Oxygen carriers standard potential

Table 9.3 Standard redox potentials of main electron carriers, total change in potential and AG° for oxidation of NADH by oxygen...

The mitochondrial respiratory chain consists of several components that catalyze the reduction of molecular oxygen in vivo. In the respiratory chain, electrons are transported through a series of electron carriers as shown in Fig. 39. This concept has been deduced based on their standard reduction potentials. The order of the reduction potentials of the coenzymes investigated here i.e., CoQ>FMN and FAD>NAD is found to be consistent with the sequence of these coenzymes in the respiratory chain. In the reduction of these coenzymes, the parts being reduced at the electrode surface are the same as those in vivo. 

To interpret the data in this table for the purpose of electron transport, we need to look at the reduction potentials of the electron carriers involved. A reaction at the top of the table tends to occur as written if it is paired with a reaction that is lower down on the table. For example, we have already seen that the final step of the electron transport chain is the reduction of oxygen to water. This reaction is at the top of Table 20.1 with a reduction potential of 0.816 V, a very positive number. If this reaction were paired directly with NAD+/NADH, what would happen The standard reduction potential for NAD+ forming NADH is given near the bottom of the table. Its reduction potential is -0.320 V. 

The sequence of the carriers in the respiratory chain has been deduced from their redox potentials, the use of inhibitors of electron transport (Section 13.7) and enzyme specificities. Since electrons normally flow from more electronegative to more electropositive values, the standard redox potentials of the carriers should become progressively more positive towards oxygen. Figure 13.3 shows the established order of the complexes but within some complexes the order of carrier participation requires elucidation. 

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