d-Glucose Transport System of Zymomonas mobilis

The properties of the d-glucose transport system of Zymomonas mobilis were determined by measuring the uptake of nonmetabolizable analogs (2-deoxy-d-glucose and d-xylose) by wild-type cells and the uptake of d-glucose itself by a mutant lacking glucokinase. d-Glucose was transported by a constitutive, stereospecific, carrier-mediated facilitated diffusion system, whereby its intracellular concentration quickly reached a plateau close to but not above the external concentration. d-Xylose was transported by the d-glucose system, as evidenced by inhibition of its uptake by d-glucose. d-Fructose was not an efficient competitive inhibitor of d-glucose uptake, indicating that it has a low affinity for the d-glucose transport system. The apparent Km of d-glucose transport was in the range of 5 to 15 mM, with a Vmax of 200 to 300 nmol min−1 mg of protein−1. The Km of Z. mobilis glucokinase (0.25 to 0.4 mM) was 1 order of magnitude lower than the Km for d-glucose transport, although the Vmax values for transport and phosphorylation were similar. Thus, glucose transport cannot be expected to be rate limiting at concentrations of extracellular glucose normally used in fermentation processes, which greatly exceed the Km for the transport system. The low-affinity, high-velocity, nonconcentrative system for d-glucose transport described here is consistent with the natural occurrence of Z. mobilis in high-sugar environments and with the capacity of Z. mobilis for rapid conversion of glucose to metabolic products with low energetic yield.