Info

300 400 500 600

Wavelength (nm)

FIGURE 19-4 Absorption spectra of cytochrome c (cyt c) in its oxidized (red) and reduced (blue) forms. Also labeled are the characteristic a, p, and y bands of the reduced form.

Protein

Protein

(c)

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.

First, the standard reduction potentials of the individual electron carriers have been determined experimentally (Table 19-2). We would expect the carriers 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, however, that the order of standard reduction potentials is not necessarily the same as the order of actual reduction potentials under cellular conditions, which depend on the concentration of reduced and oxidized forms (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)

E(V)

2H+ + 2e- —> H2

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