Radical peroxyl tyrosyl

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

In Ae absence of external reductants, MbFe(IV)=0 decays spontaneously to MbFe(I V)=0 and MbFe(III) by pathways that are poorly understood (11,14,28-32). One of the protein-based radicals of h Fe(IV)=0, a tyrosyl phenoxyl radical, has been reported to have a half-life of 45 seconds (33), and anoAer of Aese radicals, a tryptophan-centered radical, has been found to react with dioxygen to form a peroxyl radical with a half-life of 7 seconds (33-33). 

---