Nonexclusive solute transport thru protein channels model of the Na,K ATPase complex and similar channels as general transport routes

Abstract

In earlier work this author put forward a model of the Na,K ATPase complex as a general transport channel. Detailed treatment was limited to anion and monovalent cation transport. Here the functional mechanisms of the Na,K ATPase and similar protein channels as transport routes for all ionic fluxes and also amino acid, sugar and other solutes are presented. Anions, monosaccharide -OH groups and amino acid carboxyls bind to common arginyls and lose hydration water. They combine with cations which bind to adjacent side chain carboxyls, forming neutral ion pairs or positively charged complexes which have minimums in size, hydration and free polar groups. The smaller size and polarity facilitate entry into the tight, structured water channel of some 8–10 A outer bore. Solute fluxes depend on membrane redox activity which maintains channel sulfhydryls in reduced state required for proper transport. ATP binding at channels contributes to transport conformation while ATP hydrolysis gives high efflux of Na +, H + and Ca 2+ as phosphate ion pairs. This cation efflux current clears cations from inner membrane sites, increases negative potential and provides Na + and H + about the outer combining sites, while maintaining their inward gradients. Binding of many agents widens the outer bore to give larger, less selective influx.