8885dc19690-7503/1/0411:32 AM Page693mac76mac76:385 19.1 Electron-Transfer Reactions in Mitochondria potential is therefore that of the particular flavoprotein not that of isolated fad or fmn. The flavin nucleotide should be considered part of the flavoproteins active CHO CH2-CH=C—CH210-H site rather than a reactant or product in the electron- (fully oxidize transfer reaction. Because flavoproteins can participate CH. in either one- or two-electron transfers, they can serve as intermediates between reactions in which two elec H++e trons are donated (as in dehydrogenations)and those in which only one electron is accepted (as in the reduction O° of a quinone to a hydroquinone, described below) CHO Electrons Pass through a Series CH&O of Membrane- Bound carriers OH The mitochondrial respiratory chain consists of a series H +e of sequentially acting electron carriers, most of which are integral proteins with prosthetic groups capable of accepting and donating either one or two electrons CHo Three types of electron transfers occur in oxidative phosphorylation: (1) direct transfer of electrons, as in (fully reduced the reduction of Fe to Fe;(2) transfer as a hydro- gen atom(H +e and 3) transfer as a hydride ion CH), which bears two electrons. The term reducing FIGURE 19-2 Ubiquinone (Q, or coenzyme Q). Complete reductio equivalent is used to designate a single electron equiv- of ubiquinone requires two electrons and two protons, and occurs in alent transferred in an oxidation-reduction reaction two steps through the semiquinone radical intermediate In addition to NAD and flavoproteins, three types of electron-carrying molecules function in the res- piratory chain: a hydrophobic quinone (ubiquinone) and near 560 nm in type b, and near 550 nm in type c. To two different types of iron-containing proteins(cyto- distinguish among closely related cytochromes of one chromes and iron-sulfur proteins ). Ubiquinone (also type, the exact absorption maximum is sometimes used called coenzyme Q, or simply Q) is a lipid-soluble ben- in the names, as in cytochrome b56 coquinone with a long isoprenoid side chain (Fig. 19-2) The heme cofactors of a and b cytochromes are The closely related compounds plastoquinone (of plant tightly, but not covalently, bound to their associated pro chloroplasts) and menaquinone (of bacteria) play roles teins; the hemes of c-type cytochromes are covalently analogous to that of ubiquinone, carrying electrons in attached through Cys residues(Fig. 19-3). As with the membrane-associated electron-transfer chains. Ubiqui- flavoproteins, the standard reduction potential of the none can accept one electron to become the semi- heme iron atom of a cytochrome depends on its inter quinone radical (Qh) or two electrons to form ubiquinol action with protein side chains and is therefore differ (QH2)(Fig. 19-2) and, like flavoprotein carriers, it can ent for each cytochrome. The cytochromes of type a act at the junction between a two-electron donor and a and b and some of type c are integral proteins of the one-electron acceptor Because ubiquinone is both small inner mitochondrial membrane. One striking exception and hydrophobic, it is freely diffusible within the lipid is the cytochrome c of mitochondria, a soluble protein bilayer of the inner mitochondrial membrane and can that associates through electrostatic interactions with huttle reducing equivalents between other, less mobile the outer surface of the inner electron carriers in the membrane. and because it car- membrane. We encountered ries both electrons and protons, it plays a central role cytochrome c in earlier dis in coupling electron flow to proton movement. cussions of protein structure The cytochromes are proteins with characteristic (see Fig. 4-18) strong absorption of visible light, due to their iron- In iron-sulfur proteins, containing heme prosthetic groups (Fig. 19-3). Mito- first discovered by Helmut chondria contain three classes of cytochromes, desig- Beinert, the iron is present not nated a, b, and c, which are distinguished by differences in heme but in association in their light-absorption spectra. Each type of cyto- with inorganic sulfur atoms or chrome in its reduced (Fe) state has three absorp- with the sulfur atoms of Cys tion bands in the visible range(Fig. 19-4). The longest- residues in the protein, or wavelength band is near 600 nm in type a cytochromes, both. These iron-sulfur(Fe-s) Helmut Beinertpotential is therefore that of the particular flavoprotein, not that of isolated FAD or FMN. The flavin nucleotide should be considered part of the flavoprotein’s active site rather than a reactant or product in the electron￾transfer reaction. Because flavoproteins can participate in either one- or two-electron transfers, they can serve as intermediates between reactions in which two elec￾trons are donated (as in dehydrogenations) and those in which only one electron is accepted (as in the reduction of a quinone to a hydroquinone, described below). Electrons Pass through a Series of Membrane-Bound Carriers The mitochondrial respiratory chain consists of a series of sequentially acting electron carriers, most of which are integral proteins with prosthetic groups capable of accepting and donating either one or two electrons. Three types of electron transfers occur in oxidative phosphorylation: (1) direct transfer of electrons, as in the reduction of Fe3
to Fe2
; (2) transfer as a hydro￾gen atom (H

e); and (3) transfer as a hydride ion (:H), which bears two electrons. The term reducing equivalent is used to designate a single electron equiv￾alent transferred in an oxidation-reduction reaction. In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the res￾piratory chain: a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cyto￾chromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben￾zoquinone with a long isoprenoid side chain (Fig. 19–2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiqui￾none can accept one electron to become the semi￾quinone radical (QH) or two electrons to form ubiquinol (QH2) (Fig. 19–2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it car￾ries both electrons and protons, it plays a central role in coupling electron flow to proton movement. The cytochromes are proteins with characteristic strong absorption of visible light, due to their iron￾containing heme prosthetic groups (Fig. 19–3). Mito￾chondria contain three classes of cytochromes, desig￾nated a, b, and c, which are distinguished by differences in their light-absorption spectra. Each type of cyto￾chrome in its reduced (Fe2
) state has three absorp￾tion bands in the visible range (Fig. 19–4). The longest￾wavelength band is near 600 nm in type a cytochromes, near 560 nm in type b, and near 550 nm in type c. To distinguish among closely related cytochromes of one type, the exact absorption maximum is sometimes used in the names, as in cytochrome b562. The heme cofactors of a and b cytochromes are tightly, but not covalently, bound to their associated pro￾teins; the hemes of c-type cytochromes are covalently attached through Cys residues (Fig. 19–3). As with the flavoproteins, the standard reduction potential of the heme iron atom of a cytochrome depends on its inter￾action with protein side chains and is therefore differ￾ent for each cytochrome. The cytochromes of type a and b and some of type c are integral proteins of the inner mitochondrial membrane. One striking exception is the cytochrome c of mitochondria, a soluble protein that associates through electrostatic interactions with the outer surface of the inner membrane. We encountered cytochrome c in earlier dis￾cussions of protein structure (see Fig. 4–18). In iron-sulfur proteins, first discovered by Helmut Beinert, the iron is present not in heme but in association with inorganic sulfur atoms or with the sulfur atoms of Cys residues in the protein, or both. These iron-sulfur (Fe-S) 19.1 Electron-Transfer Reactions in Mitochondria 693 O• CH C H R OH CH3 CH3O (CH2 O CH3O CH3 CH2)10 Ubiquinone (Q) (fully oxidized) Semiquinone radical ( •QH) Ubiquinol (QH2) (fully reduced) H

e O CH3O CH3 CH3O H

e OH OH R CH3O CH3 CH3O FIGURE 19–2 Ubiquinone (Q, or coenzyme Q). Complete reduction of ubiquinone requires two electrons and two protons, and occurs in two steps through the semiquinone radical intermediate. Helmut Beinert 8885d_c19_690-750 3/1/04 11:32 AM Page 693 mac76 mac76:385_reb: