8885dc19690-7503/1/0411:32 AM Page695mac76mac76:385 19.1 Electron-Transfer Reactions in Mitochondria Protein FIGURE 19-5 Iron-sulfur centers. The Fes centers of iron-sulfur proteins may be as simple as (a), with a single Fe ion surrounded by the S atoms of four Cys residues. Other centers include both and Cys S atoms, as in(b)2Fe-2S or (c) d) The ferredoxin of the cyanobacterium Anabaena 7120 has one Fe-25 center(PDB ID 1FRD) Fe is red, inorganic S2 is yellow, and the S of Cys is orange. (Note that in these designations only the inorganic S atoms are counted. For example, in the 2Fe-2S center (b), each Fe ion is actually surrounded by four S atoms. )The exact standard reduction potential of the iron in these centers depends on the type of center and its interaction with the associated protein First, the standard reduction potentials of the in- Q- cytochrome b - cytochrome C1 - cytochrome dividual electron carriers have been determined ex- C- cytochrome a -)cytochrome a3-O2. Note, how- perimentally (Table 19-2). We would expect the carri- ever, that the order of standard reduction potentials is ers to function in order of increasing reduction not necessarily the same as the order of actual reduc potential, because electrons tend to flow spontaneously tion potentials under cellular conditions, which depend from carriers of lower e to carriers of higher E. The on the concentration of reduced and oxidized forms order of carriers deduced by this method is NADH-)(p 510). A second method for determining the sequence TABLE 19-2 Standard Reduction Potentials of Respiratory Chain and Related Electron Carriers Redox reaction(half-reaction) -0.414 NAD++H++2 NADH 0.320 NADPH -0.324 NADH dehydrogenase(FMN)+ 2H+ 2e -NADH dehydrogenase(FMNH2) Ubiquinone 2HT 2e ubiquinol Cytochrome b(Fea+)+ 0.077 Cytochrome C,(Fe +)+e- cytochrome c,(Fe) Cytochrome c(Fe)+e-cytochrome c(Fe4) 0.254 Cytochrome a(Fe)+e- cytochrome a(Fe") Cytochrome a3(Fe)+e- cytochrome a3(Fe 0.35 202+2H++2e一→H20 0.8166First, the standard reduction potentials of the in￾dividual electron carriers have been determined ex￾perimentally (Table 19–2). We would expect the carri￾ers to function in order of increasing reduction potential, because electrons tend to flow spontaneously from carriers of lower E to carriers of higher E . The order of carriers deduced by this method is NADH → Q → cytochrome b → cytochrome c1 → cytochrome c → cytochrome a → cytochrome a3 → O2. Note, how￾ever, that the order of standard reduction potentials is not necessarily the same as the order of actual reduc￾tion potentials under cellular conditions, which depend on the concentration of reduced and oxidized forms (p. 510). A second method for determining the sequence 19.1 Electron-Transfer Reactions in Mitochondria 695 S (c) Cys Cys Fe S S S S S S S Fe Fe Fe Cys S Cys (b) Cys C S ys Fe S Fe S S Cys Cys S Cys S S S S Cys (a) Cys Cys Fe Protein (d) FIGURE 19–5 Iron-sulfur centers. The Fe-S centers of iron-sulfur proteins may be as simple as (a), with a single Fe ion surrounded by the S atoms of four Cys residues. Other centers include both inorganic and Cys S atoms, as in (b) 2Fe-2S or (c) 4Fe-4S centers. (d) The ferredoxin of the cyanobacterium Anabaena 7120 has one 2Fe-2S center (PDB ID 1FRD); Fe is red, inorganic S2 is yellow, and the S of Cys is orange. (Note that in these designations only the inorganic S atoms are counted. For example, in the 2Fe-2S center (b), each Fe ion is actually surrounded by four S atoms.) The exact standard reduction potential of the iron in these centers depends on the type of center and its interaction with the associated protein. TABLE 19–2 Standard Reduction Potentials of Respiratory Chain and Related Electron Carriers Redox reaction (half-reaction) E  (V) 2H

2e 8n H2 0.414 NAD

H

2e 8n NADH 0.320 NADP

H

2e 8n NADPH 0.324 NADH dehydrogenase (FMN)
2H

2e 8n NADH dehydrogenase (FMNH2) 0.30 Ubiquinone
2H

2e 8n ubiquinol 0.045 Cytochrome b (Fe3
)
e 8n cytochrome b (Fe2
) 0.077 Cytochrome c1 (Fe3
)
e 8n cytochrome c1 (Fe2
) 0.22 Cytochrome c (Fe3
)
e 8n cytochrome c (Fe2
) 0.254 Cytochrome a (Fe3
)
e 8n cytochrome a (Fe2
) 0.29 Cytochrome a3 (Fe3
)
e 8n cytochrome a3 (Fe2
) 0.35 1 2 O2
2H

2e 8n H2O 0.8166 8885d_c19_690-750 3/1/04 11:32 AM Page 695 mac76 mac76:385_reb: