Ion channels of biological membranes: prediction of single channel conductance

The Poisson-Boltzmann equation was solved numerically for models of the pore regions of the Shaker K+ channel and of two glycoporins (LamB and ScrY) to yield electrostatic potential profiles along the pore axes. From these potential profiles, single-channel current-voltage (I-V) relations were calculated. The importance of a proper treatment of the ionisation state of two rings of aspartate sidechains at the mouth of the K+ channel pore emerged from such calculations. The calculated most likely state, in which only two of the eight aspartate sidechains were deprotonated, yielded better agreement with experimental conductance data. An approximate calculation of single-channel conductances based simply on pore geometry yielded very similar conductance values for the two glycoporins. This differed from an experimentally determined conductance ratio of ScrY:LamB=10:1. Preliminary electrostatics calculations appeared to reproduce the observed difference in conductance between the two glycoporins, confirming that single-channel conductance is determined by electrostatic as well as geometric considerations.