Question

Describe the evidence for chemiosmotic coupling in mitochondria.

Answer 1

The chemiosmotic hypothesis was proposed by Peter Mitchell.

It states that a proton-motive force was responsible for the synthesis of ATP.

In this, H⁺ are be pumped across the inner mitochondrial membrane as electrons went through electron transfer chain.

This result in a H⁺ gradient with an lower pH in the intermembrane space and a higher pH in the matrix of the mitochondria.

Evidence supporting the chemiosmotic hypothesis is

• Electron transport generates proton gradient across mitochondrial intermembrane space and matrix.
• The pH measured on the outside is lower than that measured inside the mitochondria (matrix).
• H⁺ gradient is only needed to synthesize the ATP.
• Electron transport is not required as long as there is another mechanism for the generating pH gradient.
• A reconstitution experiment carried out by Racker and Stoeckenius
• Reconstitution of purple membrane vesicles catalyzing light-driving proton uptake and ATP formation showed that the generation of a proton gradient can result in ATP synthesis in a totally artificial system.
• In this experiment, a mitochondrial ATPase complex from beef heart was inserted into an artificial lipid bilayer.
• And also inserted in this bilayer was a membrane fragment containing the protein, bacteriorhodopsin, from the purple Halobacterium, so the bacteriorhodopsin gives the membrane a purple color.
• Bacteriorhodopsin is a light-driving proton pump. Therefore, shining light on this purple membrane formed a proton gradient, which was used by the beef heart mitochondrial ATPase to synthesize ATP.
• The electron transfer chains and the ATPase are asymmetrically oriented in inner mitochondrial membrane.
• An asymmetric orientation is a requirement to establish a pH gradient across the membrane.
• A random arrangement would not result in a net gradient of protons and therefore, no H⁺ for the synthesis of ATP.
• Some compounds called uncouplers collapse the pH gradient. In the presence of these compounds electron transport continues, but no ATP synthesis occurs.