Tissue respiration: General characteristics
Tissue respiration is the set of reactions involving the oxidation of organic substrates by oxygen, yielding water, CO₂, and energy. The substrate acting as a hydrogen donor undergoes dehydrogenation; the detached electrons are sequentially transferred to molecular oxygen, which is reduced to water.
About 60% of the released energy is dissipated as heat, while the remaining ~40% is stored in high-energy ATP bonds during oxidative phosphorylation. Thus, respiration performs a dual function: ATP synthesis and body temperature maintenance.
Dehydrogenases and primary hydrogen donors
The removal of hydrogen from substrates is catalyzed by dehydrogenases, which are localized mainly in the mitochondrial matrix. Based on the type of coenzyme, they are divided into two groups:
- NAD⁺-dependent dehydrogenases — the NAD⁺ coenzyme is loosely bound, reduced to NADH, and dissociates from the enzyme. Example: oxidation of malate to oxaloacetate.
- FAD-dependent dehydrogenases — the FAD coenzyme is covalently bound to the protein, so a second hydrogen acceptor is required — ubiquinone (CoQ). Example: oxidation of succinate to fumarate by succinate dehydrogenase with the formation of FADH₂.
Primary hydrogen donors include catabolic products: pyruvate, isocitrate, α-ketoglutarate, malate, glutamate (for NAD⁺-dependent pathways), succinate, and acyl-CoA (for FAD-dependent pathways).
Electron transport chain
The ETC is a system of carriers embedded in the inner mitochondrial membrane. The components are arranged in order of increasing redox potential, which ensures a directed flow of electrons from NADH/FADH₂ (low redox potential) to oxygen (approximately +0.8 V).
There are five complexes:
- Complex I is NADH dehydrogenase (NADH-CoQ reductase);
- Complex II — succinate dehydrogenase;
- Complex III — CoQH₂ dehydrogenase (cytochrome bc₁);
- Complex IV is cytochrome c oxidase, which reduces O₂ to H₂O;
- Complex V is ATP synthase.
Mobile carriers between the complexes are ubiquinone (between I/II and III) and cytochrome c (between III and IV).
Proton gradient and chemiosmotic coupling
Electron transport at complexes I, III, and IV is accompanied by the release of energy, part of which is used to pump protons from the matrix into the intermembrane space. As a result, a proton electrochemical potential (Δμ̃H⁺) is formed, the main component of which is the proton gradient (ΔpH).
When a sufficient level of Δμ̃H⁺ is reached, ATP synthase is activated: protons return to the matrix through its proton channel, and the energy of their movement is used to synthesize ATP from ADP and inorganic phosphate. Structurally, ATP synthase consists of the membrane component F₀ (channel) and the matrix component F₁ (catalytic subunits that bind ADP and Pi).
Oxidative phosphorylation coefficient
The P/O ratio is the number of ATP molecules synthesized per oxygen atom reduced to water (transfer of two electrons):
- during the oxidation of NADH, electrons pass through complexes I, III, IV — P/O ≈ 3;
- during the oxidation of FADH₂, electrons enter at the level of CoQ, bypassing complex I — P/O ≈ 2.
Real P/O values are slightly lower than theoretical ones because part of the Δμ̃H⁺ energy is dissipated as heat.
Respiratory control
The rate of tissue respiration and phosphorylation are coupled and regulated by the concentration of ADP. During active ATP utilization in the cell, the level of ADP rises; it enters the mitochondria and serves as a substrate for ATP synthase. Enhanced ATP synthesis reduces the proton gradient, which stimulates the activity of ETC complexes and oxygen consumption. This dependence of the respiration rate on [ADP] is called respiratory control.
Uncouplers of oxidative phosphorylation
Uncouplers are substances that transport protons across the inner mitochondrial membrane, bypassing ATP synthase. They reduce Δμ̃H⁺, so the energy of oxidation is dissipated as heat, and ATP synthesis decreases.
- 2,4-dinitrophenol — a classic artificial uncoupler;
- free fatty acids are natural uncouplers.
When the body cools, the release of norepinephrine activates tissue lipase, increasing fatty acid levels; enhanced uncoupling increases heat production. This mechanism forms the basis of non-shivering thermogenesis in brown adipose tissue.
Respiratory chain inhibitors
- Barbiturates, rotenone inhibit complex I (NADH dehydrogenase);
- Antimycin A inhibits complex III (KoQH₂ dehydrogenase);
- Carbon monoxide (CO), hydrogen sulfide (H₂S), cyanides (CN⁻) inhibit complex IV (cytochrome c oxidase), blocking oxygen reduction; cyanide poisoning leads to rapid death;
- Oligomycin inhibits ATP synthase (complex V): protons accumulate in the intermembrane space but are not used for ATP synthesis.
Key points
- Tissue respiration is the oxidation of substrates with electron transfer to O₂ and the formation of H₂O, CO₂, and energy.
- Dehydrogenases are divided into NAD⁺- and FAD-dependent; FAD-dependent ones transfer hydrogen to ubiquinone.
- The inner mitochondrial membrane ETC consists of five complexes arranged in order of increasing redox potential.
- Complexes I, III, and IV pump protons into the intermembrane space, forming Δμ̃H⁺.
- ATP synthase uses the energy of the proton gradient to synthesize ATP from ADP and Pi.
- P/O = 3 for NADH and 2 for FADH₂.
- Respiratory control regulates the respiration rate via ADP concentration.
- Uncouplers (2,4-dinitrophenol, fatty acids) enhance heat production; ETC inhibitors (rotenone, antimycin A, cyanides, oligomycin) block ATP synthesis.