Spectral and kinetic properties of the Fet3 protein from Saccharomyces cerevisiae, a multinuclear copper ferroxidase enzyme.

High affinity iron uptake in Saccharomyces cerevisiae requires Fet3p. Fet3p is proposed to facilitate iron uptake by catalyzing the oxidation of Fe(II) to Fe(III) by O2; in this model, Fe(III) is the substrate for the iron permease, encoded by FTR1. Here, a recombinant Fet3p has been produced in yeast that, lacking the C-terminal membrane-spanning domain, is secreted directly into the growth medium. Solutions of this Fet3p at >1 mg/ml have the characteristic blue color of a type 1 Cu(II)-containing protein, consistent with the sequence homology that placed this protein in the class of multinuclear copper oxidases that includes ceruloplasmin. Fet3p has an intense absorption at 607 nm (epsilon = 5500 M-1 cm-1) due to this type 1 Cu(II) and a shoulder in the near UV at 330 nm (epsilon = 5000 M-1 cm-1) characteristic of a type 3 binuclear Cu(II) cluster. The EPR spectrum of this Fet3p showed the presence of one type 1 Cu(II) and one type 2 Cu(II) (A parallel = 91 and 190 x 10(-4) cm-1, respectively). Copper analysis showed this protein to have 3.85 g atom copper/mol, consistent with the presence of one each of the three types of Cu(II) sites found in multinuclear copper oxidases. N-terminal analysis demonstrated that cleavage of a signal peptide occurred after Ala-21 in the primary translation product. Mass spectral and carbohydrate analysis of the protein following Endo H treatment indicated that the preparation was still 15% (w/w) carbohydrate, probably O-linked. Kinetic analysis of the in vitro ferroxidase reaction catalyzed by this soluble Fet3p yielded precise kinetic constants. The Km values for Fe(II) and O2 were 4.8 and 1.3 microM, respectively, while kcat values for Fe(II) and O2 turnover were 9.5 and 2.3 min-1, consistent with an Fe(II):O2 reaction stoichiometry of 4:1.