Leakage of electrons from the electron transport chain (ETC) gives rise to reactive oxygen species (ROS) like superoxide (O2−·), hydrogen peroxide (H2O2), and hydroxyl radicals (OH·). These ROS cause damage to DNA, proteins, and initiate lipid peroxidation. To counteract the harmful effects of ROS, the cell employs enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase (refer to page 148 for more details).

During electron transport, the transfer of electrons from an electron donor (reducing agent) to an electron acceptor (oxidizing agent) along the ETC results in the release of free energy. This free energy is utilized to pump H+ ions at Complexes I, III, and IV. The electrons can be transferred in different forms, including as hydride ions to NAD+, as hydrogen atoms to FMN, CoQ, and FAD, or as electrons to cytochromes (1).

Redox reactions involve the oxidation (loss of electrons) of one substance accompanied by the reduction (gain of electrons) of another substance. For instance, Figure 6.11 illustrates the oxidation of NADH to NAD+ by NADH dehydrogenase at Complex I, along with the reduction of FMN to FMNH2. These reactions can be expressed as two separate half reactions, one representing oxidation and the other representing reduction (see Fig. 6.11). NAD+ and NADH form a redox pair, as do FMN and FMNH2. The tendency of a redox pair to lose electrons can be quantified using a constant called the standard reduction potential, denoted as E0 (in volts) (1).

The standard reduction potential allows us to order various redox pairs based on their E0 values, ranging from most negative to most positive. A more negative E0 indicates a greater tendency of the reductant member of the pair to lose electrons, while a more positive E0 indicates a greater tendency of the oxidant member of the pair to accept electrons. Consequently, electrons flow from the redox pair with the most negative E0 to the one with the most positive E0. Figure 6.12 presents the E0 values for some components of the ETC, arranged in order of increasingly positive E0 values (2).

The change in standard free energy (∆G0) is directly related to the magnitude of the change in E0, given by the equation: ∆G0 = -nF∆E0, where n represents the number of electrons transferred (1 for a cytochrome, 2 for NADH, FADH2, and CoQ) and F is the Faraday constant (23.1 kcal/volt mol) (3).

The phosphorylation of ADP to ATP has a ∆G0 of +7.3 kcal/mol. As a pair of electrons is transported from NADH to O2 through the ETC, 52.6 kcal of energy is released. This provides more than enough energy to generate three ATP molecules from three ADP and three Pi (3 × 7.3 = 21.9 kcal/mol). This energy production is sometimes expressed as a P/O ratio (ATP synthesized per O atom reduced) of 3:1. The remaining calories are utilized for additional reactions or released as heat. For FADH2, the P/O ratio is 2:1 due to bypassing Complex I (4).

The transfer of electrons along the ETC is energetically favorable due to the strong electron-donating ability of NADH and the high affinity of O2 for accepting electrons. However, this electron flow does not directly result in ATP synthesis.

According to the chemiosmotic hypothesis (also known as the Mitchell hypothesis), the free energy generated by the electron transport through the ETC is used to drive the synthesis of ATP from ADP and Pi (1).
Leakage of electrons from the ETC produces reactive oxygen species (ROS), such as superoxide (O2−·), hydrogen peroxide (H2O2), and hydroxyl radicals (OH·). ROS damage DNA and proteins and cause lipid peroxidation. Enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase are cellular defenses against ROS (see p. 148).
D. Free energy release during electron transport
The free energy released as electrons are transferred along the ETC from an electron donor (reducing agent or reductant) to an electron acceptor (oxidizing agent or oxidant) is used to pump H+ at Complexes I, III, and IV. [Note: The electrons can be transferred as hydride ions to NAD+; as hydrogen atoms to FMN, CoQ, and FAD; or as electrons to cytochromes.] 1.
Redox pairs: Oxidation (loss of electrons) of one substance is always accompanied by reduction (gain of electrons) of a second. For example, Figure 6.11 shows the oxidation of NADH to NAD+ by NADH dehydrogenase at Complex I, accompanied by the reduction of FMN, the prosthetic group, to FMNH2. Such redox reactions can be written as the sum of two separate half reactions, one an oxidation and the other a reduction (see Fig. 6.11). NAD+ and NADH form a redox pair, as do FMN and FMNH2. Redox pairs differ in their tendency to lose electrons. This tendency is a characteristic of a particular redox pair and can be quantitatively specified by a constant, E0 (the standard reduction potential), with units in volts.
2.
Standard reduction potential: The E0 of various redox pairs can be ordered from the most negative E0 to the most positive. The more negative the E0 of a redox pair, the greater the tendency of the reductant member of that pair to lose electrons. The more positive the E0, the greater the tendency of the oxidant member of that pair to accept electrons. Therefore, electrons flow from the pair with the more negative
E0 to that with the more positive E0. The E0 values for some members of the ETC are shown in Figure 6.12. [Note: The components of the chain are arranged in order of increasingly positive E0 values.] 3. Relationship of ∆G0 to ∆E0: The ∆G0 is related directly to the magnitude of the change in E0: where n = number of electrons transferred (1 for a cytochrome, 2 for NADH, FADH2, and CoQ)
F = Faraday constant (23.1 kcal/volt mol) ∆E0 = E0 of the electron-accepting pair minus the E0 of the electron-donating pair ∆G0 = change in the standard free energy 4. ∆G0 of ATP: The ∆G0 for the phosphorylation of ADP to ATP is +7.3 kcal/mol. The transport of a pair of electrons from NADH to O2 through the ETC releases 52.6 kcal. Therefore, more than sufficient energy is available to produce three ATP from three ADP and three Pi (3 × 7.3 = 21.9 kcal/mol), sometimes expressed as a P/O ratio (ATP made per O atom reduced) of 3:1. The remaining calories are used for ancillary reactions or released as heat. [Note: The P:O for FADH2 is 2:1 because
Complex I is bypassed.]
VI. PHOSPHORYLATION OF ADP TO ATP
The transfer of electrons down the ETC is energetically favored because NADH is a strong electron donor and O2 is an avid electron acceptor. However, the flow of electrons does not directly result in ATP synthesis.
A. Chemiosmotic hypothesis
The chemiosmotic hypothesis (also known as the Mitchell hypothesis) explains how the free energy generated by the transport of electrons by the ETC is used to produce ATP from ADP + Pi.
1.