Intramolecular metalloproteins

Table 10. A comparison of rate constants for intramolecular electron transfer in Ru(NH3)j-modiiied electron transport metalloproteins, modifications at surface histidine present in native proteins, pH 7. Values of AE° by determined measurements on modified protein except as indicated...

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

Table 5.12 Intramolecular Electron Transfer Rate Constants in Metalloproteins at 25°C...

The ionization is the rds. The spontaneous rate is considered to be accelerated by both inter-molecular (from the buffer) and intramolecular (from His-64) proton transfer. Cadmium can replace the native zinc from metalloenzymes and Cd nmr is being used increasingly to probe metal ligation sites in metalloproteins. The cadmium product often does not show enzyme activity however. 

Chang IJ, Gray HB, Winkler JR. High-driving-force electron-transfer in metalloproteins - intramolecular oxidation of ferrocytochrome-c by Ru(2,2 -bpy)2(lm)(His-33)3+. J Am Chem Soc 1991 113 7056-7. 

The rate constants also hold a useful clue to concentration and time ranges for new investigations of intermolecular ET processes of cyt c4 in solution, from which the intramolecular rate constants may be substantiated. Voltammetry, therefore, stands forward as a powerful tool towards otherwise elusive ET dynamics in composite metalloproteins. 

Chi, Q.J., Zhang, J.D., Jensen, P.S., Nazmudtinov, R.R., and Ulstrup, J. (2008) Surface-induced intramolecular electron transfer in multi-centre redox metalloproteins the di-haem protein cytochrome q in homogeneous solution and at electrochemical surfaces. Journal of Physics Condensed Matter, 20, 374124. 

Over the past several years, we have developed a technique that has proven extremely valuable in the study of electron transfer between redox sites in metalloproteins. We have reported kinetic studies of the reaction of cytochrome c with cytochrome c peroxidase (i-3), cytochrome oxidase (4), cytochrome bs (5, 6) plastocyanin (7), and cytochrome Ci (8). In addition, we have been able to show (9,10) that intramolecular electron transfer in cytochrome bs covalently... 

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. 

The study of electron transfer (ET) in metalloproteins has taken advantage of the foundation laid by research on simple chemical systems (173,174,189). It was recognized early that parameters such as ET distance and driving force would have to be carefully controlled in order to provide experimental information that could be used in a meaningful way to evaluate theoretical models (108-110). Since it proved to be difficult to accomplish this goal through intermolecular ET studies, attention turned to measurements of intramolecular ET rates in systems in which the donor and acceptor are separated by a fixed and known distance. 

Work on intramolecular ET complexes has included investigations of intervalence spectra in binuclear complexes (29,160,178). Recent research has been focused on measurements of the distance and driving force dependence of intramolecular ET rates in donor-acceptor complexes. Concurrently, methods to monitor ET in metalloproteins and protein-protein complexes have been developed. Our aim in this chapter is to compare and contrast ET data from proteins and donor-acceptor complexes in an attempt accurately to represent the current level of understanding of ET processes as they relate to biological systems. Our emphasis will be on the potential control of ET by variation in fundamental parameters such as the ET distance, driving force, and intervening medium. 

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. 

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

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