NEW ASPECTS OF THE INTERACTION OF COPPER (II) WITH SERUM ALBUMIN: VOLTAMMETRIC AND MICROCALORIMETRIC STUDIES

Structural organization of serum albumins - the most abundant globular proteins in serum plasma - gives rise to their extraordinary binding and functional capacity. Various classes of ligands, including the metal ions can be captured and transported by albumins. Metal binding to human serum albumin, HSA, that is an essential multipurpose target for the modern biomedicine, to its bovine equivalent, BSA, and other mammalian analogs have been extensively explored in the context of metabolism of essential metal ions, like Cu2+. Taking into account structural similarity of human and bovine serum albumins, the later was selected as a relevant model in laboratory studies due to its low cost and wide availability. In the present work metal binding properties of BSA with copper ions (Cu2+) were explored using combined voltammetric and thermodynamic examinations. According to voltammetric data, addition of equal amount of BSA (1.8x10-3)M to the solution (0.2 M KCl) containing (1.8x 10-3) M CuCl2 results that two pairs of redox peaks belonging to the Cu2+ /Cu+ (E0 = 0,16 V) and Cu+/Cu0 (E0 = -0.2 V) electronic transformations disappear and a new weak single reductive peak, at Epk=-0,55V attributable to the Cu2+/Cu+ transition is shown. BSA- Cu2+ complex formation is presumably responsible for this dramatic shift of Cu2+ reduction process to much more negative potential. The chelating environment of "N-terminal" sequence of: Asp-Thr-His- of BSA, assisted by direct participation of the sulfuric group of a Cys-34 residue, is presumably responsible for the entrapment and "locking" the copper ion, in an "abnormal", redox inactive condition (showing virtually no voltammetric activity). Our DSC data confirmed the complex formation process in the solutions containing the equal (1.8x10-3) M concentrations of both, BSA and CuCl2 and clearly shows small but distinct conformational stabilization with respect of two thermodynamic parameters, the melting temperature and melting enthalpy.