-terminal domain of Fps1p constitutively release glycerol. We showed that cells producing truncated Fps1p constructs during glucose fermentation compensate for glycerol loss by increasing glycerol production. Interestingly, the strain with a deregulated Fps1 glycerol channel had a different phenotype to the strain overexpressing GPD genes and showed poor growth during fermentation. Overexpression of GPD1 in this strain increased the amount of glycerol produced but led to a pronounced growth defect.