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Does Tyrosine Protect S. Coelicolor Laccase from Oxidative Degradation?

submitted on 18.07.2020 and posted on 20.07.2020 by Patrycja Kielb, Harry B. Gray, Jay R. Winkler
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 (CuT1) and then are transferred over 13 Å to a trinuclear Cu site (TNC: (CuT3)2CuT2) 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. Canters and coworkers have shown that when O2 reacts with a reduced SLAC variant lacking the CuT1 center, a Tyr108· radical near the TNC forms rapidly. We have found that ascorbate reduces the Tyr108· radical in wild-type SLAC about 10 times faster than it reacts with the CuT12+ center, possibly owing to radical transfer along a Tyr/Trp chain. Aerobic oxidation of two reduced SLAC mutants (Y108F and W132F) leads to the formation of a long-lived (~15 min) Tyr· radical with distinct absorption at 408 nm. The diffusion of redox equivalents away from the primary enzymatic pathway in SLAC may indicate a poorly optimized enzyme or a mechanism to protect against protein damage.


Electron Transfer in Iron and Copper Oxygenases and Oxidases

National Institute of Diabetes and Digestive and Kidney Diseases

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California Institute of Technology


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