Fig. 8.

Predicted Na1 and Na2 sites shown in a homology model of hSGLT1 (Fig. 2). Na⁺ ions are colored yellow. (A) Na2 site. The Na2 site is 10 Å from the sugar binding site. Na⁺ binding is predicted to involve H bonds with carbonyl oxygens of A76, I79, and S389 (bond length <4 Å) and the side chain hydroxyl (OH⁻) from S392 (bond length 5 Å). The Na⁺ bond with D204 is predicted to occur via a water bridge (35). No Na⁺ bonding is shown to the side chain of S393 in accordance with the experimental data. (B) Na1 site. A Na⁺ ion is placed ∼2.5 Å from the carbonyl oxygen of N78 and H83 [corresponding residues are in the LeuT Na1 site (6)]. Also shown are distances between Na⁺ and Y290 (3.3 Å), W291 (6.3 Å), E102 (7.5 Å), and the O2 of glucose (3.5 Å).