Replacement of the axial copper ligand methionine with lysine in amicyanin converts it to a zinc-binding protein that no longer binds copper

► M98K mutation of the axial copper ligand of the type I site of amicyanin converts it to a Zn-binding protein. ► Anomalous difference Fourier map clearly confirms the presence of zinc and the absence of copper. ► Addition of copper to M98K apoamicyanin results in denaturation of the protein. ► The ligand set for Zn2+ at the type I site of M98K amicyanin is unique. ► A hydrogen bond to water appears to allow Lys98 to ligate Zn2+.