Incorporation of transmembrane hydroxide transport into the chemiosmotic theory

Abstract

A cornerstone of textbook bioenergetics is that oxidative ATP synthesis in mitochondria requires, in normal conditions of internal and external pH, a potential difference (Δψ) of well over 100 mV between the aqueous compartments that the energy-transducing membrane separates. Measurements of Δψ inferred from diffusion of membrane-permeant ions confirm this, but those using microelectrodes consistently find no such Δψ — a result ostensibly irreconcilable with the chemiosmotic theory. Transmembrane hydroxide transport necessarily accompanies mitochondrial ATP synthesis, due to the action of several carrier proteins; this nullifies some of the proton transport by the respiratory chain. Here, it is proposed that these carriers' structure causes the path of this “lost” proton flow to include a component perpendicular to the membrane but within the aqueous phases, so maintaining a steady-state proton-motive force between the water at each membrane surface and in the adjacent bulk medium. The conflicting measurements of Δψ are shown to be consistent with the response of this system to its chemical environment.