Metal-substituted haemoproteins

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. 

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Photochemical methods offer a convenient tool to study intra- and interprotein ET because of their time resolution and selectivity. Various mechanistic and design approaches based on photochemistry of metal complexes have been undertaken. Most of the studies on protein electron transfer processes have been done for hae-moproteins using among others ruthenium complex as a photosensitizer, modified haemoproteins in which haem iron is substituted by another metal (mainly Zn), and CO-bonded haem proteins [6,7],... 

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