Accidental ferryl states in proteins

All haem proteins that utilise ferryl intermediates can react with peroxide. These proteins are high-spin in their ferric state with an easily displaced ligand (usually water) at the sixth coordination position. The majority of non-enzymatic electron-transfer haems are low-spin with two strongly bound amino- 

Although both haemoglobin [155,231] and myoglobin produce ferryl iron and free radicals upon addition of H2O2, the latter reaction has been studied more extensively. In contrast to most peroxidases, at least two myoglobin-bound free radicals have been detected immediately after peroxide addition, both by room-temperature [142] and low-temperature EPR[137]. These decay rapidly and independently of the decay of the ferryl iron, which is stable for at least an hour. It is not necessary to add exogenous reductants to reduce the radicals it is possible that some of the electrons come from elsewhere on the protein as different, more stable, free radicals can be detected one hour subsequent to peroxide treatment [137,236]. 

With ferryl myoglobin, in contrast to peroxidases, the reactions of the protein free radicals and that of the ferryl haem can be considered as uncoupled from each other. The protein has not been designed to form a cation radical for a specific reaction therefore not only is more than one cation free radical generated, but there is no control over their subsequent reactions. A similar situation can be observed in cytochrome c peroxidase mutants that have lost tryptophan-191. A different amino-acid free radical is still formed that is less stable. Indeed, even in the presence of tryptophan-191, small amounts of other free radicals are formed [237] this is further evidence that even in enzymes it is difficult to exclusively control free radical reactions. 

Ferryl myoglobin species (either the FeIV=0 itself or the protein free radicals) are capable of catalysing lipid peroxidation in model membranes [238], erythrocytes [239] and low-density lipoproteins [240]. They can also oxidise phenols, styrene, 3-carotene and ascorbate [211], At high peroxide levels, protein cross-linking is observed, followed by iron release which can result in Fenton chemistry in vitro. However, it is difficult to see how the peroxide haem ratio can ever be high enough in vivo to initiate these reactions. 

Ferryl myoglobin has been detected indirectly in vivo by the addition of sulfide to generate sulfmyoglobin. This derivative can then be detected 

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