NOVEL THIOETHER BOND REVEALED BY A 1.7-A CRYSTAL-STRUCTURE OF GALACTOSE-OXIDASE

GALACTOSE oxidase is an extracellular enzyme secreted by the fungus Dactylium dendroides. It is monomeric, with a relative molecular mass of 68,000, catalyses the stereospecific oxidation of a broad range of primary alcohol substrates and possesses a unique mononuclear copper site essential for catalysing a two-electron transfer reaction during the oxidation of primary alcohols to corresponding aldehydes 1. Recent evidence 2 arguing against a Cu(III)-Cu(I) couple 3 implies the existence of a second redox-active site proposed to involve pyrroloquinoline quinone 4 or a tyrosine radical 5,6. We now report the crystal structure of galactose oxidase at 1.7 angstrom resolution. This reveals a unique structural feature at the copper site with a novel thioether bond linking Cys 228 and Tyr 272 in a stacking interaction with Trp 290. We propose that these molecular components stabilize the protein free-radical species essential for catalysis and thus provide a 'built-in' secondary cofactor. This feature may represent a new mechanism for mediating electron transfer in metalloenzymes in the absence of exogenous cofactors.