▲ FIGURE 8-15 Heme and iron-sulfur prosthetic groups in the respiratory (electron-transport) chain. (a) Heme portion of cytochromes bL and bH, which are components of the CoQH2-cytochrome c reductase complex. The same porphyrin ring (yellow) is present in all hemes. The chemical substituents attached to the porphyrin ring differ in the other cytochromes in the respiratory chain. All hemes accept and release one electron at a time. (b) Dimeric iron-sulfur cluster (2Fe-2S). Each Fe atom is bonded to four S atoms: two are inorganic sulfur and two are in cysteine side chains of the associated protein. (Note that only the two inorganic S atoms are counted in the chemical formula.) All Fe-S clusters accept and release one electron at a time.

In the respiratory chain, electrons move through the cy-tochromes in the following order: b, c1, c, a, and a3 (see Figure 8-13). The various cytochromes have slightly different heme groups and axial ligands, which generate different environments for the Fe ion. Therefore, each cy-tochrome has a different reduction potential, or tendency to accept an electron—an important property dictating the unidirectional electron flow along the chain. Because the heme ring in cytochromes consists of alternating double-and single-bonded atoms, a large number of resonance forms exist, and the extra electron is delocalized to the heme carbon and nitrogen atoms as well as to the Fe ion. All the cytochromes, except cytochrome c, are components of multiprotein complexes in the inner mitochondrial membrane. Although cytochrome c comprises a heme-protein complex, it moves freely by diffusion in the intermembrane space.

Iron-sulfur clusters are nonheme, iron-containing prosthetic groups consisting of Fe atoms bonded both to inorganic S atoms and to S atoms on cysteine residues on a protein (Figure 8-15b). Some Fe atoms in the cluster bear a + 2 charge; others have a +3 charge. However, the net charge of each Fe atom is actually between +2 and +3 because electrons in the outermost orbits are dispersed among the Fe atoms and move rapidly from one atom to another. Iron-sulfur clusters accept and release electrons one at a time; the additional electron is also dispersed over all the Fe atoms in the cluster.

Coenzyme Q (CoQ), also called ubiquinone, is the only electron carrier in the respiratory chain that is not a protein-bound prosthetic group. It is a carrier of hydrogen atoms, that is, protons plus electrons. The oxidized quinone form of CoQ can accept a single electron to form a semiquinone, a charged free radical denoted by CoQ--. Addition of a second electron and two protons to CoQ~- forms dihydroubiquinone (CoQH2), the fully reduced form (Figure 8-16). Both CoQ and CoQH2 are soluble in phospholipids and diffuse freely in the inner mitochondrial membrane.

Ubiquinone (CoQ)

(oxidized form)

Dihydroquinone (CoQH2)

(fully reduced form)





Semiquinone (CoQ-)

(free radical)

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