Comparative electron paramagnetic resonance study of radical intermediatesin turnip peroxidase isozymes

The occurrence of isozymes in plant peroxidases is poorly understood Turnip roots contain seven season-dependent isoperoxidases with distinct physicoc hemical properties. In the work presented here, multifrequency electron par amagnetic resonance spectroscopy has been used to characterize the Compound I intermediate obtained by the reaction of turnip isoperoxidases 1, 3, and 7 with hydrogen peroxide. The broad (2500 G) Compound I EPR spectrum of al l three peroxidases was consistent with the formation of an exchange-couple d oxoferryl-porphyrinyl radical species. A dramatic pH dependence of the ex change interaction of the [Fe(IV)=O por(.)+] intermediate was observed for all three isoperoxidases and for a pH range of 4.5-7.7. This result provide s substantial experimental evidence for previous proposals concerning the p rotein effect on the ferro- or antiferromagnetic character of the exchange coupling of Compound I based on model complexes. Turnip isoperoxidase 7 exh ibited an unexpected pH effect related to the nature of the Compound I radi cal. At basic pH, a narrow radical species (similar to 50 G) was formed tog ether with the porphyrinyl radical. The g anisotropy of the narrow radical Deltag = 0.0046, obtained from the high-field (190 and 285 GHz) EPR spectru m, was that expected for tyrosyl radicals. The broad g(x) edge of the Tyr(. ) spectrum centered at a low g(x) value (2.00660) strongly argues for a hyd rogen-bended tyrosyl radical in a heterogeneous microenvironment. The relat ionship between tyrosyl radical formation and the higher redox potential of turnip isozyme 7, as compared to that of isozyme I, is discussed.