Glycerol modulates water permeation through Escherichia coli aquaglyceroporin GlpF

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 three-dimensional potential of mean force of glycerol along the permeation path through GlpF's conducting pore. There is a deep well near the Asn-Pro-Ala (NPA) motifs (6.5 kcal/mol below the bulk level) 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 μM range. At mM concentrations, GlpF is glycerol-saturated and a glycerol residing 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).

Erythritol predicted to inhibit permeation of water and solutes through the conducting pore of P. falciparum aquaporin

“…[23] This experimentally measured value validates GlpF’s free-energy profile computed in Ref. [41] that resembles PfAQP’s profile discussed above.…”

“…The current literature does not have all the data to fully validate this theoretical prediction but a careful analysis of the existent in vitro experiments was done in Ref. [41] showing that the presence of glycerol suppresses water conduction through aquaglyceroporin. For example, Song et al[11] examined water permeability of PfAQP expressed in protoplasts.…”

“…The formulae of Ref. [41] were used for extracting the free-energy profiles from the simulation data and for computing the dissociation constants. They are restated below.…”

Ligand-modulated interactions between charged monolayer-protected Au144(SR)60gold nanoparticles in physiological saline

“…46. The starting positions of the two Au 144 S 60 cores’ COMs were set at locations (x,y,z) = (0,0,±2.5) nm.…”

Pollen-Specific Aquaporins NIP4;1 and NIP4;2 Are Required for Pollen Development and Pollination in Arabidopsis thaliana

“…Hu et al (2014) demonstrated that exogenous glycerol inhibited primary root growth. In bacteria, it has been postulated that glycerol modulates water permeation in the micromolar range, whereas at millimolar concentrations, E. coli GlpF is glycerol saturated, occluding the conducting pore (Chen, 2013). In yeast, S. cerevisiae Fps1 plays critical roles in osmoregulation by modulating the accumulation of glycerol (reviewed in Ahmadpour et al, 2014).…”