Cytochrome redox reactions

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

Such free radicals may be stabilized by binding to proteins. Redox reactions may also occur between ionic species, for example the oxidation of reduced cytochrome c by hexacyanoferrate (ferricyanide) ions. 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

A redox reaction is a special case of the equilibrium reaction of A + B in Equation 13.1 B is now a reducible group in a biomolecule with an EPR spectrum either in its oxidized or in its reduced state (or both), and A is now an electron or a pair of electrons, that is, reducing equivalents provided by a natural redox partner (a reductive substrate, a coenzyme such as NADH, a protein partner such as cytochrome c), or by a chemical reductant (dithionite), or even by a solid electrode ... 

Other redox partners Co(bipy)33+ (oxidant) and Ru(NH3)s py2+ (reductant) are likewise partially blocked by Pt(NH3)6 +. Interestingly the reaction of cytochrome c(II) with PCu(II) is also blocked by Pt(NH3)5 +, thus identifying this as a site for electron transfer with cytochrome c. This observation is con-sis tant with a preliminary report of NMR results (19). The blocking is in fact more extensive than that observed with the above complexes, which is reasonable in view of the larger size of cytochrome c. Reaction with the negatively charged dipicol-inate oxidant, Co(dipic)2, was similarly investigated, where separate association of the oxidant with Pt(NH3)6 + can be... 

Similar results were obtained for the redox reactions of a series of cobalt diimine complexes with cytochrome c (156, 157). In general a good agreement exists between the kinetically and thermodynami-... 

The temperature dependence of ET rates between cytochrome-c and the reaction center in Chromatium (Figure 1), fitted to eqs 3-5 (8), demonstrated that, unlike many redox reactions in... 

Early attempts at observing electron transfer in metalloproteins utilized redox-active metal complexes as external partners. The reactions were usually second-order and approaches based on the Marcus expression allowed, for example, conjectures as to the character and accessibility of the metal site. xhe agreement of the observed and calculated rate constants for cytochrome c reactions for example is particularly good, even ignoring work terms. The observations of deviation from second-order kinetics ( saturation kinetics) allowed the dissection of the observed rate constant into the components, namely adduct stability and first-order electron transfer rate constant (see however Sec. 1.6.4). Now it was a little easier to comment on the possible site of attack on the proteins, particularly when a number of modifications of the proteins became available. 

M. Fabian and co-workers have studied the protein s role in internal electron transfer to the catalytic center of cytochrome c oxidase using stopped-flow kinetics. Mitochondrial cytochrome c oxidase, CcO, an enzyme that catalyzes the oxidation of ferrocytochrome c by dioxygen, is discussed more fully in Section 7.8. In the overall process, O2 is reduced to water, requiring the addition of four electrons and four protons to the enzyme s catalytic center. Electrons enter CcO from the cytosolic side, while protons enter from the matrix side of the inner mitochondrial membrane. This redox reaction. 

Gray HB, Winkler JR (1996) Electron transfer in proteins. Annu Rev Biochem 65 537 Fedurco M (2000) Redox reactions of heme-containing metalloproteins dynamic effects of self-assembled monolayers on thermodynamics and kinetics of cytochrome c electron-transfer reactions. Coord Chem Rev 209 263... 

Chart 2. Examples of chiral complexes studied in redox reactions with GO, HRP, and cytochrome c peroxidase. 

In some proteins, particularly cytochrome c (a relatively small enzyme, if still vast compared with a normal ion its molecular weight is 12,400) electron transfer occurs through the modifier to the heme group. What is surprising is the rate at which this electron transfer takes place it is about the same as that of a fast redox reaction to a simple ion in solution. With such a monster reactant, one might have expected a ponderously slow reaction. 

The final group of mitochondrial redox components are one-electron carriers, small proteins (cytochromes) that contain iron in the form of the porphyrin complex known as heme. These carriers, which are discussed in Chapter 16, exist as several chemically distinct types a, b, and c. Two or more components of each type are present in mitochondria. The complex cytochrome aa3 deserves special comment. Although cytochromes are single-electron carriers, the cytochrome aa3 complex must deliver four electrons to a single 02 molecule. This may explain why the monomeric complex contains two hemes and two copper atoms which are also able to undergo redox reactions.1 2... 

There are more complex examples of metal ion catalysis. Cobalt in vitamin B12 reactions forms covalent bonds with carbons of substrates.41,42 Metals can also act as electron conduits in redox reactions. For example, in cytochrome c the iron in the heme is reversibly oxidized and reduced. 

Flavins are very versatile redox coenzymes. Flavopro-teins are dehydrogenases, oxidases, and oxygenases that catalyze a variety of reactions on an equal variety of substrate types. Since these classes of enzymes do not consist exclusively of flavoproteins, it is difficult to define catalytic specificity for flavins. Biological electron acceptors and donors in flavin-mediated reactions can be two-electron acceptors, such as NAD+ or NADP+, or a variety of one-electron acceptor systems, such as cytochromes (Fe2+/ Fe3+) and quinones, and molecular oxygen is an electron acceptor for flavoprotein oxidases as well as the source of oxygen for oxygenases. The only obviously common aspect of flavin-dependent reactions is that all are redox reactions. 

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

Although electron transfer as such is not considered as catalysis, most enzymatic redox reactions require the presence of electron-transfer proteins for fast and efficiently directed electron transfer to the active sites. The ferredoxins, azurins, and cytochromes are most well known in this respect. Variations of over 15 A in distance may occur, and as a consequence, the electron-transfer rate may vary over 10 orders of magnitude [35], Exciting developments are ongoing in this field, and are highly relevant for the bioinorganic catalytic subject. 

Reaction of Cytochrome cimu with Tris(oxalato)cobalt(III) The cytochrome c protein was also used as reductant in a study of the redox reaction with tris (oxalato)cobalt(III).284 Selection of the anionic cobalt(III) species, [Conl(ox)3]3 was prompted, in part, because it was surmised that it would form a sufficiently stable precursor complex with the positively charged cyt c so that the equilibrium constant for precursor complex formation (K) would be of a magnitude that would permit it to be separated in the kinetic analysis of an intermolecular electron transfer process from the actual electron transfer kinetic step (kET).2S5 The reaction scheme for oxidation of cyt c11 may be outlined ... 

Many other redox reactions are potentially amenable to antibody catalysis. For example, the chemistry of the P-450 cytochromes, including the hydroxylation of alkanes and the epoxidation of alkenes, can be mimicked with synthetic porphyrins. Incorporation of such molecules into antibody active sites could conceivably yield new catalysts that combine the intrinsic reactivity of the cofactor with the tailored selectivity of the binding pocket. Work is just beginning in this area, but preliminary studies with porphyrin haptens have yielded some interesting results.126-130 Novel redox chemistry can also be anticipated for antibodies containing metal ions, flavins, nicotinamide analogs, and other reactive moieties. 

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