Fet3p is a multicopper oxidase (MCO)<sub>2</sub> found in Saccharomyces cerevisiae with a structure consisting of three cupredoxin-like ò-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). Fet3p is a type I membrane protein with an orientation that places the amino-terminal oxidase domain in the exocellular space (N<sub>exo</sub>) and the carboxyl terminus in the cytoplasm (C<sub>cyt</sub>).
Part of the ferroxidase reaction, Fet3p catalyzes the oxidation of Fe(II) to Fe(III) using O<sub>2</sub> as substrate. The Fe(III) generated by Fet3p is a ligand for the iron permease, Ftr1p.