Stereospecificity of the Glucose Carrier in Sugar Beet Suspension Cells

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 three equatorial OH groups on the three carbons opposite the oxygen and for the $\text{CH}_{2}\text{OH}$ group. Hexopyranose isomers with the same orientation at the three OH groups (carbons 2, 3, and 4 of C-1 D-glucose), but not with the $\text{CH}_{2}\text{OH}$ 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 $\text{CH}_{2}\text{OH}$ group, but it had no effect on D-glucose uptake perhaps because of an interference of the OH group adjacent to the $\text{CH}_{2}\text{OH}$ 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. However, in starved cells they compete with D-glucose uptake, but the competition is for the available energy and not the carrier binding site.