Docking Studies Show That D-Glucose and Quercetin Slide through the Transporter GLUT1

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On a three-dimensional templated model of GLUT1 (Protein Data Bank code 1SUK), a molecular recognition program, AUTODOCK 3, reveals nine hexose-binding clusters spanning the entire "hydrophilic" channel. Five of these cluster sites are within 3-5 A of 10 glucose transporter deficiency syndrome missense mutations. Another three sites are within 8 A of two other missense mutations. D-glucose binds to five sites in the external channel opening, with increasing affinity toward the pore center and then passes via a narrow channel into an internal vestibule containing four lower affinity sites. An external site, not adjacent to any mutation, also binding phloretin but recognizing neither D-fructose nor L-glucose, may be the main threading site for glucose uptake. Glucose exit from human erythrocytes is inhibited by quercetin (K(i) = 2.4 mum) but not anionic quercetin-semiquinone. Quercetin influx is retarded by extracellular D-glucose (50 mm) but not by phloretin and accelerated by intracellular D-glucose. Quercetin docking sites are absent from the external opening but fill the entire pore center. In the inner vestibule, Glu(254) and Lys(256) hydrogen-bond quercetin (K(i) approximately 10 microm) but not quercetin-semiquinone. Consistent with the kinetics, this site also binds D-glucose, so quercetin displacement by glucose could accelerate quercetin influx, whereas quercetin binding here will competitively inhibit glucose efflux. Beta-D-hexoses dock twice as frequently as their alpha-anomers to the 23 aromatic residues in the transport pathway, suggesting that endocyclic hexose hydrogens, as with maltosaccharides in maltoporins, form pi-bonds with aromatic rings and slide between sites instead of being translocated via a single alternating site.