Functional studies of human GLUT5: effect of pH on substrate selection and an analysis of substrate interactions

The adsorption of D-fructose within the lumen of the human small intestine is thought to be mediated by the GLUT5 isoform of the human facilitative sugar transporter family. This isoform has been expressed in oocytes and shown to be capable of D-fructose transport. Some debate remains regarding the absolute substrate specificity of this isoform. To that end, we have undertaken an analysis of the functional properties of this protein when expressed in Xenopus oocytes. We have examined the pH dependence of transport activity, the ability to transport D-fructose versus deoxyglucose, and employed a range of sugar analogues to probe the nature of the exofacial substrate binding site. Our data show that the human GLUT5 isoform functions exclusively as a D-fructose transporter between pH 4.5 and 8. The Km for D-fructose was found to be 15 +/- 4 mM at pH 7. 5, and was relatively unaltered even at pH 4.5. Analysis of the effects of a range of compounds on GLUT5 function suggests that this isoform transports D-fructose preferentially in the furanose ring form.