Probing the copper(II) binding features of angiogenin. Similarities and differences between a N-terminus peptide fragment and the recombinant human protein.

The angiogenin protein (hAng) is a potent angiogenic factor and its cellular activities may be affected by copper ions even if it is yet unknown how this metal ion is able to produce this effect. Among the different regions of hAng potentially able to bind copper ions, the N-terminal domain appears to be an ideal candidate. Copper(II) complexes of the peptide fragments encompassing the amino acid residues 4-17 of hAng protein were characterized by potentiometric, UV-vis, CD, and EPR spectroscopic methods. The results show that these fragments have an unusual copper(II) binding ability. At physiological pH, the prevailing complex species formed by the peptide encompassing the protein sequence 4-17 is [CuHL], in which the metal ion is bound to two imidazole and two deprotonated amide nitrogen atoms disposed in a planar equatorial arrangement. Preliminary spectroscopic (UV-vis, CD, and EPR) data obtained on the copper(II) complexes formed by the whole recombinant hAng protein, show a great similarity with those obtained for the N-terminal peptide fragments. These findings indicate that within the N-terminal domain there is one of the preferred copper(II) ions anchoring site of the whole recombinant hAng protein.