Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter II. GENERAL LAWS OF METABOLISM

CHAPTER 9. BIOENERGETIC PROCESSES: ELECTRON TRANSPORT; OXIDATIVE PHOSPHORYLATION IN MITOCHONDRIA

9.4. OXIDATIVE PHOSPHORYLATION AND MITOCHONDRIAL ATP SYNTHASE

Oxidative Phosphorylation is the process by which the chemical energy released during Electron transport along the mitochondrial respiratory (electron transport) chain is captured and used for the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (P_i).

The synthesis of ATP from ADP and inorganic phosphate P_i is termed coupling between Respiration (electron transport in Cell/35.html">Mitochondria) and oxidative phosphorylation.

Class="center">Release of chemical energy in the Respiratory Chain and sites of ATP formation

Electron transport in the mitochondrial respiratory chain from primary Donors of reducing equivalents (substrates of Biological Oxidation) to molecular oxygen is accompanied by a decrease in Free energy (ΔG), the value of which can be calculated using the equation:

where: n is the number of transported electrons (equal to two during The formation of a Water molecule);

F is the Faraday constant [23,062 kcal/V · mol];

ΔE₀' is the standard potential difference between the electron donor and electron acceptor systems (at pH = 7.0).

Taking into account the data in Table 9.1, let us calculate the release of chemical energy under conditions of The transfer of two electrons from pyridine-dependent dehydrogenases to oxygen:

Summation of the two equations yields:

which corresponds to

The synthesis of one ATP molecule from ADP and P_i requires an expenditure of chemical energy equal to + 7.3 kcal (+ 30.5 kJ). Obviously, The energy released during electron transport in the mitochondrial respiratory chain is sufficient for the synthesis of several ATP molecules. Direct biochemical studies have proven that during The oxidation of substrates via NADH-coenzyme Q reductase, 3 ATP molecules are formed, while the action of succinate-coenzyme Q reductase yields 2 ATP molecules.

The oxidative phosphorylation coefficient is The ratio of The amount of bound (esterified) inorganic phosphate (in moles) to the amount of oxygen taken up by mitochondria (in moles) (designated as P_i/O) — which is quantitatively equal to the number of ATP molecules formed per pair of reducing equivalents transferred to one atom of oxygen, i.e., ATP/O — Table 9.2.

Table 9.2. Values of the oxidative phosphorylation coefficient during the oxidation of various substrates in mitochondria

Sites of coupling between Electron Transport and oxidative phosphorylation

The synthesis of ATP from ADP and P_i can occur only at specific sites of the Mitochondrial Electron Transport chain where the magnitude of the chemical energy released during The transport of a pair of electrons between two redox systems (Components of the respiratory chain) is sufficient for the synthesis of 1 ATP molecule (i.e., > 7.3 kcal or 30.5 kJ) — Table 9.3.

Table 9.3. Sites of the mitochondrial respiratory chain where the release of chemical energy is sufficient for the synthesis of an ATP molecule

The indicated segments of the Electron Transport Chain are referred to as the sites of coupling between respiration (electron transport) and oxidative phosphorylation, as shown in Figure 9.3.

Fig. 9.3. Localization of coupling sites (1, 2, 3) of respiration and oxidative phosphorylation in The electron transport chain of The inner mitochondrial membranes.

Chemiosmotic Theory of Oxidative Phosphorylation

The MOLECULAR MECHANISMS OF ATP generation during biological oxidation in mitochondria are explained by the chemiosmotic theory (proposed by P. Mitchell).

Fig. 9.4. Peter D. Mitchell (born 1920), British biochemist. Author of the chemiosmotic theory of oxidative and photosynthetic phosphorylation. Nobel Prize laureate (1978).

The main postulate of the chemiosmotic theory is that the coupling of electron transport in mitochondria with the biochemical system of ATP synthesis is driven by the electrochemical proton gradient (Δμ_Н+) generated during the functioning of the electron transport chain.

The chemiosmotic theory postulates that:

1. The functioning of the respiratory (electron transport) chain in the inner (coupling) membranes of mitochondria is accompanied by the generation of an electrochemical proton (H⁺) gradient across these membranes.

2. Individual components of the electron transport chain act as proton pumps that drive vectoral transport of protons (perpendicular to the membrane plane) directed from the matrix to the outer surface of the membrane.

3. The electrochemical proton potential across the coupling membranes, created by the action of the respiratory chain proton pumps, serves as the driving force for ATP synthesis from ADP and P_H.

4. There exists an enzyme system that utilizes

the energy of the electrochemical proton potential for ATP synthesis via proton translocation across the mitochondrial membrane from the outer surface to the matrix.

This enzyme system, which completes the proton cycle across the mitochondrial coupling membranes, is the proton ATPase (H⁺-ATPase) or ATP synthase.

ATP synthase is a protein with a quaternary Structure consisting of several protein subunits that form the F₀ and F₁ components (F₀F₁-ATPase).

5. Any physical, chemical, or biological factors that disrupt the integrity of the mitochondrial coupling membranes and dissipate the electrochemical gradient energy will impair ATP synthesis, thus acting as uncouplers of electron transport and oxidative phosphorylation.

Thus, According to the chemiosmotic theory, coupling between electron transfer in the respiratory chain and ATP synthesis is achieved through the formation of an H⁺ concentration gradient between the two surfaces of the mitochondrial membrane during the operation of proton pumps. ATP synthase, by transporting protons back (down their electrochemical gradient), releases chemical energy which is utilized to form the high-energy bonds of ATP.

The scheme of the proton cycle across the mitochondrial coupling membranes as the driving force of Oxidative phosphorylation is presented in Figure 9.5.

Fig. 9.5. Chemiosmotic Mechanism of coupling electron transport in the respiratory chain with ATP synthesis by ATP synthase via the action of vectorially oriented (perpendicular to the membrane plane) proton pumps.

Fig. 9.6. Schematic representation of the intra-membrane Organization of electron transport complexes in the form of "loops" that cross the membrane twice.

The ability of mitochondrial electron carriers to translocate protons across the membrane is determined by the Specific features of their intra-membrane topography. It is believed that the respiratory chain is embedded in the coupling membrane as three redox "loops" corresponding to the three electron-transfer complexes — I, III, and IV. Each pair of reducing equivalents (2H⁺ + 2e^-), passing through a "loop", transports two H⁺ ions from the matrix into the external medium. The reverse transport of H⁺ ions through the proton channel of ATP synthase channels this electrochemical energy into ATP synthesis: