ENERGETICS AND KINETICS OF MALTOSE TRANSPORT IN SACCHAROMYCES-CEREVISIAE - A CONTINUOUS-CULTURE STUDY

In Saccharomyces cerevisiae, maltose is transported by a proton sympor t mechanism, whereas glucose transport occurs via facilitated diffusio n. The energy requirement for maltose transport was evaluated with a m etabolic model based on an experimental value of Y(ATP) for growth on glucose and an ATP requirement for maltose transport of 1 mol . mol-1. The predictions of the model were verified experimentally with anaero bic, sugar-limited chemostat cultures growing on a range of maltose-gl ucose mixtures at a fixed dilution rate of 0.1 h-1. The biomass yield (grams of cells . gram of sugar-1) decreased linearly with increasing amounts of maltose in the mixture. The yield was 25% lower during grow th on maltose than during that on glucose, in agreement with the model predictions. During sugar-limited growth, the residual concentrations of maltose and glucose in the culture increased in proportion to thei r relative concentrations in the medium feed. From the residual maltos e concentration, the in situ rates of maltose consumption by cultures, and the K(m) of the maltose carrier for maltose, it was calculated th at the amount of this carrier was proportional to the in situ maltose consumption rate. This was also found for the amount of intracellular maltase. These two maltose-specific enzymes therefore exert high contr ol over the maltose flux in S. cerevisiae in anaerobic, sugar-limited, steady-state cultures.