8885dc19690-7503/1/0411:32 AM Page694mac76mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation CHs CHECH CHs CHCH C -Fe- CH. CH2CO0 CH2CH2CO0 CH2CH2CO0 CH2 CHCO0 on protoporphyrin IX CHeCH FIGURE 19-3 Prosthetic groups of cytochromes. Each group consists of four five-membered CH。CH called a porphyrin. The four nitrogen atoms are CH3 CHs CHS ordinated with a central Fe ion either Ch, CH, Coo- cytochromes and in hemoglobin and myoglobin (see Fig. 4-17). Heme c is covalently bound he protein of cytochrome c through thioether CHO CHOCH.COO bonds to two Cys residues. Heme a, found in the a-type cytochromes, has a long isoprenoid tail attached to one of the five-membered rings. The conjugated double-bond system(shaded pink) of visible light by these hemes centers range from simple structures with a single Fe atom coordinated to four Cys-SH groups to more com- Reduced plex Fe-S centers with two or four Fe atoms (Fig. 19-5) Rieske iron-sulfur proteins (named after their dis coverer,John S Rieske) are a variation on this theme Oxidized in which one fe atom is coordinated to two his residues rather than two Cys residues. All iron-sulfur proteins participate in one-electron transfers in which one iron atom of the iron-sulfur cluster is oxidized or reduced At least eight Fe-s proteins function in mitochondrial electron transfer. The reduction potential of Fe-s pro- teins varies from -0.65 V to +0.45 V, depending on the microenvironment of the iron within the protein. In the overall reaction catalyzed by the mitochon drial respiratory chain, electrons move from NADH,suc- cinate, or some other primary electron donor through flavoproteins, ubiquinone, iron-sulfur proteins, and cy- Wavelength (nm) chromes, and finally to O. A look at the methods used to determine the sequence in which the carriers act is of cytochrome c (cyt c)in its ox instructive, as the same general approaches have been dized(red) and reduced(bl ns. Also labeled are the character- used to study other electron- transfer chains, such as stic a, B, and y bands of the reduced form. those of chloroplastscenters range from simple structures with a single Fe atom coordinated to four Cys OSH groups to more com￾plex Fe-S centers with two or four Fe atoms (Fig. 19–5). Rieske iron-sulfur proteins (named after their dis￾coverer, John S. Rieske) are a variation on this theme, in which one Fe atom is coordinated to two His residues rather than two Cys residues. All iron-sulfur proteins participate in one-electron transfers in which one iron atom of the iron-sulfur cluster is oxidized or reduced. At least eight Fe-S proteins function in mitochondrial electron transfer. The reduction potential of Fe-S pro￾teins varies from 0.65 V to
0.45 V, depending on the microenvironment of the iron within the protein. In the overall reaction catalyzed by the mitochon￾drial respiratory chain, electrons move from NADH, suc￾cinate, or some other primary electron donor through flavoproteins, ubiquinone, iron-sulfur proteins, and cy￾tochromes, and finally to O2. A look at the methods used to determine the sequence in which the carriers act is instructive, as the same general approaches have been used to study other electron-transfer chains, such as those of chloroplasts. 694 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Fe N N N N CH3 CH3 CH2CH2COO CH2 CH CH2 CH CH3 Heme A (in a-type cytochromes) Fe N N N N CH3 CH3 CH3 CH3 CH2CH2COO CH2 CH2 CHO CH2 CH2 CH Iron protoporphyrin IX (in b-type cytochromes) COO CH3 OH Cys S Cys Fe N N N N CH3 CH3 CH3 CH3 CH2CH2COO CH2CH2 Heme C (in c-type cytochromes) COO CH3 CH CH2 S CH CH CH2 CH3 CH3 CH3 CH3 COO FIGURE 19–3 Prosthetic groups of cytochromes. Each group consists of four five-membered, nitrogen-containing rings in a cyclic structure called a porphyrin. The four nitrogen atoms are coordinated with a central Fe ion, either Fe2
or Fe3
. Iron protoporphyrin IX is found in b-type cytochromes and in hemoglobin and myoglobin (see Fig. 4–17). Heme c is covalently bound to the protein of cytochrome c through thioether bonds to two Cys residues. Heme a, found in the a-type cytochromes, has a long isoprenoid tail attached to one of the five-membered rings. The conjugated double-bond system (shaded pink) of the porphyrin ring accounts for the absorption of visible light by these hemes. 100 Relative light absorption (%) 50 300 400 500 600 Wavelength (nm) Oxidized cyt c Reduced cyt c   0 FIGURE 19–4 Absorption spectra of cytochrome c (cyt c) in its oxi￾dized (red) and reduced (blue) forms. Also labeled are the character￾istic ,
, and bands of the reduced form. 8885d_c19_690-750 3/1/04 11:32 AM Page 694 mac76 mac76:385_reb: