GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

11. MAIN MECHANISMS OF METABOLISM AND ENERGY CONVERSION IN MICROORGANISMS

11.6. ELECTRON TRANSPORT CHAIN AND PHOSPHORYLATION (ATP SYNTHESIS) DURING ELECTRON TRANSFER

11.6.5. Mechanism of ATP synthesis during electron transfer

How is ATP synthesized during electron transport in the Respiratory Chain? Where does the energy that is subsequently stored in the high-energy bonds of ATP come from? What is The Mechanism of ATP synthesis during electron transfer? The answer to these questions is provided by the chemiosmotic hypothesis, proposed by the British scientist P. Mitchell in 1961. Its main principles are as follows (Fig. 11.11):

the membrane in which the respiratory chain is localized is impermeable to protons and hydroxyl ions;

reducing equivalents donated by substrates are transferred to Cell/30.html">The Plasma Membrane or The inner mitochondrial membrane. The arrangement of the respiratory chain components within the membrane is such that during electron Transport from the substrate to oxygen, protons are bound on the inside of the membrane and released to the outside. One can imagine that electrons follow a zig-zag path through the membrane, thereby transporting protons from the inside to the outside. For instance, The oxidation of NADH in the respiratory chain is accompanied by the release of protons on the outer side of the membrane (Fig. 11.11,a)

Class="center">Fig. 11.11. Chemiosmotic hypothesis of Oxidative Phosphorylation in Mitochondria

the consequence of this proton translocation is the establishment across the membrane of a pH gradient (ΔpH) and an electrical potential gradient (Δψ), with a positive potential on the outside and a negative potential on the inside (Fig. 11.11,b). Consequently, the interior of mitochondria or the inner side of the bacterial plasma membrane is electronegative relative to the external environment and exhibits a higher pH. The transmembrane difference in electrical charge (electrical potential) and the chemical gradient of hydrogen ion concentration (pH gradient) create a transmembrane electrochemical proton gradient (proton-motive force, proton potential). The proton-motive force may be generated solely by the pH gradient or solely by the electrical charge gradient;

to dissipate the potential that arises at specific sites of the plasma membrane (or inner mitochondrial membrane), an H⁺-dependent ATP synthase enzyme complex is embedded, which catalyzes the synthesis or Hydrolysis of ATP (Fig. 11.11, c). The ATP synthesis reaction is coupled to The transport of protons down the proton potential gradient (from the outer side of the membrane to the inner), resulting in its dissipation. The energy generated in this process (the energy produced by the proton-motive force) is stored in the high-energy bonds of ATP.

The ATP hydrolysis reaction is accompanied by the translocation of protons against the gradient, resulting in the accumulation of protons outside the membrane and the generation or increase of Δμ_H⁺. A mutual interconversion of the two forms of energy takes place: Δμ_H⁺ ⇄ ATP. Therefore, it can be said that the ATP synthase complex acts as a "proton pump".

It should be noted that the energy of the proton-motive force is utilized not only for ATP synthesis. This form of energy powers many membrane-localized processes in Bacteria: Active Transport, flagellar motility, reverse electron transfer, and others. For certain processes, this form of energy is more convenient than ATP because the energy of the proton-motive force is not packaged into discrete portions like ATP, meaning there is no lower threshold for its formation. It can be formed and consumed under conditions where ATP synthesis is impossible.

Thus, microorganisms possess two forms of energy: energy in the form of ATP and energy of the proton-motive force. ATP is formed via substrate-level phosphorylation and phosphorylation during electron transfer. The energy of the proton-motive force is generated during electron transport in the respiratory chain or during ATP hydrolysis mediated by the ATP synthase enzyme complex.