The Electron Pathway to Cu(II) in Ceruloplasmin

Abstract

Abstract The reduction mechanism of type 1 Cu(II) of the human plasma copper protein, ceruloplasmin (EC 1.12.3.1), was investigated using the pulse radiolysis method. The hydrated electron, used as reducing agent, was found to decay by a bimolecular process. At the same rate, a transient absorption band, centered at 410 nm, appeared. It was followed by a slower increase of absorption due to a broad band with a maximum at 305 nm. The first band is assigned to the adduct of e-aq with the disulfide groups (RSSR- radical ion). The second band is probably due to the adduct of e-aq with an aromatic amino acid residue (histidyl). Both bands decay in a first order process at the same rate at which type 1 Cu(II) is reduced ((9 ± 1) x 102 s-1). The pathway of the reduction equivalent to this oxidation-reduction active center, may therefore be delineated at least for reduction by the very potent e-aq; it proceeds from the bimolecular encounter step, in which the electron is attached to residues on the protein surface (peptide bonds and disulfide groups) by an internal transfer to an imidazol residue and ultimately to the type 1 Cu(II) center.