Cytochrome protein redox reactions

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 ... 

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... 

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... 

The binding of small inorganic ions such as [Fe(CN)6]3 and [Cr(en)3]3+ can be used to explore binding sites on electron-transfer proteins. Thus redox inactive [Cr(en)3]3+ inhibits the oxidation of cytochrome bs with [Co(NH3)6]3+ and other oxidizing agents, and also blocks association with cytochrome c. The reaction of cytochrome b5 with the oxidants occurs by a mechanism involving association of the reactants prior to electron transfer. The reaction may then be inhibited by redox... 

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 ... 

As shown in Figure 7.4, flavins can undergo a one-electron reduction to the semiquinone radical or a two-electron reduction to dihydroflavin. This means that flavins can act as intermediates between obligatory two-electron redox reactions involving nicotinamide nucleotides (Section 8.4.1) and obligatory one-electron reactions involving cytochromes, iron-sulfur proteins, and ubiquinone (Section 14.6). 

As imphed by equation (3), and by the location of the O2 reduction site in the structure, proton transfer across the cytochrome oxidase protein is required for function, which necessitates proton-conducting pathways for three specific purposes, that is, to transfer the four substrate protons from the A-side of the membrane into the site of O2 reduction, for uptake of the four pumped protons (per O2 reduced) that are translocated across the membrane coupled to the redox reaction, and for release of these protons to the opposite side of the membrane (exit pathway). Site-directed mutagenesis data indicated the presence of two proton transfer pathways from the A-side of the membrane toward the binuclear heme... 

Redox catalysis is the catalysis of redox reactions and constitutes a broad area of chemistry embracing biochemistry (cytochromes, iron-sulfur proteins, copper proteins, flavodoxins and quinones), photochemical processes (energy conversion), electrochemistry (modified electrodes, organic synthesis) and chemical processes (Wacker-type reactions). It has been reviewed altogether relatively recently [2]. We will essentially review here the redox catalysis by electron reservoir complexes and give a few examples of the use of ferrocenium derivatives. 

Ruthenium complexes are excellent reagents for protein modification and electron-transfer studies. Ru +-aquo complexes readily react with surface His residues on proteins to form stable derivatives [20, 21]. Low-spin pseudo-octahedral Ru-complexes exhibit small structural changes upon redox cycling between the Ru + and Ru + formal oxidation states [3, 22]. Hence, the inner-sphere barriers to electron transfer (Ai) are small. With the appropriate choice of ligand, the Ru + + reduction potential can be varied from <0.0 to >1.5 V versus NHE [23]. Ru-bpy complexes bound to Lys and Cys residues have been employed to great advantage in studies of protein-protein ET reactions. The kinetics of electron transfer in cytochrome 65/cytochrome c [24], cytochrome c/cytochrome c peroxidase [12], and cytochrome c/cytochrome c oxidase [25] complexes have been measured with the aid of laser-initiated ET from a Ru-bpy label. 

The ability of clathrochelates to form ion pairs and covalently attached complexes is utilized in biochemistry [315-321], The stereoselectivity of the redox reactions of plastocyanine and horse heart cytochrome C with several cage complexes was reported in Ref 319. Studies on stereoselective electron transfer in different systems provide information on the importance of close ion pair association of a cage complex with protein in chiral discrimination. 

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