Compared with membranes of pure POPE or POPC, the barrier of 13.5 kJ/mol results in an increase in the permeability by a factor of 750 or 230, respectively, when assuming dense expression of GlpF. This value is significantly lower than the activation energy of 30.5 kJ/mol reported in a previous molecular dynamics (MD) study ( 9). As expected, GlpF is permeated efficiently by glycerol with a maximum barrier of 13.5 kJ/mol. Results Solute Permeability.īoth glycerol and urea encounter substantial barriers between 27 and 34 kJ/mol against permeation through POPE or POPC, underlining the need for membrane channels if a substantial glycerol or urea flux is required by the metabolism. The results reveal a comprehensive mechanism underlying the permeation characteristics of aquaporins and aquaglyceroporins. The direct evaluation of interactions that govern the selectivity enables one to identify the molecular determinants of selectivity. This approach also allows one to rationalize permeation experiments on aquaporins and aquaglyceroporins embedded in oocyte membranes or liposomes. The physiological relevance of permeation through aquaporins is investigated by comparing the pathway through the aquaporin pores with the pathway across two distinct model membranes. Because these solutes differ strongly in hydrophobicity and size, the permeation barriers allow one to derive a unifying picture of the selectivity of aquaporins and aquaglyceroporins. Potentials of mean force (PMFs) for O 2, CO 2, NH 3, glycerol, urea, and water were derived by using the technique of umbrella sampling simulations. Therefore, we applied molecular dynamics simulations to determine permeation barriers of a wide range of solutes permeating through human AQP1 (hAQP1) and GlpF as typical members of the two aquaporin subfamilies. So far, however, a unifying picture that explains the diverse permeation properties of both aquaporins and aquaglyceroporins has not evolved. This interpretation has been questioned by recent experiments on oocyte permeability ( 23). The highly conserved arginine, together with a nearby histidine in water-specific aquaporins, was considered to be essential for the isolation of water molecules from their solvation shell in the bulk ( 5). The residues in the ar/R region differ in the aquaporin family, rendering the constriction site diverse in size and hydrophobicity. This interpretation has been supported by theoretical studies ( 21, 22) and mutation experiments ( 23). Close to the extracellular exit of the channel, the aromatic/arginine (ar/R) constriction region forms the narrowest part of the pore and is therefore generally assumed to be important for the channel selectivity. The dipoles of the half helices generate an electrostatic barrier in the NPA region, which is, together with desolvation effects, essential for proton exclusion ( 17– 20). 1 A) with their two asparagine side chains pointing into the pore are located at the end of two half helices. In the center of the pore, two conserved Asn-Pro-Ala motifs (NPA compare Fig. Two main constriction sites have been identified in the aquaporin channels ( 4– 6). Hence, a hydrophobic effect, together with steric restraints, determines the selectivity of aquaporins. Surprisingly, not solute-channel but water-channel interactions were found to be the key determinant underlying the selectivity mechanism of aquaporins. This finding renders AQP1 a selective filter for small polar solutes, whereas GlpF was found to be highly permeable for small solutes and permeable for larger solutes. For small solutes permeating through AQP1, a remarkable anticorrelation between permeability and solute hydrophobicity was observed, whereas the opposite trend was observed for permeation through the membrane. Using molecular dynamics simulations, we calculated potentials of mean force for solute permeation along the aquaporin channels and compared them with the alternative pathway across the lipid bilayer. We studied the selectivity of aquaporin-1 (AQP1) and the bacterial glycerol facilitator, GlpF, for O 2, CO 2, NH 3, glycerol, urea, and water. Aquaporins and aquaglyceroporins form a family of pore proteins that facilitate the efficient and selective flux of small solutes across biological membranes.
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