Stereospecificity of the Glucose Carrier in Sugar Beet Suspension Cells

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Abstract
The stereospecificity of the binding site on the glucose carrier system in sugar beet suspension culture cells was determined using a series of aldo and keto hexose sugars and sugar alcohols. Specificity was determined as competition with [14C]glucose transport and glucose/proton symport. The binding site of the glucose carrier system was specific for the stereo orientation of the 3 equatorial OH groups on the 3 C atoms opposite the O atom and for the CH2OH group. Hexopyranose isomers with the same orientation at the 3 OH groups (carbons 2, 3, and 4 of C-1 D-glucose), but not with the CH2OH group, have only little (1-C D-glucose), or no effect (1-C D-idose and myoinositol) on D-glucose uptake. The C-1 L-sorbose molecule matches the C-1 D-glucose at many points including the stereo configuration of the CH2OH group, but it had no effect on D-glucose uptake perhaps because of an interference of the OH group adjacent to the CH2OH substituent. The D-glucose analogs, 3-O-methylglucose and glucosamine, were the most effective in binding to the glucose carrier. The isomers D-fructose, D-galactose, and D-mannose have separate distinctive proton cotransport systems. In starved cells they compete with D-glucose uptake, but the competition is for the available energy and not the carrier binding site.