text: The catalytic cycle of galactose oxidase (GOase) illustrates the involvement of non-innocent ligands. GOase oxidizes primary alcohols into aldehydes using O and releasing HO. The active site of the enzyme GOase features a tyrosyl coordinated to a Cu ion. In the key steps of the catalytic cycle, a cooperative Brønsted-basic ligand-site deprotonates the alcohol, and subsequently the oxygen atom of the tyrosinyl radical abstracts a hydrogen atom from the alpha-CH functionality of the coordinated alkoxide substrate. The tyrosinyl radical participates in the catalytic cycle:  1e-oxidation is effected by the Cu(II/I) couple and the 1e oxidation is effected by the tyrosyl radical, giving an overall 2e change. The radical abstraction is fast. Anti-ferromagnetic coupling between the unpaired spins of the tyrosine radical ligand and the d Cu center gives rise to the diamagnetic ground state, consistent with synthetic models.