In this article we will discuss about the role of protons in the synthesis of ATP in energy transducing membranes, explained with the help of a suitable diagram.
It is known that electron carrier chains are implanted in specialized membranes. Whenever an energy recovery and a synthesis of ATP are coupled with the reoxidation of a chain of e– carriers, the membrane is called “energy transducer”; it plays an essential role in the synthesis of ATP, in mitochondria (inner membrane) as well as in chloroplasts (thylakoids) and bacteria (plasmic membrane, respiration or photosynthesis).
In order to explain the phosphorylating oxidations coupled with the respiratory chain, it was tried for years to find a mechanism of the type of glyceraldehyde-3-phosphate dehydrogenase coupled with 3-phosphogiycerate kinase. This hypothetical mechanism which did not take the membrane into consideration was refuted after years of research.
Around 1961, the role of protons in the mechanism of phosphorylating oxidations was suggested independently by P. Mitchell and R.J.P. Williams. Since then, Peter Mitchell in collaboration with Jennifer Moyle developed the elegant and convincing chemosmotic theory. Numerous experimental arguments have been advanced in its favour.
This theory proposes a general mechanism for the synthesis of ATP linked with the transducer membranes (mitochondria, chloroplasts, bacteria). The membrane has a functional asymmetry. The chain of e– carriers implanted in the membrane has an anisotropic functioning and owing to the orientation of its sites, it causes the translocation of protons and their ejection on a specific face of the membrane.
Since the membrane is impermeable to ions except at the level of specific and controlled translocators, the functioning of the chain creates a gradient of protons on either side of the membrane and a protomotive force which are utilized for the synthesis of ATP thanks to the recapture of the ejected protons by a transmembrane ATPase-ATP synthetase.
On the face of the membrane where they have been ejected, the protons penetrate into the proton canal of the enzymatic complex and thus proceed towards the sites of ATP-synthesis, oriented towards the other face of the membrane. ATP is thus synthesised at the cost of the gradient of protons (see fig. 3-17).
This mechanism is valid even for the simplest transducing membranes of photosynthesizing bacteria (Halobacterium halobiuni). The plasmic membrane of these bacteria is strewn with purple membranes consisting of a single protein, a photosensitive bacteriorhodopsin which, under the effect of light, catalyzes the ejection of protons outside, thus creating a gradient of protons and a protomotive force.
The pumping of protons through canals can permit the synthesis of ATP thanks to a transmembrane ATPase-ATP synthetase, or the protons gradient can be utilized directly for the active transports of molecules or ions across the membrane. A complete chain of electron carriers is therefore not indispensable.
However, at night, in the absence of light, the respiratory chain implanted in the plasmic membrane (outside the purple membranes) also catalyzes the outward ejection of protons which will be pumped again and utilized for the synthesis of ATP.
The theory of localized protons advanced by R.J.P. Williams (1978) is based on the same concept though it presents some different aspects on the location of ejected protons and their migration.
Lastly, let us cite the conformational hypothesis suggested by P.D. Boyer around 1964. According to this hypothesis, the functioning of the chain of electron carriers, by “energizing” the membrane, produces conformational changes which are utilized, at least partly, for the synthesis of ATP.