The edge edge separation of the two hemes is 20 a the

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The edge-edge separation of the two hemes is 20 A. The ZnP + cation radical produced in the k f step is a powerful oxidant; back electron transfer (k b ) will thus occur and regenerate the starting material. The reactions shown in Equation (6.32) have been investigated in mixed- metal [Zn, Fe] hemoglobins. 123 - 125 A hemoglobin molecule can be viewed as two independent electron-transfer complexes, each consisting of an al-f32 sub- unit pair (Figure 6.31), since the al-a2, f31-f32, and al-f31 distances are prohib- itively long (> 30 A). Both [a(Zn), f3(Fe)] and [a(Fe), f3(Zn)] hybrids have been studied. The ZnP and FeP are nearly parallel, as in the cytochrome b 5 -cytochrome c model com- plex. Long-range electron transfer eZnP* ~ Fe3+) between the al and f32 sub- units has been observed (the heme-edge/heme-edge distance is ~20 A). The driving force for the forward electron-transfer step is ~0.8 eV, and k f (see Equation 6.32) is ~ 100 S -I at room temperature, but decreases to ~9 s -I in the low-temperature region (Figure 6.32). Below 140-160 K the vibrations that induce electron transfer "freeze out"; nuclear tunneling is usually associated with such slow, temperature-independent rates. A complete analysis of the full temperature dependence of the rate requires a quantum-mechanical treatment 126,127 of A. i rather than that employed in the Marcus theory. It is interesting to note that the heme b vinyl groups (see Figure 6.6) for a given [al(Fe), f3iZn)] hybrid point toward each other and appear 125 to facilitate electron transfer. 351
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. ,jI ••••• ~~. ~ ~ -. ~ .. ..... ~, C~ ·1 .. ~~ 352 6 I ELECTRON TRANSFER 120 c- 90 c- 'I !:!'.- CD 60 c- 30 c- I ~·1Ii I ""Ii", 100 150 - I 200 temperature (K) I 250 I 300 Figure 6.32 Temperature dependence of the forward electron-transfer rate, k f , for [a(Zn), I3(Fe lll H z O»); adapted from Reference 124. D. Cytochrome c Cytochrome c occupies a prominent place in the mitochondrial electron-trans- port chain. Its water solubility, low molecular weight (12.4 kDa), stability, and ease of purification have allowed many experiments, which, when taken to- gether, present a detailed picture of the structure and biological function of this electron carrier. 128-133 X-ray structures 134 of oxidized and reduced tuna cytochrome c are very similar; most of the differences are confined to changes in the orientations of the side chains of some surface-exposed amino acids and sub-Angstrom adjust- ments of some groups in the protein interior. Upon reduction, the heme active site becomes slightly more ordered (Figure 6.33). Two-dimensional NMR studies 135-137 confirm this interpretation of the x-ray data, and further establish that the crystal and solution structures of cytochrome c differ in only minor respects. Cytochrome c exhibits several pH-dependent conformational states. In par- ticular, an alkaline transition with a pK a ~ 9.1 has been observed for ferricy- tochrome c. This transition is believed to be associated with the dissociation of Met-80; the reduction potential decreases dramatically, 138 and the 695-nm ab- sorption band, associated with a sulfur ...... ,. iron charge-transfer transition, dis- appears. The 2H NMR resonance due to CZH 3
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