Tyrosine or Tryptophan? Modifying a Metalloradical Catalytic Site by Removal of the Cys–Tyr Cross‐Link in the Galactose 6‐Oxidase Homologue GlxA

The concerted redox action of a metal ion and an organic cofactor is a unique way to maximize the catalytic power of an enzyme. An example of such synergy is the fungal galactose 6‐oxidase, which has inspired the creation of biomimetic copper oxidation catalysts. Galactose 6‐oxidase and its bacterial homologue, GlxA, possess a metalloradical catalytic site that contains a free radical on a covalently linked Cys–Tyr and a copper atom. Such a catalytic site enables for the two‐electron oxidation of alcohols to aldehydes. When the ability to form the Cys–Tyr in GlxA is disrupted, a radical can still be formed. Surprisingly, the radical species is not the Tyr residue but rather a copper second‐coordination sphere Trp residue. This is demonstrated through the introduction of a new algorithm for Trp‐radical EPR spectra simulation. Our findings suggest a new mechanism of free‐radical transfer between aromatic residues and that the Cys–Tyr cross‐link prevents radical migration away from the catalytic site. Bindung gebrochen und immer noch radikal: Radikalische Kupfer‐Oxidasen verfügen über einen ungewöhnlichen Cys‐Tyr‐Redoxkofaktor. Dieser enthält ein stabiles Proteinradikal, das für ihre katalytische Aktivität entscheidend ist. Nach dem Bruch der Cys‐Tyr‐Bindung verbleibt immer noch ein Radikal, erstaunlicherweise aber nicht an Tyr, sondern an einem π‐gestapelten Trp‐Rest.