title: Electrostatic attraction of weak monoacid anions increases probability for protonation and passage through aquaporins.
abstract: A positive electrostatic field emanating from the center of the aquaporin (AQP) water and solute channel is responsible for the repulsion of cations. At the same time, however, a positive field will attract anions. In this regard, l-lactate/lactic acid permeability has been shown for various isoforms of the otherwise highly water and neutral substrate selective AQP family. The structural requirements rendering certain AQPs permeable for weak monoacids and the mechanism of conduction have remained unclear. Here, we show by profiling pH-dependent substrate permeability, measurements of media alkalization, and proton decoupling that AQP9 acts as a channel for the protonated, neutral monocarboxylic acid species. Intriguingly, the obtained permeability rates indicate an up to 10 times higher probability of passage via AQP9 than given by the fraction of the protonated acid substrate at a certain pH. We generated AQP9 point mutants showing that this effect is independent from properties of the channel interior but caused by the protein surface electrostatics. Monocarboxylic acid-conducting AQPs thus employ a mechanism similar to the family of formate-nitrite transporters for weak monoacids. On a more general basis, our data illustrate semiquantitatively the contribution of surface electrostatics to the interaction of charged molecule substrates or ligands with target proteins, such as channels, transporters, enzymes, or receptors.
authors.affiliation: From the Department of Pharmaceutical and Medicinal Chemistry, Christian-Albrechts-University of Kiel, 24118 Kiel, Germany.
authors.forename: Monja
authors.initials: M
authors.lastname: Rothert
authors.affiliation: From the Department of Pharmaceutical and Medicinal Chemistry, Christian-Albrechts-University of Kiel, 24118 Kiel, Germany.
authors.forename: Deike
authors.initials: D
authors.lastname: Rönfeldt
authors.affiliation: From the Department of Pharmaceutical and Medicinal Chemistry, Christian-Albrechts-University of Kiel, 24118 Kiel, Germany ebeitz@pharmazie.uni-kiel.de.
authors.forename: Eric
authors.identifier: 0000-0001-5912-6626
authors.initials: E
authors.lastname: Beitz
mesh_terms: D020346:Aquaporins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002264:Carboxylic Acids / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D017354:Point Mutation; D055672:Static Electricity
chemical_list: C109669:AQP9 protein, human; C491927:Aqp7 protein, mouse; D020346:Aquaporins; D002264:Carboxylic Acids
keywords: aquaporin; electrostatics; lactic acid; membrane transport; permeability; substrate specificity