Structural Insights into Sodium-Dependent Sugar Transporters and their Inhibition Mechanism

Sodium-dependent glucose transporters (SGLTs) are members of the large solute carrier (SLC) family of proteins that exploit the sodium ion concentration gradient to transport a myriad of small molecules across the plasma membrane. In humans, there are six SGLT subtypes labeled 1-6 that are expressed widely in the small intestine, kidney, lung, muscle, and brain. Due to their role in sugar reabsorption, SGLTs are currently exploited as drug targets for the treatment of type 2 diabetes, especially hSGLT2, which is responsible for 98% of glucose reabsorption in the kidneys. Current inhibitors are chemical derivatives of the naturally occurring small molecule phlorizin, which is expressed in the bark of fruit trees, such as apple and pear. The structural basis of binding is not known, in part, because high-resolution structures of mammalian SGLTs do not exist. However, the inward-facing structure of the bacterial homologue from vibrio parahaemolyticus (vSGLT) has been solved both in apo and in complex with galactose, and our collaborators recently solved the outward-facing structure of a closely related homologue (unpublished). Here we combine homology modeling, virtual screening techniques and molecular dynamics simulations to achieve two goals: 1) model the outward-facing state of SGLTs, and 2) predict the binding mode of phlorizin and its derivatives hSGLT1 and 2. As a result, the spectroscopic data (double electron-electron resonance) probing outward facing state of SGLTs validate our homology model and mutagenesis studies testing binding to hSGLT1 and 2 are in agreement with our predicted binding modes.