Tohoku J. Exp. Med., 2007, 212(4)

The Mechanism of Suicide-Inactivation of Tyrosinase: A Substrate Structure Investigation

EDWARD J. LAND,¹ CHRISTOPHER A. RAMSDEN¹ and PATRICK A. RILEY²

¹Lennard-Jones Laboratories, School of Physical and Geographical Sciences, Keele University, Staffordshire, U.K.
²Totteridge Institute for Advanced Studies, London, U.K.

Tyrosinase is a copper-containing mono-oxygenase, widely distributed in nature, able to catalyze the oxidation of both phenols and catechols to the corresponding ortho-quinones. Tyrosinase is characterised by a hitherto unexplained irreversible inactivation which occurs during the oxidation of catechols. Although the corresponding catechols are formed during tyrosinase oxidation of monophenols, inactivation in the presence of monophenolic substrates is minimal. Previous studies have established the kinetic features of the inactivation reaction which is first-order in respect of the enzyme concentration. The inactivation reaction exhibits the same pH-profile and saturation properties as the oxidation reaction, classing the process as a mechanism-based suicide inactivation. The recent elucidation of the crystallographic structure of tyrosinase has stimulated a new approach to this long-standing enigma. Here we report the results of an investigation of the tyrosinase-catalysed oxidation of a range of hydroxybenzenes which establish the structural requirements associated with inactivation. We present evidence for an inactivation mechanism based on catechol hydroxylation, with loss of one of the copper atoms at the active site. The inactivation mechanism involves two linked processes occurring in situ: (a) catechol presentation resulting in α-oxidation, and (b) deprotonation of an adjacent group. On the basis of our experimental data we believe that a similar mechanism may account for the inhibitory action of resorcinols.

keywords —— tyrosinase; suicide-inactivation; catecholase; cresolase; α-oxidation; deprotonation

© 2007 Tohoku University Medical Press

===============================

Tohoku J. Exp. Med., 2007, 212, 341-348

Correspondence: Prof. P.A. Riley, Totteridge Institute for Advanced Studies, The Grange, Grange Avenue, London N20 8AB, U.K.

e-mail: rebc900@ucl.ac.uk