ROLE OF WATER IN PROTON CONDUCTANCE ACROSS MODEL AND BIOLOGICAL-MEMBRANES

Proton conductance across model and biological membranes can be understood in terms of proton translocation along chains of hydrogen-bonded water molecules. This translocation mechanism accounts for the unexpectedly high permeability of lipid bilayers to proton flux, which seems to occur through rare transient defects in the bilayer barrier. The nature of the proton-conducting channel of the F1F0 adenosine 5'-triphosphate (ATP) synthase is unknown, but may use a similar translocation mechanism. We have tested the gramicidin channel as a model of such proton conductance. The channel consists of a single chain of hydrogen-bonded water molecules, and its proton conductance at saturation is near 140 pA, or 10(9) H+/s. Assuming that an adequate supply of protons is made available to the putative channel, this rate easily supports proton transport requirements of the F1F0 ATP synthase during ATP synthesis. However at neutral pH ranges diffusion of free protons probably could not maintain an adequate supply at the channel mouth. Other sources of protons must therefore be postulated.