Anomeric specificity of glucose uptake systems in Lactococcus cremoris, Escherichia coli, and Saccharomyces cerevisiae: mechanism, kinetics, and implications.

The mechanism and kinetics of the glucose uptake systems of three representative microorganisms are studied during cultivation in a chemostat. The three microorganisms are Lactococcus cremoris, Escherichia coli, and Saccharomyces cerevisiae. Two models describing respectively competitive and independent uptake of the two glucose anomers are tested on experimental data where alpha- and beta-glucose are determined by flow injection analysis after pulse addition of the pure anomers to a chemostat. The very accurate experimental results are used to give a convincingly clear model discrimination for all three microorganisms. The uptake of glucose by S. cerevisiae occurs by a competitive mechanism with preference for alpha-glucose (K(alpha) = 32 mg/L and K(beta) = 48 mg/L). Surprisingly, the glucose uptake by the two bacteria is shown to be mediated by anomer specific transport systems with no competitive inhibition from the other glucose anomer. This novel finding has not been described in the literature on the phosphotransferase system. In L. cremoris the relative uptake rates of the glucose anomers match the equilibrium composition exactly (36% alpha-glucose). In E. coli the relative uptake rate of alpha-glucose at glucose unlimited growth is 26%, which means preference for beta-glucose. However, the saturation constants of the two sites in E. coli are K(alpha) = 2 mg/L and K(alpha) = 15 mg/L, and a preference for alpha-glucose is exhibited at very low glucose concentrations. The results are of considerable importance in relation to enzyme based on-line measurements during fermentations as well as to the modeling of glucose limited growth and product formation.