Role of hydration in the conformational transitions between unliganded and liganded forms of loop 13 of the Na+/glucose cotransporter 1.

SGLT1 as a Na+/glucose cotransporter is inhibited by phlorizin, a phloretin 2'-glucoside that has strong interactions with the C-terminal loop 13 (residues 541-638). Here we investigated the effect of a partial substitution of glycerol for water in the medium on the stability and phlorizin-binding function of loop 13 using fluorescence spectroscopy. Increasing the glycerol concentration promoted an increase in the stability of the protein to urea. The ability of loop 13 to expose hydrophobic surface promoted by phlorizin binding was partially lost in the presence of glycerol (20%). Glycerol also led to a decrease in the phlorizin affinity of loop 13 in solution. Approximately 15 molecules of water were taken up to cover additional surface area (137.7+/-27.9A(2)) upon formation of the loop 13-phlorizin complex. Together these results demonstrate quantitatively that the stability and phlorizin affinity of loop 13 are critically dependent on protein hydration.