PATHWAY ANALYSIS OF OXYGEN UTILIZATION AND TRICARBOXYLIC-ACID CYCLE ACTIVITY IN SACCHAROMYCES-CEREVISIAE GROWING ON GLUCOSE

A method is proposed to quantitatively analyze the oxygen utilization of Saccharomyces cerevisiae growing on glucose through the three reducing-equivalent-generating routes, i.e., biomass synthesis, TCA cycle (including glycolysis) and product formation. Calculations with literature data showed that, under conditions of purely oxidative and glucose-limited growth, the TCA cycle activity of S. cerevisiae accounts for over 70% and biosynthesis for 30% of the total oxygen consumption or less, independent of the assumed route for the generation of NADPH(2) required for biosynthesis. Under conditions of acetate production (but without ethanol formation) reduced NADH(2) formation via TCA cycle activity was calculated, indicating saturation or depression of the TCA cycle activity. With the onset of ethanol production at high growth rate, product formation can contribute up to 7-9% of the total oxygen consumption and the contribution of biosynthesis appeared to decline. Markedly lower values (30-72%) for NADH(2) formation via TCA cycle were obtained for cultures under transition from glucose limitation to glucose excess. It was found that the presence of glucose in cultures represses the activity of TCA cycle but not the respiratory capacity of S. cerevisiae. Under these conditions biosynthesis and product formation may contribute to the major portion of oxygen consumption. It was also found that the presence of high ethanol concentration in cultures obviously inhibits the activity of the TCA cycle. From these calculations it can be concluded that bottlenecks in the oxidative glucose metabolism of S. cerevisiae occur at the level of the TCA cycle or beyond. This is in accordance with recent experimental results.