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Among aquaglyceroporins that transport both water and glycerol across thecell membrane, Escherichia coli glycerol uptake facilitator (GlpF) is the mostthoroughly studied. However, one question remains: Does glycerol modulate waterpermeation? This study answers this fundamental question by determining thechemical-potential profile of glycerol along the permeation path through GlpF'sconducting 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 toGlpF's perfect steric arrangement: The glycerol-protein van der Waalsinteractions are attractive near the NPA but repulsive elsewhere in theconducting pore. In light of the single-file nature of waters and glycerolslining 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 theconducting pore. Therefore, water permeation is fully correlated to glyceroldissociation 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 whichhave not been given the proper attention they deserved: The Arrheniusactivation barriers were found to be 7 kcal/mol for water permeation and 9.6kcal/mol for glycerol permeation; The presence of up to 100 mM glycerol did notaffect the kinetics of water transport with very low permeability, in apparentcontradiction with the existent theories that predicted high permeability (0 Mglycerol).

Glycerol Modulates Water Permeation through Escherichia coli Aquaglyceroporin GlpF