Determinants of sugar-induced influx in the mammalian fructose transporter GLUT5

• Published in: eLife 2023-07, Vol.12
• Main Authors: McComas, Sarah E, Reichenbach, Tom, Mitrovic, Darko, Alleva, Claudia, Bonaccorsi, Marta, Delemotte, Lucie, Drew, David
• Format: Article
• Language: English
• Subjects: Affinity; Animals; Binding sites; Biochemical analysis; biochemistry; Biochemistry and Chemical Biology; Biological Transport; Catalysis; chemical biology; Fructose; Fructose - metabolism; fructose transport; Gating; Glucose; Glucose - metabolism; Glucose transport; Glucose Transport Proteins, Facilitative - metabolism; Glucose transporter; Glucose transporter type 5; Homeostasis; Humans; Isoforms; Malaria, Falciparum; Mammals - metabolism; MD simulations; mechanism; Monosaccharides; Plasmodium falciparum; Proteins; S. cerevisiae; Salt; Substrate preferences; Sugar; Sugars
• ISSN: 2050-084X; 2050-084X
• DOI: 10.7554/eLife.84808

In mammals, glucose transporters (GLUT) control organism-wide blood-glucose homeostasis. In human, this is accomplished by 14 different GLUT isoforms, that transport glucose and other monosaccharides with varying substrate preferences and kinetics. Nevertheless, there is little difference between the sugar-coordinating residues in the GLUT proteins and even the malarial transporter HT1, which is uniquely able to transport a wide range of different sugars. HT1 was captured in an intermediate 'occluded' state, revealing how the extracellular gating helix TM7b has moved to break and occlude the sugar-binding site. Sequence difference and kinetics indicated that the TM7b gating helix dynamics and interactions likely evolved to enable substrate promiscuity in HT1, rather than the sugar-binding site itself. It was unclear, however, if the TM7b structural transitions observed in HT1 would be similar in the other GLUT proteins. Here, using enhanced sampling molecular dynamics simulations, we show that the fructose transporter GLUT5 spontaneously transitions through an occluded state that closely resembles HT1. The coordination of D-fructose lowers the energetic barriers between the outward- and inward-facing states, and the observed binding mode for D-fructose is consistent with biochemical analysis. Rather than a substrate-binding site that achieves strict specificity by having a high affinity for the substrate, we conclude GLUT proteins have allosterically coupled sugar binding with an extracellular gate that forms the high-affinity transition-state instead. This substrate-coupling pathway presumably enables the catalysis of fast sugar flux at physiological relevant blood-glucose concentrations.