CHEMIOSMOTIC HYPOTHESIS

CHEMIOSMOTIC HYPOTHESIS:

 A theory postulated by the biochemist Peter Mitchell in 1961 to describe ATP synthesis by way of a proton electrochemical coupling. According to this hypothesis proton motive force was responsible for driving the synthesis of ATP. In this hypothesis, protons would be pumped across the inner mitochondrial membrane as electron went through the Electron Transport Chain (ETS). This would result in a proton gradient with a lower pH in the inter-membrane space and an elevated pH in the matrix of the mitochondria. An intact inner mitochondrial membrane, impermeable to protons is a requirement for this model. The proton gradient and membrane potential are the proton motive force i.e. holds to derive ATP synthesis. pH gradient act 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. His comprehensive review on chemiosmotic coupling known as Grey book has been reported here with permission to offer milestone record and easy access to this important contribution to the biochemical literature. This acceptance by the scientific community is a result of accumulating experimental evidence supporting hypothesis.   

Experimental Evidences:

Some of the evidence supporting Mitchell’s chemiosmotic hypothesis is as follows:

1)      Electron transport generates a proton gradient. The pH measured on the centriole is lower than that measured inside the mitochondria.

2)      Only a proton gradient is needed to synthesize ATP, Electron transport is not required as long as there is another mechanism for generating p H gradient.

3)      A reconstitution experiment carried out by Racker andStoeckenius showed that the generation of 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, a membrane fragment containing the protein bacteriorhodopsin from the purple bacteria holobacterium was also inserted. This is known as purple bacteria because bacteriorhodopsin gives the membrane a purple colour. Bacteriorhodopsin is a light driven proton pump, therefore shining light on this artificial purple membrane found a proton gradient which measured by the beef heart mitochondria ATPase to synthesize ATP.

The Electron Transport Chain and the ATPase are asymmetrically oriented in the inner mitochondril membrane. An asymmetric orientation is a requirement to establish a p H gradient. A random arrangement would not result in a net gradient of protons and therefore no proton motive force for the synthesis of ATP.

Compounds called uncouplers, were found to collapse the pH gradient by shuttling protons back across the membrane through the compounds. One such uncoupler is dinitrophenol. In the presence of uncoupler electron transport continues but no ATP synthesis occurs.

Uncoupling of electron transfers and ATP synthesis is useful to an organism. Such uncoupling can generate an energetically wasteful by-product as heat. This occurs normally in many hibernating animals, in newborn humans and in mammals adapted to the cold. It occurs in a specialized tissue known as brown adipose tissue. An uncoupling protein called thermogenin can accomplish this uncoupling and thus allow heat to be generated.

We need to consider the processes the processes that take place during the activation of electrons and their transport to determine the steps that cause a proton gradient to develop.

         i.            We know that splitting of water molecule takes place on the inner side of membrane, the protons or H+ ion that are produced by the splitting of water accumulate within the lumen of thylakoids.

       ii.            As electrons move through the photosystems, protons are transported across the membrane. This happens because the primary acceptor of electrons is located towards the outer side of the membrane and transfers its electrons not to an electron carrier but to a hydrogen carrier. Hence, this molecule removes a proton from the stroma while transporting an electron.

      iii.            When this molecule passes on its electrons to the electron carrier on the inner side or the lumen side of the membrane.

     iv.            The NADP reductase enzyme is located on the stroma side of the membrane. Along with the electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP+ to NADPH+H+. These protons are also removed from the stroma.

       v.            Thus protons in the stroma decrease in number and accumulate in the lumen. This creates a proton gradient across the thylakoid membrane as well as a membrane decrease in p H in the lumen.

     vi.            The breakdown of this gradient leads to release of energy. The movement of protons across the membrane to the stroma results in the breakdown of gradient. The movement of protons takes place through the trans-membrane channel of the F₀ of the ATPase.

Structure of ATPase:

The ATPase enzyme consists of two parts; one part is called the F₀ and is embedded in the membrane. This forms two trans- membrane channel which carries out facilitated diffusion of protons across the membrane. The other portion is called F1, it protrudes on the outer surface of thylakoid membrane on the lumen side. The breakdown of the gradient provides enough energy to cause a change in the F1 particle of the ATPase which results in synthesis of several molecule of energy packed ATP.

It can be said that chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is utilized to pump protons across a membrane to create a gradient or protons within the thylakoid membrane. ATP ase has a channel, this channel allows diffusion of protons back across the membrane. The diffusion of protons release enough energy to activate ATPase enzyme. The ATPase enzyme catalyzes the formation of ATP, NADPH and the ATP is used in the biosynthesis reaction which takes place in the stroma. This reaction is responsible for fixing CO2 and for the synthesis of sugars.

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