Spectroscopic identification of ferryl haem proteins

Various haem proteins such as horseradish peroxidase, cytochrome-c oxidase and cytochrome peroxidase have been shown to be oxidizable to states which correspond formally to Fe(IV) (Compound II) and Fe(V) (Compound I) oxidation levels (Yamada and Yamazaki, 1974  

Witt and Chan, 1987 Rao et al., 1988) described as ferryl states. The Fe(IV) state has been designated as an iron-oxo complex (Fe02+) the Fe(V) state is a similar complex, the extra electron being lost from the porphyrin jr-system (P +-Fe02+) (Felton et al., 1976 Rakshit et al., 1976). 

It has been proposed that the second oxidizing equivalent is accepted by tyrosine-103 on the surface of the protein (Ortiz de Montel-lano, 1983) forming a tyrosine phenoxyl radical which rapidly takes up oxygen forming the peroxyl species (Davies, 1990) as depicted in Fig. 4.4. 

It is important to bear in mind that ferrylmyoglobin has two forms the radical species as described above, and the non-radical iron-oxo ferryl form in the iron(IV) state. 

The former can be detected by ESR spectroscopy (Fig. 4.5), by spin-trapping with DMPO or by stopped-flow analysis (Chapter 1). 

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