Visualizing proton conductance in the gramicidin channel

Abstract

In this month's Biophysical Journal, Pomes and Roux (1996) provide a molecular dynamics simulation of protonic conductance along a single strand of hydrogen-bonded water in the gramicidin channel. This result complements an earlier report by Sagnella and Voth (1996) and is significant because protons do not depend on diffusion alone to move through aqueous media. Instead a can hop along chains of hydrogen-bonded water molecules, so that the measured ionic mobility of protons in water is several times higher than that of sodium or potassium ions. Proton conduction by hopping along water chains was first proposed by Hladky and Haydon (1972) to explain the anomalously high conductance through the gramicidin channel, which is approximately 15 times that of potassium ions. Although conductance through the gramicidin channel is intrinsically interesting, it may also provide a useful model system for biological systems of more general interest. The term proton was coined by Nagle and Morowitz (1978) to describe possible conductance pathways in proteins in which hydrogen-bonded side chains of amino acids could provide pathways for protons. Although a wire composed purely of amino acid side chains has not yet been discovered, there are now a number of proposed protonic con-