A spectroscopic and voltammetric study of the pH-dependent Cu(II) coordination to the peptide GGGTH: relevance to the fifth Cu(II) site in the prion protein
Résumé
The GGGTH sequence has been proposed to be the minimal sequence involved in the binding of a fifth Cu(II) ion in addition to the octarepeat region of the prion protein (PrP) which binds four Cu(II) ions. Coordination of Cu(II) by the N- and C-protected Ac-GGGTH-NH2 pentapeptide (P5) was investigated by using potentiometric titration, electrospray ionization mass spectrometry, UV–vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and cyclic voltammetry experiments. Four different Cu(II) complexes were identified and characterized as a function of pH. The Cu(II) binding mode switches from NO3 to N4 for pH values ranging from 6.0 to 10.0. Quasi-reversible reduction of the [CuII(P5)H−2] complex formed at pH 6.7 occurs at E 1/2=0.04 V versus Ag/AgCl, whereas reversible oxidation of the [CuII(P5)H−3]− complex formed at pH 10.0 occurs at E 1/2=0.66 V versus Ag/AgCl. Comparison of our EPR data with those of the rSHaPrP(90-231) (Burns et al. in Biochemistry 42:6794–6803, 2003) strongly suggests an N3O binding mode at physiological pH for the fifth Cu(II) site in the protein.