Does Tyrosine Protect S. coelicolor Laccase from Oxidative Degradation or Act as an Extended Catalytic Site?

We have investigated the roles of tyrosine (Tyr) and tryptophan (Trp) residues in the four-electron reduction of oxygen catalyzed by Streptomyces coelicolor laccase (SLAC). During normal enzymatic turnover in laccases, reducing equivalents are delivered to a type 1 Cu center (Cu_T1) and then are transferred over 13 Å to a trinuclear Cu site (TNC: (Cu_T3)₂Cu_T2) where O₂ reduction occurs. The TNC in SLAC is surrounded by a large cluster of Tyr and Trp residues that can provide reducing equivalents when the normal flow of electrons is disrupted. Prior studies by Canters and co-workers [J. Am. Chem. Soc. 2009, 131 (33), 11680-11682] have shown that when O₂ reacts with a reduced SLAC variant lacking the Cu_T1 center, a Tyr108^• radical near the TNC forms rapidly. We have found that the Tyr108^• radical is reduced 10 times faster than Cu_T1²⁺ by excess ascorbate, possibly because of radical transfer along Tyr/Trp chains.