Cytochrome model systems

An important model system for electron transfer studies is the electron carrier cytochrome c (Cyt c). Its redox center is a heme, coordinated by a histidine and... 

Cytochromes are electron-transfer proteins having one or several haem groups. Cytochrome c binds to the protein by one or, more commonly two, thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cytochrome c has been proved to be a useful model system for studying the relationship between protein structure and thermostability due to the availability of its three-dimensional structure from a wide variety of organisms, both mesophiles and thermophiles. 

Willner I, Willner B (1991) Artificial Photosynthetic Model Systems Using Light-Induced Electron Transfer Reactions in Catalytic and Biocatalytic Assemblies. 159 153-218 Woggon W-D (1997) Cytochrome P450 Significance, Reaction Mechanisms and Active Site Analogues. 184 39 - 96... 

Extraction and purification of proteins by employing RMs have been extensively studied using model systems (proteins from commercial suppliers) such as cytochrome C, ribonuclease, a-chymotrypsin, etc. However, very few reports are available on the extraction studies of proteins from the real systems of fermentation broths using RMs [ 15,43]. As we know that fermentation broths containing proteins are very much more complicated media compared to model proteins and mixture of model proteins, there is a need to investigate extraction behavior of proteins present in these real systems. 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

An iron porphyrin is the prosthetic group in the oxygen transport and storage proteins, hemoglobin and myoglobin. Consequently there has been much interest in porphyrin complexes, especially of first row transition metals, as model systems for oxygen transport and storage. Much interest has also been shown in metal porphyrins as models for oxidases, in particular cytochrome P-450. 

Cytochrome c model systems, 847 Cytochrome P-450 reactions, 844 Cytochrome enzymes, 772 Cytochrome c oxidase copper complexes, 772 Cytochromes, 982 Cytosine... 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

By H NMR monitoring of the oxidation of benzene oxide-oxepine with dimethyldioxirane (DMDO), a significant by-product, oxepine 4,5-dioxide, was identified <1997CRT1314>. This fact supports the hypothesis that the route from oxepine to muconaldehyde proceeds via oxepine 2,3-oxide with a minor pathway leading to symmetrical oxepine 4,5-oxide. The DMDO oxidations provide model systems for the cytochrome P450-dependent metabolism of benzene and atmospheric photooxidation of benzenoid hydrocarbons. 

This leads us finally to a brief discussion of our recent work on metalloproteins, and their model systems, metalloporphyrins. Here, the basic longterm objectives are to obtain a better understanding of how CO and 02, and their isoelectronic counterparts the isocyanides (RNC) and nitrosoalkanes/nitrosoarenes (RNO), bind to Fe, as well as to probe the structure of cytochrome c, a small protein involved not only in electron-transfer, but in apoptosis, or programmed cell death (44). 

Structural model systems are difficult to construct for cytochrome P-450 because thiolates do not bind strongly to Fe (III) porphyrin units, and oxidation of the thiolate to disulfides accompanied by reduction to Fe(II) is common (Equation 12.5). A range of models for the Fe(III) resting state have been prepared using open iron (III) porphyrins with alkyl or aryl thiolates, however. 

V.B. Model Systems for Cytochrome P-450 Using Single Oxygen Atom Donors... 

Figure 12 Examples of metalloporphyrins used in model systems of cytochrome P-450. TPP, fMeso-tetraphenyIporphyri n TDCPP, /wesotetrakis-(2,6-dichlorophenyl) porphyrin TDCPCl8P, /weiotetrakis-(2,6-dichlorophenyl)-P-octachloroporphyrin.

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