6.2 The electron transport chain

Subsections:
6.2.1 Redox cofactors in the electron transport chain
6.2.2 The redox potential of a cofactor measures its affinity for electrons
6.2.3 Hydrogen and electron carriers
6.2.4 The ubiquinone cycle
6.2.5 Cytochrome C oxidase (complex IV)

Figure 6.2-1 shows structural sketches of the four protein complexes that form the respiratory chain. Each of the four complexes has a specific role in the electron transport process:

Complex I accepts hydrogen from NADH + H⁺ and is therefore also called NADH dehydrogenase. The NADH is oxidized back to NAD⁺ and thereby readied for the next round of reduction in the TCA or by pyruvate dehydrogenase. The two hydrogen protons are expelled, as are apparently two additional protons. The electrons are thereafter transferred to the small carrier molecule ubiquinone (cf. Figure 6.2.1-1c).
Complex II accepts hydrogen from succinate. It was mentioned before (cf. section 5.2.1) that it is identical with succinate dehydrogenase, which illustrates that the two pathways are really functionally one. The electrons are again transferred to coenzyme Q, but apparently no proton extrusion occurs at complex II.
Complex III reoxidizes coenzyme Q and expels protons. According to the coenzyme Q cycle model presented below, four protons are being expelled at this stage for each electron transported, but in some sources the number of protons are expelled is given as two. This example shows that there is still quite a bit of uncertainty about the mechanistic details of the electron transport chain. The electrons are being delivered to the small electron carrier protein cytochrome C.
Complex IV reoxides cytochrome C and is therefore also called cytochrome C oxidase. The electrons are transferred to oxygen, and the considerable free energy associated with this electron transfer step is utilized to expel up to 4 protons from the mitochondrial matrix.