Proton-coupled energy transduction by biological membranes. Principles, pathways and praxis

Abstract

A brief outline of certain features of the chemiosmotic hypothesis of the mechanism of free-energy transfer between the reactions of electron transport and adenosine triphosphate synthesis catalysed by biological membranes is given. Pulses of electron transport induced by the addition of small quantities of oxygen to suspensions of the bacterium Paracoccus denitrificans lead to vectorial H⁺ movements into the aqueous phase external to the organisms, where they may be detected with a glass pH electrode. The stoichiometry of the number of protons translocated into the bulk phase external to the organisms, per oxygen atom reduced, is essentially unchanged when the amount of oxygen reduced is varied, in a manner inconsistent with the predictions of the chemiosmotic-coupling hypothesis. These and other observations lead to the view that the energy-coupling proton-transfer processes utilised in reactions such as electron-transport phosphorylation are confined to the membrane phase. Mechanisms which most easily account for this are discussed.