Novel thioether bond revealed by a 1.7 Å crystal structure of galactose oxidase

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Abstract

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 aldehydes1. Recent evidence2 arguing against a Cu(III)-Cu(I) couple3 implies the existence of a second redox-active site proposed to involve pyrroloquinoline quinone4 or a tyrosine radical5,6. We now report the crystal structure of galactose oxidase at 1.7 Å 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.

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Ito, N., Phillips, S., Stevens, C. et al. Novel thioether bond revealed by a 1.7 Å crystal structure of galactose oxidase. Nature 350, 87–90 (1991) doi:10.1038/350087a0

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