Peroxidase protein hydroperoxide formation

Concerning the mode of formation of ES, we prefer the concept that the substrate in a monolayer is chemisorbed to the active center of the enzyme protein, just as the experimental evidence pertaining to surface catalysis by inorganic catalysts indicates that in these reactions chemisorbed, not physically adsorbed, reactants are involved. Such a concept is supported by the demonstration of spectroscopically defined unstable intermediate compounds between enzyme and substrate in the decomposition by catalase of ethyl hydroperoxide,11 and in the interaction between peroxidase and hydrogen peroxide.18 Recently Chance18 determined by direct photoelectric measurements the dissociation con-... 

Previously, we have examined the formation of amino acid hydroperoxides following exposure to different radical species [100]. We observed that valine was most easily oxidised, but leucine and lysine are also prone to this modification in free solution. Scheme 12 illustrates the mechanism for formation of valine hydroperoxide. However, tertiary structure becomes an important predictor in proteins, where the hydrophobic residues are protected from bulk aqueous radicals, and lysine hydroperoxides are most readily oxidised. Hydroperoxide yield is poor from Fenton-derived oxidants as they are rapidly broken down in the presence of metal ions [101]. Like methionine sulphoxide, hydroperoxides are also subject to repair, in this case via glutathione peroxidase. They can also be effectively reduced to hydroxides, a reaction supported by the addition of hydroxyl radical in the presence of oxygen. Extensive characterisation of the three isomeric forms of valine and leucine hydroxides has been undertaken by Fu et al. [102,103], and therefore will not be discussed further here. 

When free peroxidases or catalase react with hydroperoxides, stable green substances, called compounds I and II, are formed they have absorption spectra of yr-cation-radical type [Blumberg (7), Brill (16)]. This has been taken as evidence that stable Fem porphyrin cation radicals are formed by peroxide oxidation in these particular heme proteins [Borg (10), Dolphin (51)]. It is not known why the hydrogen peroxide does not attack the porphyrin ring, as is normal with hemoproteins, nor is it certain whether this radical formation has any biological implication. 

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