Thermodynamics of electron transport
In the electron transport chain (ETC), particles move from a negative redox potential (-0.4 V) to a positive one (+0.8 V). As electrons pass along the chain, their free energy decreases. At certain stages, a sharp drop in free energy occurs—it is precisely at these points (coupling sites) that optimal conditions for ATP molecule synthesis are created.
The energy released during the passage of electrons through Complexes I, III, and IV is used for the active transport of hydrogen protons ($H^+$) from the mitochondrial matrix into the intermembrane space. As a result, a proton electrochemical potential ($\Delta\mu H^+$) is generated, which is based on the difference in proton concentrations on both sides of the inner membrane.
Mechanism of ATP synthase and nucleotide exchange
When the proton gradient reaches a critical level, ATP synthase (Complex V) comes into play. A special ion channel opens within this enzyme. Protons rush down the gradient from the intermembrane space back into the mitochondrial matrix.
The energy of the electrochemical potential is transformed into the energy of chemical bonds. The phosphorylation reaction takes place: ADP + $P_i \rightarrow$ ATP
For the newly synthesized ATP to be utilized by the cell in energy-requiring processes, it must be exported into the cytoplasm. This is performed by ATP-ADP translocase, a carrier protein embedded in the inner membrane of mitochondria. It operates via an antiporter mechanism: it exports an ATP molecule from the matrix while simultaneously importing an ADP molecule for subsequent resynthesis.
Oxidative phosphorylation ratio (P/O)
The efficiency of ATP synthesis is evaluated using the P/O ratio. This is the ratio of the amount of inorganic phosphate consumed to the amount of oxygen atoms that were absorbed and reduced to water.
The values of this ratio depend on which coenzyme donates hydrogen to the respiratory chain:
- P/O = 3 (Maximum value): Observed when hydrogen is supplied by NADH. Electrons pass through three coupling sites (Complexes I, III, and IV), each generating a gradient sufficient for the synthesis of one ATP molecule.
- P/O = 2: Occurs if hydrogen is supplied by FAD-dependent dehydrogenases (e.g., in the succinate oxidation reaction). Electrons bypass Complex I, are transferred directly to ubiquinone, and pass through only two coupling sites—Complexes III and IV.
Note: In a living cell, actual P/O values are always slightly lower than the theoretical maxima (3 and 2), as part of the electrochemical potential energy is inevitably dissipated as heat.
Reactions at coupling sites
Each of the three key complexes performs its specific reaction accompanied by proton pumping:
- Complex I (NADH dehydrogenase). Oxidizes NADH and reduces ubiquinone (Q) to $QH_2$.
- Complex III (Q-cytochrome c oxidoreductase). Transfers electrons from ubiquinol ($QH_2$) to cytochrome c.
- Complex IV (Cytochrome c oxidase). Transfers electrons from cytochrome c directly to oxygen, reducing it to form a water molecule.
Complex II (Succinate dehydrogenase) lacks proton pump function, therefore it does not participate in gradient generation and is not a coupling site.
5 stages of energy transformation
Energy transformation in the body from a substrate to a high-energy bond is a strict sequence of five stages:
- Chemical bond energy of oxidized substances (substrates).
- Electron energy within reduced coenzymes (NADH and $FADH_2$).
- Energy of electrons transported along the ETC to oxygen (redox potential).
- Energy of the transmembrane electrochemical potential ($\Delta\mu H^+$ gradient).
- Energy of high-energy chemical bonds in the ATP molecule.
Briefly: answers to topical questions
Which tissue respiration inhibitors block Complex I?
Complex I (NADH dehydrogenase) is blocked by tissue respiration inhibitors such as rotenone and amobarbital.
These substances interrupt the respiratory chain at the very beginning by blocking electron transfer from coenzyme NADH to ubiquinone. As a result of their addition, mitochondrial respiration (oxygen consumption) drops sharply or ceases entirely. This block can be bypassed by adding succinate. It is oxidized by succinate dehydrogenase (Complex II), and electrons are delivered to ubiquinone, bypassing the blocked first complex and restoring respiration.
What are uncouplers of oxidative phosphorylation and how do they work?
Uncouplers are substances that disrupt the coupling between tissue respiration and phosphorylation, causing energy to be dissipated as heat.
Upon the addition of 2,4-dinitrophenol (2,4-DNP), the respiration rate increases to a maximum: it removes the limiting factor—the proton gradient—and the ETC operates at maximum idle speed.
Examples of uncouplers:
- 2,4-dinitrophenol (2,4-DNP).
- Free fatty acids — during cold adaptation, they act as uncouplers of tissue respiration and oxidative phosphorylation by participating in proton transport across the mitochondrial membrane.
- Unconjugated (indirect) bilirubin — acts as an uncoupler of oxidative phosphorylation and decreases ATP synthesis in neurons.
What else to study on this topic
- Detailed chemical structure of Complexes I, III, IV, and V
- Mechanism of the rotary motor inside ATP synthase
- Structure and redox properties of NADH and FADH2 coenzymes
- Role of ubiquinone (coenzyme Q) and cytochrome c as mobile carriers
- Uncoupling of tissue respiration and oxidative phosphorylation
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