Galactose oxidase is a copper containing enzyme which oxidises primary alcohols to aldehydes by the reduction of molecular oxygen to hydrogen peroxide. The enzyme catalyses the oxidation of many substrates, including dihydroxyacetone, small molecules and olysaccharides by use of a protein radical cofactor, a novel metalloradical complex, formed by a unique structural feature at the copper site with a novel thioether bond linking Cys 228 and Tyr 272 formed from a post-translational modification, which is in a stacking interaction with Trp 290. Galactose oxidase is remarkable in the extent to which free radicals are involved in all aspects of the enzyme function: serving as a key feature of the active site structure, defining the characteristic reactivity of the complex, and directing the biogenesis of the Tyr-Cys cofactor during protein maturation.