Docking studies show that D-glucose and quercetin slide through the transporter GLUT1

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 angstrom of 10 glucose transporter deficiency syndrome missense mutations. Another three sites are within 8 angstrom 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 mu M) 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 = 10 mu M) 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.