Glycerol Modulates Water Permeation through Escherichia coli Aquaglyceroporin GlpF

TL;DR

This study investigates how glycerol influences water transport through E. coli GlpF, revealing that glycerol modulates water permeation by occluding the pore at higher concentrations, supported by chemical-potential profiling and experimental data.

Contribution

It provides a detailed chemical-potential profile of glycerol in GlpF and demonstrates the modulation of water permeation by glycerol, a novel insight into aquaglyceroporin function.

Findings

Glycerol modulates water permeation in the microM range.

Glycerol saturation occludes the GlpF pore at mM concentrations.

Water permeation correlates with glycerol dissociation, with an activation barrier of 6.5 kcal/mol.

Abstract

Among aquaglyceroporins that transport both water and glycerol across the cell membrane, Escherichia coli glycerol uptake facilitator (GlpF) is the most thoroughly studied. However, one question remains: Does glycerol modulate water permeation? This study answers this fundamental question by determining the chemical-potential profile of glycerol along the permeation path through GlpF's conducting pore. There is a deep well near the Asn-Pro-Ala (NPA) motifs (dissociation constant 14 microM) and a barrier near the selectivity filter (10.1 kcal/mol above the well bottom). This profile owes its existence to GlpF's perfect steric arrangement: The glycerol-protein van der Waals interactions are attractive near the NPA but repulsive elsewhere in the conducting pore. In light of the single-file nature of waters and glycerols lining up in GlpF's amphipathic pore, it leads to the following conclusion: Glycerol modulates water permeation in the microM range. At mM concentrations, GlpF is glycerol-saturated and a glycerol dwelling in the well occludes the conducting pore. Therefore, water permeation is fully correlated to glycerol dissociation that has an Arrhenius activation barrier of 6.5 kcal/mol. Validation of this theory is based on the existent in vitro data, some of which have not been given the proper attention they deserved: The Arrhenius activation barriers were found to be 7 kcal/mol for water permeation and 9.6 kcal/mol for glycerol permeation; The presence of up to 100 mM glycerol did not affect the kinetics of water transport with very low permeability, in apparent contradiction with the existent theories that predicted high permeability (0 M glycerol).