Analysis of the structural requirements of sugar binding to the liver, brain and insulin-responsive glucose transporters expressed in oocytes.

We have expressed the liver (GLUT 2), brain (GLUT 3) and insulin-responsive (GLUT 4) glucose transporters in oocytes from Xenopus laevis by microinjection of in vitro-transcribed mRNA. Using a range of halogeno- and deoxy-glucose analogues, and other hexoses, we have studied the structural basis of sugar binding to these different isoforms. We show that a hydrogen bond to the C-3 position is involved in sugar binding for all three isoforms, but that the direction of this hydrogen bond is different in GLUT 2 from either GLUT 1, 3 or 4. Hydrogen-bonding at the C-4 position is also involved in sugar recognition by all three isoforms, but we propose that in GLUT 3 this hydrogen bond plays a less significant role than in GLUT 2 and 4. In all transporters we propose that the C-4 position is directed out of the sugar-binding pocket. The role of the C-6 position is also discussed. In addition, we have analysed the ability of fructopyranose and fructofuranose analogues to inhibit the transport mediated by GLUT2. We show that fructofuranose analogues, but not fructopyranose analogues, are efficient inhibitors of transport mediated by GLUT 2, and therefore suggest that GLUT 2 accommodates D-glucose as a pyranose ring, but D-fructose as a furanose ring. Models for the binding sites of GLUT 2, 3 and 4 are presented.