A Deeper Insight in Metal Binding to the hCtr1 N-terminus Fragment: Affinity, Speciation and Binding Mode of Binuclear Cu(2+) and Mononuclear Ag(+) Complex Species

Ctr1 regulates copper uptake and its intracellular distribution. The first 14 amino acid sequence of the Ctr1 ectodomain Ctr1((1-14)) encompasses the characteristic Amino Terminal Cu(2+) and Ni(2+) binding motif (ATCUN) as well as the bis-His binding motif (His5 and His6). We report a combined thermodynamic and spectroscopic (UV-vis, CD, EPR) study dealing with the formation of Cu(2+) homobinuclear complexes with Ctr1((1-14)), the percentage of which is not negligible even in the presence of a small Cu(2+) excess and clearly prevails at a M/L ratio of 1.9. Ascorbate fails to reduce Cu(2+) when bound to the ATCUN motif, while it reduces Cu(2+) when bound to the His5-His6 motif involved in the formation of binuclear species. The histidine diade characterizes the second binding site and is thought to be responsible for ascorbate oxidation. Binding constants and speciation of Ag(+) complexes with Ctr1((1-14)), which are assumed to mimic Cu(+) interaction with N-terminus of Ctr1((1-14)), were also determined. A preliminary immunoblot assay evidences that the anti-Ctr1 extracellular antibody recognizes Ctr1((1-14)) in a different way from the longer Ctr1((1-25)) that encompasses a second His and Met rich domain.