Tyrosinase-catalyzed oxidation of fluorophenols

The activity of the type 3 copper enzyme tyrosinase toward 2-, 3-, and 4-fluorophenol was studied by kinetic methods and H-1 and F-19 NMR spectroscopy. Whereas 3- and 4-fluorophenol react with tyrosinase to give products that undergo a rapid polymerization process, 2-fluorophenol is not reactive and actually acts as a competitive inhibitor in the enzymatic oxidation of 3,4-dihydroxyphenylalanine (L-dopa). The tyrosinase-mediated polymerization of 3- and 4-fluorophenols has been studied in detail. It proceeds through a phenolic coupling pathway in which the common reactive fluoro-quinone, produced stereospecifically by tyrosinase, eliminates an inorganic fluorine ion. The enzymatic reaction studied as a function of substrate concentration shows a prominent lag that is completely depleted in the presence Of L-dopa. The kinetic parameters of the reactions can be correlated to the electronic and steric effects of the fluorine substituent position. Whereas the fluorine electron withdrawing effect appears to control the binding of the substrates (K-m for 3- and 4-fluorophenols and K-1 for 2-fluorophenol), the k(cat) parameters do not follow the expected trend, indicating that in the transition state some additional steric effect rules the reactivity.