Cytochrome-linked electron transfer

A wide variety of different cytochrome-linked electron-transfer systems is encountered in bacteria respiratory chains with oxygen, nitrate or sulphate as electron acceptors, fumarate reductase systems and light-driven cyclic electron-transfer systems (Fig. 3). All these systems are composed of several electron-transfer carriers, the nature of which varies considerably in different organisms. Electron carriers which are most common in bacterial electron-transfer systems are flavoproteins (dehydrogenases), quinones, non-heme iron centres, cytochromes and terminal oxidases and reductases. One common feature of all electron-transfer systems is that they are tightly incorporated in the cytoplasmic membrane. Another important general property of these systems is that electron transfer results in the translocation of protons from the cytoplasm into the external medium. Electron transfer therefore... 

Volumes XII and XIII, covering Parts B and C of oxidation-reduction enzymes, will include chapters on members of the second great family of dehydrogenases those linked to flavin derivatives. Coverage will also include flavin-linked electron-transferring enzymes. Other sections will cover oxygenases and oxidases, including cytochrome oxidase. Chapters on catalase and peroxidase will complete the volumes. 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

Between 1945 and 1960 the links between succinate, its dehydrogenase, NADH, and the cytochrome chain were aggressively reexamined by Slater, Chance, and David Green s groups. Slater used the well-established inhibitor approach (CO, CN", azide to block cytochrome oxidase, and BAL, antimycin A, and amytal to stop electron transfer from succinate or NADH to cytochrome c) to show... 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

Ubiquinone is readily reduced to ubiquinol, a process requiring two protons and two electrons similarly, ubiquinol is readily oxidized back to ubiquinone. This redox process is important in oxidative phosphorylation, in that it links hydrogen transfer to electron transfer. The cytochromes are haem-containing proteins (see Box 11.4). As we have seen, haem is an iron-porphyrin complex. Alternate oxidation-reduction of the iron between Fe + (reduced form) and Fe + (oxidized form) in the various cytochromes is responsible for the latter part of the electron transport chain. The individual cytochromes vary structurally, and their classification... 

Some small peptide-heme complexes have been prepared, including an undecapeptide (residues 11-21)668"669 and an octapeptide (residues 14-21). TTiese are useful models as they include the two cysteine residues that covalently link the heme to the peptide, and one of the axial ligands. The axial Met-80 residue is absent, but the position can be filled by methionine or by other ligands as required.670 Work with several octapeptide complexes shows that the rates of outer-sphere electron transfer appear to be independent of the axial ligand, and faster than the reaction for cytochrome c. Other comparisons show that the orientation of the axial methionine in cytochrome c and the contacts between heme and protein are important controlling factors in the electronic structure of the heme. Aqua and hydroxo complexes of iron(III) octapeptide complexes are also useful models for studying spin equilibria in iron(III) hemoproteins.671... 

Reference has been made in Section 62.1.12 to the cytochromes in E. coli which are believed to function in electron transfer to dioxygen as the terminal acceptor, namely cytochromes o, d and a,. In addition, a number of other oxidants may be linked to the E. coli respiratory chain. 

In the photosynthetic and mitochondrial membranes the components of the transmembrane electron transport chain are not linked with covalent bonds, but fixed in a protein matrix. An example of such an arrangement of the electron transport chain in an artificial system can be found in papers by Tabushi et al. [244, 245], which deal with the dark electron transfer across the lipid membranes containing the dimers of cytochrome c3 from Desulfovibrio vulgaris. The dimer size is about 60 A, i.e. it somewhat exceeds the membrane thickness. This enables electron to move across the membrane via the cytochrome along the chain of hem fragments embedded in the protein. However, the characteristic time of the transmembrane electron transfer by this method is rather long (about 10 s). 

Furukawa Y, Matsuda F, Ishimori K, Morishima I. Investigation of the electron-transfer mechanism by cross-linking between Zn-substituted myoglobin and cytochrome bs. J Am Chem Soc 2002 124 4008-4019. 

In the binuclear haem-copper centre of cytochrome oxidases there is no cation radical formed at the active site. Instead the extra positive charge is held by the copper atom as it converts from cuprous (Cu1+) to cupric (Cu2+). In fact there is growing evidence to support the model of Mitchell [56] that it is the protonation steps associated with oxidation state changes in this copper atom (Cub) that provide the link between the electron transfer and proton translocation activities of this enzyme. 

The cytochromes are the electron carrier heme proteins occurring in the mitochondrial respiratory chain.449 There are five cytochromes linking coenzymes Q (ubiquinone) and 02 in this electron transport chain (Scheme 7). Cytochromes are also involved in energy transfer in photosynthesis. The iron atom in cytochromes cycles between the Fe11 and Fe111 states, i.e. they are one-electron carriers, in contrast to CoQ and the NADH flavins they act upon which are two-electron carriers. Thus, one molecule of reduced CoQ transfer its two high potential electrons to two molecules of cytochrome b, the next member of the electron transport chain. 

Cope [224] linked the enzymic charge-transfer behavior of cytochrome oxidase conclusively to its solid-state semiconductivity based on the fact that this molecule is embedded in a fragment of the mitochondrial wall. Pelletier and Kraut [225] demonstrated the existence of cytochrome d cytochrome c peroxidase complexes. Electron transfer tunneling is said [225] to proceed along the cytochrome c peroxidase backbone. 

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