The chemiosmotic hypothesis was proposed by Peter Mitchell in 1969. This hypothesis stated that a proton-motive force was responsible for driving the synthesis of ATP. In this hypothesis, protons would be pumped across the inner mitochondrial membrane as electrons went through the electron transfer chain. This would result in a proton gradient with an lower pH in the intermembrane space and a elevated pH in the matrix of the mitochondria. An intact inner mitochondrial membrane, impermeable to protons, is a requirement of such a model. The proton gradient and membrane potential are the proton-motive force that is used to drive ATP synthesis. In effect, the pH gradient acts as a "battery" which stores energy to produce ATP. Over the past several years, Mitchell's chemiosmotic hypothesis has been widely accepted as the mechanism of coupling of electron transport and ATP synthesis. He was awarded the Nobel Prize in Chemistry in 1978. This acceptance by the scientific community is a result of accumulating experimental evidence supporting the hypothesis.
Some of the evidence supporting Mitchell's chemiosmotic hypothesis is as follows.
- Electron transport
generates a proton gradient. The pH measured on the outside
is lower than that measured inside the mitochondria.
- Only a proton gradient is
needed to synthesize ATP. Electron transport is not
required as long as there is another mechanism for
generating a pH gradient.
- A reconstitution experiment carried out by Racker & Stoeckenius (J Biol Chem 1974 Jan 25;249(2):662-3, Reconstitution of purple membrane vesicles catalyzing light-driven proton uptake and adenosine triphosphate formation) showed that the generation of a proton gradient can result in ATP synthesis in a totally artificial system. In their experiment, a mitochondrial ATPase complex from beef heart was inserted into an artificial lipid bilayer. Also inserted in this bilayer was a membrane fragment containing the protein, bacteriorhodopsin* [pic], from the purple bacteria Halobacterium, so called because the bacteriorhodopsin gives the membrane a purple color. Bacteriorhodopsin is a light-driven proton pump. Therefore, shining light on this artificial "purple membrane" formed a proton gradient, which was used by the beef heart mitochondrial ATPase to synthesize ATP.
Compounds called uncouplers were found to collapse the pH gradient by shuttling protons back across the membrane through the compounds. One such uncoupler, dinitophenol is shown below. In the presence of the uncoupler electron transport continues, but no ATP synthesis occurs.
