Untersuchungen zum Reaktionsmechanismus einer pflanzlichen Oxidase, von Monooxygenasen und von Modellverbindungen für Dioxygenasen

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The main objects of research in this proposal are the three metalloenzymes catechol oxidase, tyrosinase, and catechol dioxygenase. The aim of this project is to investigate the possible existence of radical reaction pathways for each of these three enzymes. Catechol oxidase and tyrosinase are related dinuclear copper enzymes which catalyze the oxidation of monophenols or o-diphenols to o-quinones. Our group has determined the X-ray structure of catechol oxidase from sweet potato. The structure which involves an unusual covalent cysteine-histidine bond close to the active site indicates the possibility of a radical reaction pathway. This hypothesis shall be investigated by EPR measurements of the apoenzyme as well as by other spectroscopic methods and kinetic experiments. Different tyrosinases will also be isolated and crystallized. A full single crystal X-ray structure determination is also intended in order to compare these two dicopper enzymes. The proposed mechanism for the reaction pathway of tyrosinase involving radical intermediates will be investigated in detail. Catechol dioxygenase is a non-heme iron enzyme catalyzing the oxidative cleavage of the aromatic rings of catechols. The hypothetical mechanism of this enzyme involves short-lived alkyl-peroxy radicals. From fast kinetic studies on the enzyme and on biomimetic model complexes intermediate semiquinone species shall be trapped by radicals such as NO in order to get insight into the mechanism.