Copper-binding Properties of Bovine Serum Albumin and Its Amino-terminal Peptide Fragment

A 24-residue peptide, called the fragment, has been isolated from a peptic digest of bovine serum albumin. This peptide has the same terminal sequence, Asp-Thr-, as does bovine serum albumin, and will similarly bind a single ion of copper(II) at its terminal α-amino group. Spectral and titrimetric data are reported which suggest that the Cu(II)-binding site for both albumin and peptide is a chelate locus involving multiple nitrogenous ligands in the neutral pH range. Addition of copper(II) to the peptide decreases the slope of its titration curve in the pH range, 6 to 7, and decreases its catalytic effect on the hydrolysis of p-nitrophenyl acetate. These effects lead to the proposal that a histidyl residue occupies position 3 in the peptide chain and that the unique binding site for Cu(II) in both the Asp fragment and intact albumin is the amino-terminal sequence, Asp-Thr-His-. The bulk of the albumin molecule has relatively little influence on the properties of this site.