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Appl Environ Microbiol. 1985 January; 49(1): 151-157
Copyright © 1985, American Society for Microbiology. All Rights Reserved.

D-Glucose Transport System of Zymomonas mobilis

Anthony A. DIMarco and Antonio H. Romano^*

Microbiology Section, The University of Connecticut, Storrs, Connecticut 06268

ABSTRACT

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 K_m of D-glucose transport was in the range of 5 to 15 mM, with a V_max of 200 to 300 nmol min^–1 mg of protein^–1. The K_m of Z. mobilis glucokinase (0.25 to 0.4 mM) was 1 order of magnitude lower than the K_m for D-glucose transport, although the V_max 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 K_m 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.

^* Corresponding author.

Present address: Department of Microbiology, University of Illinois at Urbana-Champaign, Urbana, IL 61801.

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