Reactions with Metalloproteins

The electronic effects in energy and electron transfer reactions, including excited state systems, have been discussed in a review by Endicott. The trends observed in the rate constants for the quenching of the doublet E) excited state of [Cr(bpy)3] by a series of organochromium complexes, [Cr(H20)5R], indicate an outer-sphere electron transfer mechanism. The different reactivity patterns found for the oxidations of [(H20)Co([14]aneN4)R] complexes by [Ru(Bpy)3] and [ Cr(bpy)3] point to electron and energy transfer mechanisms, respectively. The reductive quenching of [ Cr(bpy)3] by Fe produces [Cr(bpy)3], which also quenches the excited state in the absence of added 

The rate constants for the electron transfer quenching of Eu and the cryptate complexes [ Eu([2.2.1]cryptand)] and [ Eu([2.2.1]cryptand)] 2F by [Fe(CN)6], [Ru(CN)6] , and [Os(CN)6] parallel the potentials of the reductants. The trend in for each reductant is attributed to control of the rate by water displacement in the first two cases and by electronic factors in the last reaction. The corresponding reactions with [Cr(CN)e] and the quenching of the analogous Tb species proceed by energy transfer mechanisms. 

The electron exchange rate constants [Rh(dmpe)3] / couples have been determined to be 2 x 10 and 4 x 10 M s , respectively, from the appliction of the Marcus cross-relationship to the reactions with several ruthenium(II) pentaammine complexes. The relative values are consistent with the differences in the M—P bond distance changes (Ado = 0.068 A for Tc and 0.054 A for Re) determined by EXAFS measurements. 

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. 

---

It is known that superoxide reacts very slowly with all amino-acids since all rate constants are below 100 mol l 1 s (89). Hence its reactivity with proteins without prosthetic group is low (89). One exception seems to be collagen, in which proline residues are oxidized into hydroxyproline (90). On the other hand, superoxide reacts efficiently with free radicals such as tryptophanyl radical (91). Reaction is fast with metalloproteins. It proceeds mostly by oxidizing or reducing the metal center. Some characteristics and rate constants of reactions with metalloproteins are given in table 7. It is obvious that products are often unknown and that the mechanism is sometimes unclear. It seems that there is no reaction with transferrin (92) and horseradish and lacto-peroxidase compounds II (93). The reason is unknown. 

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. 

The aim in solution studies on metalloprotein is to be able to say more about intermolecular electron transfer processes, first of all by studying outer-sphere reactions with simple inorganic complexes as redox partners. With the information (and experience) gained it is then possible to turn to protein-protein reactions, where each reactant has its own complexities... 

The problem of the definition of charge and consideration of the sizes of reactants (the diameters of the reactant ions and the activated complex are assumed equal in the derivation of (2.184)) is most acute with reactions of metalloproteins. Probably the most nsed expression for the effect of ionic strength on such reactions is ... 

The substitution process permeates the whole realm of coordination chemistry. It is frequently the first step in a redox reaction and in the dimerization or polymerization of a metal ion, the details of which in many cases are still rather scanty (e.g. for Cr(III) ). An understanding of the kinetics of substitution can be important for defining the best conditions for a preparative or analytical procedure. Substitution pervades the behavior of metal or metal-activated enzymes. The production of apoprotein (demetalloprotein and the regeneration of the protein, as well as the interaction of substrates and inhibitors with metalloproteins are important examples. ... 

Mauk, A. G., Scott, R. A., and Gray, H. B. (1980). Distances of electron transfer to and from metalloprotein redox sites in reactions with inorganic complexes. J. Am. Chem. Soc. 102,4360-4363. 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

---