Tissue respiration and oxidative phosphorylation

Tissue respiration: General characteristics

Tissue respiration is a set of oxidation reactions of organic substrates in which hydrogen atoms are transferred to molecular oxygen to form water and release energy. About 60% of this energy is dissipated as heat, and about 40% is stored in high-energy ATP bonds during oxidative phosphorylation.

The removal of hydrogen from substrates is carried out by dehydrogenase enzymes, localized predominantly in the mitochondrial matrix. According to the type of coenzyme, they are divided into two groups:

NAD-dependent dehydrogenases — the NAD⁺ coenzyme is loosely bound to the protein and easily dissociates in its reduced form, NADH (for example, during the oxidation of malate to oxaloacetate, isocitrate, glutamate, pyruvate);
FAD-dependent dehydrogenases — the FAD coenzyme is covalently bound to the enzyme, therefore a second hydrogen acceptor is required — ubiquinone (CoQ). A classic example is succinate dehydrogenase, which oxidizes succinate to fumarate with the formation of FADH₂.

Primary hydrogen donors — catabolic products: pyruvate, isocitrate, α-ketoglutarate, malate, glutamate for NAD-dependent enzymes; succinate and acyl-CoA for FAD-dependent ones.

Electron transport chain

The transfer of electrons from reduced coenzymes to oxygen is carried out by a system of carriers embedded in the inner mitochondrial membrane and forming the electron transport chain (ETC). It consists of five complexes:

Complex I — NADH dehydrogenase (NADH:ubiquinone oxidoreductase);
Complex II — succinate dehydrogenase;
Complex III — CoQH₂ dehydrogenase (cytochrome bc₁);
Complex IV — cytochrome c oxidase;
Complex V — ATP synthase.

The components are arranged in order of increasing redox potential — from low in NADH to high (about +0.8 V) in oxygen. This ensures a unidirectional flow of electrons and stepwise energy release.

Proton gradient and ATP synthesis

The transfer of electrons through complexes I, III, and IV is coupled with the translocation of protons from the matrix into the intermembrane space. As a result, an electrochemical proton potential Δμ_H⁺ is formed, consisting of chemical (concentration) and electrical components, with the proton gradient playing the leading role.

Upon reaching a critical level of Δμ_H⁺, ATP synthase (Complex V) is activated: protons return to the matrix through its channel (F₀ subunit), and the energy of their movement is used by the catalytic F₁ subunit to phosphorylate ADP to ATP. This coupling of electron transport and ATP synthesis via the proton gradient is described by Mitchell's chemiosmotic theory.

Oxidative phosphorylation coefficient (P/O) — the number of ATP molecules synthesized per pair of electrons transferred to one oxygen atom:

during NADH oxidation, P/O ≈ 3 (electrons pass through complexes I, III, IV);
during FADH₂ oxidation, P/O ≈ 2 (electrons enter at the level of ubiquinone, bypassing complex I).

Respiratory control

The rate of tissue respiration and oxidative phosphorylation is strictly coupled to the energy demands of the cell. During active ATP consumption, the concentration of ADP in the matrix increases. ADP is a substrate for ATP synthase, which enhances its activity, reduces the proton gradient, relieves the inhibition of electron transport, and increases oxygen consumption and substrate oxidation. This dependence of the respiration rate on ADP concentration is called respiratory control.

Uncouplers and inhibitors

Uncouplers — lipophilic compounds that transport protons from the intermembrane space into the matrix, bypassing ATP synthase. They reduce Δμ_H⁺; while substrate oxidation and oxygen consumption continue, the energy is dissipated as heat. Uncouplers include:

2,4-dinitrophenol — a classic artificial uncoupler;
free fatty acids — natural uncouplers; the action of adrenaline activates lipase, increasing the concentration of fatty acids, which enhances heat production (an important thermoregulatory mechanism).

Specific ETC inhibitors:

barbiturates, rotenone — block Complex I (NADH dehydrogenase);
antimycin A — inhibits Complex III (CoQH₂ dehydrogenase);
cyanides, carbon monoxide (CO), hydrogen sulfide (H₂S) — block Complex IV (cytochrome c oxidase), halting oxygen reduction; cyanide poisoning develops rapidly and leads to death;
oligomycin — inhibits ATP synthase: protons accumulate in the intermembrane space, but ATP synthesis stops.

Key points

Tissue respiration — oxidation of substrates with hydrogen transfer to oxygen; ~40% of energy is stored in ATP, ~60% is dissipated as heat.
Dehydrogenases are divided into NAD-dependent and FAD-dependent; FAD is covalently bound to the protein and transfers hydrogen to ubiquinone.
The ETC of the inner mitochondrial membrane consists of five complexes arranged in order of increasing redox potential.
Complexes I, III, and IV pump protons into the intermembrane space, forming the electrochemical potential Δμ_H⁺.
ATP synthase uses the energy of the reverse proton flow to phosphorylate ADP (chemiosmotic coupling).
The P/O ratio is 3 for NADH and 2 for FADH₂.
Respiratory control — regulation of the respiration rate by ADP concentration.
Uncouplers (2,4-dinitrophenol, fatty acids) enhance heat production; inhibitors block specific ETC complexes or ATP synthase.

Tissue respiration and oxidative phosphorylation: electron transport chain