Stimulated glycolytic flux increases the oxygen uptake rate and aerobic ethanol production, during oxido-reductive growth of Saccharomyces cerevisiae

Abstract

The influence of overproduction of the glycolytic enzyme 3-phosphoglycerate kinase (PGK) in bakers' yeast Saccharomyces cerevisiae YT6 was investigated in glucose-limited aerobic chemostat experiments. Below the critical value of the dilution rate, the switch point of purely oxidative glucose metabolism to oxido-reductive yeast growth ( D crit), the physiology of the PGK-overproducing mutant showed no differences when compared with the wild type YT6. However, at μ values above the D crit, both the respiration and ethanol production of the mutant were increased. Coincidental higher concentrations of end-products of the reductive glucose metabolism (like ethanol, acetate and glycerol) and metabolic intermediates (pyruvate) were found in the culture fluid of the mutant. The increased specific oxygen uptake rate in the PGK overproducer, as a consequence of the enhanced glycolytic flux and accumulation of pyruvate, shows that the respiratory capacity in the wild type strain is not at its maximal level during oxido-reductive growth. However, its ability to react upon a sudden excess of glucose by an upshift in the D or a glucose pulse, can be the limiting factor causing immediate ethanol and acetate production. The high glycolytic rate compared with the respiratory activity may result in the high reductive response. In this view the coupling of anabolic and catabolic activity is very important. During oxido-reductive growth the maximal PGK yield per mol glucose ( Y psm) is low when compared with purely oxidative growth and is even more decreased by an enhanced reduction activity at the higher dilution rates. This in contrast to the biomass yield per mol ATP ( Y ATP), which is only slightly decreased at D crit and is lower for the overproducing mutant in comparison with the wild type strain YT6.