An Evaluation of the Mitchell Hypothesis of Chemiosmotic Coupling in Oxidative and Photosynthetic Phosphorylation

Abstract

The Mitchell hypothesis of chemiosmotic coupling in oxidative phosphorylation is examined in the light of experimental data on oxidative phosphorylation at present available. The following objections are brought against the theory: Data on the magnitude of the phosphate potential against which ATP can be synthesized by the respiratory chain require, on the basis of chemiosmotic coupling, very effective proton and/or cation extrusion and/or anion uptake by the mitochondria. Experimental evidence for this is lacking. Indeed it has been shown by Chance and Mela that mitochondria in the controlled state do not extrude protons. Although reversible ATP‐driven proton (chloroplasts) or cation (mitochondria, erythrocytes) pumps have been demonstrated, this is insufficient evidence for the postulate that the establishment of a proton gradient is the primary energy‐conserving event of the chloroplast or mitochondrial respiratory chain. Indeed, it is general experience that cations are required for the extrusion of protons by mitochondria. The stoicheiometry of proton extrusion by mitochondria and of proton uptake by chloroplasts, and the kinetics of the latter process are also difficult to reconcile with the chemiosmotic hypothesis. Further experimental evidence is presented confirming that, under the conditions of the oxygen‐pulse experiments of Mitchell and Moyle, the extrusion of H+ is not associated with the oxidation of mitochondrial NADH. The respiratory chain included in the chemiosmotic hypothesis is difficult to reconcile with our present knowledge of the chain. It is concluded that the chemiosmotic theory, in its present form, is untenable.