A view of hydrogen/hydroxide flux across lipid membranes.
A topic emerging roughly 30 years ago and engendering an incompletely resolved controversy is reviewed in this article: the relatively high permeability and pH independence associated with H(+)/OH(-) passive movements across lipid membranes. We summarize the expected characteristics of simple H(+)/OH(-) diffusion and those of a reaction between H(+) and OH(-) being attracted from opposite surfaces and condensing in an interfacial zone of the membrane. An interfacial H(+)/OH(-) reaction mechanism gives the experimentally observed behavior of an H(+)/OH(-) flux that is independent of the pH measurement range. This mechanism assumes that H(+) and OH(-) within the interfacial zone become electrostatically aligned on opposite sides of the hydrophobic membrane core. Electrostatic attraction and charge delocalization among a small cluster of water molecules surrounding the ions reduce the Born energy for H(+)/OH(-) insertion into lipid. This transmembrane condensation model predicts the magnitude of the experimentally determined H(+)/OH(-) flux, which is significantly greater than that of other monovalent ions. The consequences of an elevated H(+)/OH(-) permeability compared to other ions and the relative pH independence of this flux have consequences for understanding the chemical evolution of life.
Nichols
J Wylie
JW
Department of Physiology, Emory University School of Medicine, Atlanta, GA 30322-3110, USA.
Abercrombie
R F
RF
D000818:Animals; D004058:Diffusion; D006801:Humans; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000378:metabolism*; D008563:Membrane Lipids / Q000378:metabolism*
D006878:Hydroxides; D008563:Membrane Lipids; D006859:Hydrogen; C031356:hydroxide ion