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.

Human adipose glycerol flux is regulated by a pH gate in AQP10

“…2 ), induction of lipolysis resulted in internal acidification and stimulation of glycerol release. Only a few reports hint at a pH-dependence of mammalian aquaglyceroporin-mediated flux 9 , 18 , 19 . We therefore assessed the pH effect on water and glycerol flow across membrane vesicles prepared from human adipocytes challenged with osmotic gradients (Fig.…”

Molecular Basis of Aquaporin-7 Permeability Regulation by pH

“…hAQP7 pH-dependency has been previously evaluated in Madin-Darby canine kidney (MDCKII) cells by measuring 14 C-glycerol uptake in the pH range of 5.5 to 8.0 and, although glycerol uptake was significantly reduced at pH 5.0, no meaningful physiological relevance was attributed to this effect [ 61 ]. A recent study also showed pH-dependence of murine AQP7 when expressed in yeast, with a loss of glycerol permeability at pH 4 and below [ 62 ]. This prompted us to investigate the pH-sensitivity of hAQP7 using our optimized yeast-cell system where channel permeability was fully characterized as shown above.…”

The Ionophores CCCP and Gramicidin but Not Nigericin Inhibit Trypanosoma brucei Aquaglyceroporins at Neutral pH

“…At an IC 50 of 2 µM, the potency of CCCP on the TbAQP2 glycerol permeability is remarkable, since CCCP is usually used in the two-digit micromolar concentration range. This effect is clearly independent of the action of CCCP on the transmembrane pH gradient, because neutral solute permeability of AQP channels is not affected by the proton availability [ 37 , 38 ]. Therefore, the CCCP inhibition of TbAQP2 must be seen as an unexpected and serious side effect that occurs at conventionally used concentrations.…”