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|:: Luigi Casella ::|
New mechanistic insight into the activity of tyrosinase from enzymatic and biomimetic studies
Dipartimento di Chimica Generale, Università di Pavia, 27100 Pavia, Italy
Tyrosinase catalyzes the efficient oxidation of phenolic and catecholic substrates to quinones, 1 but important details of the mechanisms by which these reactions occur are still completely unknown. For instance, the mode of substrate binding and dioxygen activation at the dinuclear copper center are still subject to speculation. 2 We have studied the activity of tyrosinase in the temperature range between 0° and 30° C in a mixed aqueous-organic solvent to the end of obtaining the activation parameters associated with the key steps of the enzymatic reactions and the thermodynamic parameters associated with substrate binding. This investigation was extended to a family of phenolic and catecholic substrates structurally related to tyrosine. A main result is that the activation free energy of the enzymatic reactions is dominated by the enthalpic term, which occurs in the relatively narrow range of 61 ± 9 kJ mol -1, independently of substrate and reaction type (monophenolase or diphenolase). The activation entropies are small and generally negative and contribute no more than 10% to the activation free energy. The substrate binding parameters are characterized by large and negative enthalpy and entropy contributions, which are typical for polar protein-substrate interactions. The enzymatic studies have been integrated with model studies, using biomimetic dinuclear copper complexes which efficiently perform catechol oxidation or phenol hydroxylation reactions. Also for these systems the thermodynamic parameters characterizing the monophenolase and diphenolase reactions have been obtained, but also reaction intermediates could be spectroscopically characterized by low temperature experiments. The comparison between enzymatic and model studies enables to build a structural model for the course of the reactions and understand the key features which rule the enzyme reactions.
 Land, E. J.; Ramsden, C. A.; Riley P. A. Acc. Chem. Res.2003, 36, 300-308.
 H. Decker, R. Dillinger, F. Tuczek, Angew. Chem. Int. Ed.2000, 39, 1591-1595.