Fet3p is a multicopper oxidase (MCO)₂ found in Saccharomyces cerevisiae with a structure consisting of three cupredoxin-like β-barrel domains and four copper ions located in three distinct metal sites (T1 in domain 3, T2, and the binuclear T3 at the interface between domains 1 and 3).^[1][2] Fet3p is a type I membrane protein with an orientation that places the amino-terminal oxidase domain in the exocellular space (N_exo) and the carboxyl terminus in the cytoplasm (C_cyt).

Part of the ferroxidase reaction, Fet3p catalyzes the oxidation of Fe(II) to Fe(III) using O₂ as substrate. The Fe(III) generated by Fet3p is a ligand for the iron permease, Ftr1p.