Rolf Karl-Heinz Kinne

Human sodium-independent glucose cotransporter 1 (hGLUT1) has been studied for its tetramerization and multimerization at the cell surface. Homozygous or compound heterozygous mutations in hGLUT1 elicit GLUT1-deficiency syndrome (GLUT1-DS), a metabolic disorder, which results in impaired glucose transport into the brain. The reduced cell surface expression or loss of function have been shown for some GLUT1 mutants. However, the mechanism by which deleterious mutations affect protein structure, conformational stability and GLUT1 oligomerization is not known and require investigation. In this review, we combined previous knowledge of GLUT1 mutations with hGLUT1 crystal structure to analyze native interactions and several natural single-point mutations. The modeling of native hGLUT1 structure confirmed the roles of native residues in forming a range of side-chain interactions. Interestingly, the modeled mutants pointed to the formation of a variety of non-native novel interactions, altering interaction networks and potentially eliciting protein misfolding. Self-aggregation of the last part of hGLUT1 was predicted using protein aggregation prediction tool. Furthermore, an increase in aggregation potential in the aggregation-prone regions was estimated for several mutants suggesting increased aggregation of misfolded protein. Protein stability change analysis predicted that GLUT1 mutant proteins are unstable. Combining GLUT1 oligomerization behavior with our modeling, aggregation prediction, and protein stability analyses, this work provides state-of-the-art view of GLUT1 genetic mutations that could destabilize native interactions, generate novel interactions, trigger protein misfolding, and enhance protein aggregation in a disease state.

This article reviews the role of segments of SLCs located outside the plasma membrane bilayer (ectodomains) using the inhibition of SGLTs (SLC5 family) by the aromatic glucoside phlorizin as a model system.

Current advances in structural biology provide valuable insights into structure–function relationship of membrane transporters by solving crystal structures of bacterial homologs of human transporters. Therefore, scientists consider bacterial transporters as useful structural models for designing of drugs targeted in human diseases. The functional homology between Vibrio parahaemolyticus Na⁺/galactose transporter (vSGLT) and Na⁺/glucose cotransporter SGLT1 has been well established a decade ago. Now the crystal structure of vSGLT is considered quite valuable in explaining not only the cotransport mechanisms, but it also acts as a representative protein in understanding the protein stability and amino acid interactions within the core structure. We investigated the molecular mechanisms of genetic variations in SGLT1 that cause glucose–galactose malabsorption (GGM) defects using the crystal structure of vSGLT as a model sugar transporter. Our in silico mutagenesis and modeling analysis suggest that the GGM genetic variations lead to conformational changes either by structure destabilization or by formation of unnecessary interaction within the core structure of SGLT1 thereby explaining the genetic defects in Na⁺ dependent sugar translocation across the cell membrane.

Recently, the idea has been developed to lower blood glucose levels in diabetes by inhibiting sugar reabsorption in the kidney . The main target is thereby the early proximal tubule where secondary active transport of the sugar is mediated by the sodium-d-glucose cotransporter SGLT2 . A model substance for the inhibitors is the O-glucoside phlorizin which inhibits transport competitively. Its binding to the transporter involves at least two different domains: an aglucone binding site at the transporter surface, involving extramembranous loops , and the sugar binding /translocation site buried in a hydrophilic pocket of the transporter. The properties of these binding sites differ between SGLT2 and SGLT1 , which mediates sugar absorption in the intestine . Various O-, C-, N- and S-glucosides have been synthesized with high affinity and high specificity for SGLT2 . Some of these glucosides are in clinical trials and have been proven to successfully increase urinary glucose excretion and to decrease blood sugar levels without the danger of hypoglycaemia during fasting in type 2 diabetes .