Cytochrome metal substitution

We have already noted that extensive conformational studies suggest that the metal substituted cytochromes c (eg Zncytc, porphcytc) are essentially isostructural with Fecytc. Thus we expect that for the Zn, Fe, and H. porphyrin cyto chromes metal-porphyrin dependent variations in structure will not greatly affect the general trend observed for the dependence of rate on AG. 

The replacement of native haem with metal-substituted porphyrin can be performed in two ways. In the first, iron ions are removed from the protein by treating native protein with anhydrous HF, followed by insertion of the appropriate metal to metal-free protein [57-59]. In the second, haem is removed either chemically or by recombination (preparing a proper recombinant protein), and then protein is reconstituted with metal-substituted porphyrins [60-64]. The Zn-substituted metal-loproteins such as cytochrome c [65-67], myoglobin [59,61, 62, 64, 68, 69], and haemoglobin [64, 68,70] have been extensively used to study photoinduced ET (PET) between modified proteins and their physiological redox partners. Interestingly, in haemoglobin with a and /3 subunits it was possible to determine ET parameters for... 

A more sophisticated method involves the combination of metal-substituted proteins with electron acceptors covalently attached to the amino acid residue located at the protein surface. The [Ru(NH3)5]3+ complex attached to the histidine residue had been used in Zn-substituted myoglobin [71,72] or cytochrome b562 [73] as an electron acceptor. This design allows the distance between donor and acceptor to be fixed and is very useful for experimental analysis of intramolecular electron transfer in proteins. 

Hoffman and coworkers have employed metal-substituted CcP and cytochrome c to explore the ET kinetics between these two proteins. " Results from four-dimensional quenching studies, temperature and ionic-strength dependences, species variations, and electrostatic modeling provide compelling... 

Other modifications to cytochrome c involve metal substitutions. A particularly well-known example involves the comparison of diamagnetic Co" cytochrome c and Fe cytochrome c by NMR techniques. This showed the absence of conformational change, subsequently confirmed by the high resolution crystallographic studies. The nickel-, copper- and manganese-substituted cytochromes have all been studied. The reduced Mn cytochrome c was not oxidized by mam-... 

We are at the discovery stage for determining the ability of various bacteria to reduce metals and nonessential compounds. Mechanisms for these reductions generally have not yet been established, and it is apparent that much is unknown. A number of questions pertaining to reduction are raised Which elements and compounds are reduced at the cell surface Why are some of the compounds not reduced at the cell surface but become reduced at the plasma membrane or in the cytoplasm What is the nature of the nonenergetic reactions in the cytoplasm of the bacterial cell What are the physiologic substrates for the cytochromes and which reactions occur because of substitution of chemicals due to similar structural features ... 

As with any metalloprotein, the chemical and physical properties of the metal ion in cytochromes are determined by the both the primary and secondary coordination spheres (58-60). The primary coordination sphere has two components, the heme macrocycle and the axial ligands, which directly affect the bound metal ion. The pyrrole nitrogen donors of the heme macrocycle that are influenced by the substitutents on the heme periphery establish the base heme properties. These properties are directly modulated by the number and type of axial ligands derived from the protein amino acids. Typical heme proteins utilize histidine, methionine, tyrosinate, and cysteinate ligands to affect five or six coordination at the metal center. 

An analogous approach has been applied to study the electron transfer reaction between yeast cytochrome c peroxidase (CCP) and cytochrome c (cyt c) by employing the complex between zinc-substituted CCP (ZnCCP) and native cyt c [70]. The two heme planes in this complex are nearly parallel at a metal-metal distance of 25 A and an edge-to-edge distance of 17-18 A. 

Quantum yields have been measured for the photoaquation of a large range of substituted pyridine complexes of the type [Ru(NH3)5(pyX)]2+.53 The marked dependence of quantum yields on the nature of X indicates that metal-to-ligand-charge-transfer (MLCT) excited states are not involved in the photosubstitution. Presumably, a ligand-field excited state is responsible. Evidence has been reported for a simple outer-sphere reduction of cytochrome c by [Ru(NH3)6]2+.54 Such a... 

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