Metalloprotein electron transfer reactions

Measurements of the rates of oxidation-reduction reactions began in the late 1940s. A great deal of the early experimental work was carried out by inorganic chemists, and by the 1970s the reactivity patterns of many complexes had been uncovered." Chemists studying the mechanisms of metalloprotein electron-transfer reactions frequently seek parallels with the redox behavior of less-complicated inorganic complexes. 

Fedurco M. 2000. Redox reactions of heme-containing metalloproteins Dynamic effects of self-assemhled monolayers on thermod3mamics and kinetics of c)dochrome c electron-transfer reactions. Coord Chem Rev 209 263-331. 

Outer-sphere electron transfer reactions involving the [Co(NH3)6]3+/2+ couple have been thoroughly studied. A corrected [Co(NH3)6]3+/2+ self-exchange electron transfer rate (8 x 10-6M-1s-1 for the triflate salt) has also been reported,588 which is considerably faster than an earlier report. A variety of [Co(NH3)g]3+/2+ electron transfer cross reactions with simple coordination compounds,589 organic radicals,590,591 metalloproteins,592 and positronium particles (electron/ positron pairs)593 as redox partners have been reported. 

Studies on 1 1 electron-transfer reactions of metalloproteins with inorganic complexes are, in a number of cases, at a stage where the site or sites on the protein at which electron transfer occurs can be specified. 

Rate Constants and Reactivity. Electron-transfer reactions of plastocyanin (and other metalloproteins) are so efficient that only a narrow range of redox partners (having small driving force) can be employed. Rates are invariably in the stopped-flow range, Table I. Unless otherwise stated parsley plastocyanin... 

Blue copper proteins are a family of metalloproteins that have been found to play an important role in a number of electron-transfer reactions in nature. Solomon and coworkers have studied a range of blue copper enzymes in detail to produce a thorough description of how molecular and electronic structure interact to provide the function of these enzymes (26,158). 

The mechanism of the regulation of electron transfer in metalloproteins has been investigated 61) and two relevant examples have been discussed in the first one the molecular mechanism controlling the electron transfer reactions is restricted to the immediate chemical environment of the metal center (azurin), while in the second one it involves a conformational transition of the whole quaternary structure of the enzyme. The power of the kinetic approach in detecting significant intermediates was emphasized 6t>. The Cu metal complex site of azurin has a distorted tetrahedral... 

A number of transition metal complexes exhibit rich photoredox properties, which allow studies of photocleavage of DNA via guanine oxidation [10], photoinduced electron-transfer reactions in metalloproteins [11], and the use... 

Grove TZ, Kostic NM. Metalloprotein association, self-association, and dynamics governed by hydrophobic interactions simultaneous occurrence of gated and true electron-transfer reactions between cytochrome and cytochrome c6 from Chlamydomonas reinhardii. J Am Chem Soc 2003 125 10598-607. 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

H. SigelandA. Sigeleds, MetalIonsinBiologicalSystems , Vol. 27, Electron Transfer Reactions in Metalloproteins, Marcel Dekker, New York, 1991. 

Our research on the electron transfer reactions of metalloproteins is supported by the National Science Foundation, the National Institutes of Health, and the Arnold and Mabel Beckman Foundation. 

The opportunity of obtaining direct electrochemistry of cytochrome c and other metalloproteins at various electrode materials such as modified gold and pyrolytic graphite has led to numerous reports of heterogeneous electron transfer rates and mechanisms between the protein and the electrode. In all the reports, Nicholson s method (37) was employed to calculate rate constants, which were typically within the range of 10" -10 cm sec with scan rates varying between 1 and 500 mV sec This method is based on a macroscopic model of the electrode surface that assumes that mass transport of redox-active species to and from the electrode occurs via linear diffusion to a planar disk electrode and that the entire surface is uniformly electroactive, i.e., the heterogeneous electron transfer reaction can take place at any area. 

Isied, S. S. In Metal Ions in Biological Systems Electron Transfer Reactions in Metalloproteins , Sigel, H. Sigel, A., Eds. Dekker New York, 1991 Vol. 27,1-56. 

Sigcl, H., and A. Sigel, eds. Electron transfer reactions in metalloproteins. Metal Ions Biol. Syst. 27 (1991). 

These intramolecular electron transfer processes provide an opportunity to examine electron transfer within the protein environment. Addition of a reduc-tant, such as aniline, results in efficient reaction of the Ru(III) with the reduc-tant to form Ru(II), which leaves the heme iron in the reduced state. If a redox active metalloprotein is present in the solution, electron transfer between the reduced heme and the added protein can be observed. Production of reduced heme iron and removal of the Ru(III) intermediate can be accomplished within a few hundred nanoseconds, which allows the study of extremely rapid interprotein electron transfer reactions. 

Pulse radiolysis has been used to study elementary reactions of importance in photosynthesis. Early experiments provided rate constants for electron transfer reactions of carotenoid radical cations and radical anions with chlorophyll pigments.More recent experiments dealt with intramolecular electron transfer in covalently bound carotenoid-porphyrin and carotenoid-porphyrin-quinone compounds. Intramolecular electron transfer reactions within metalloproteins have been studied by various authors much of that work has been reviewed by Buxton, and more recent work has been published. Pulse radiolysis was also used to study charge migration in stacked porphyrins and phthalocyanines. Most of these studies were carried out by pulse radiolysis because this techruque allowed proper initiation of the desired processes and pemtitted determination of very high reaction rate constants. The distinct character of radiolysis to initiate reactions with the medium, in contrast with the case of photolysis, and the recent developments in pulse radiolysis techniques promise continued application of this technique for the study of porphyrins and of more complex chemical systems. 

The investigation of the kinetics and mechanisms of electron transfer reactions between metalloproteins and with inorganic redox reactants continues to be a rapidly growing field. The Proceedings of the 3rd International Conference on Bioinorganic Chemistry (1987) have been published in a special issue of Recueil des Travaux Chimiques des Pay-Bas, and include a section on metalloprotein electron transfer. The subject of long-distance electron transfer in metalloproteins has also been reviewed. 

Table 2.3. Intramolecular Electron Transfer Reactions Involving Metalloproteins at 25 °C ... 

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