Cu(II) ion coordination to SPARC: a model study on short peptide fragments

SPARC (secreted protein, acidic and rich in cysteine) is a glycoprotein of the extracellular matrix that mediates the cell-matrix interactions. It plays also a role in angiogenesis, tumorigenesis, caractogenesis and wound healing. The human SPARC consists of three distinct modules. Module II is follistatin-like and its hydrolysis gives rise to a number of oligopeptides that can regulate angiogenesis in vivo and the biological activity of which has been related to their association with endogenous or exogenous copper ion. In order to completely understand the biological role of metal complexes formed by SPARC and its fragments, more information is needed on their stoichiometry, stability and structure in solution. In the present paper a potentiometric and spectroscopic investigation on Cu(II) complexes with the three SPARC(122-126), SPARC(121-126) and SPARC(120-126) fragments, protected at both their amino and carboxylic ends, is reported. These peptides (Ac-HKLHL-NH2, Ac-GHKLHL-NH2 and Ac-KGHKLHL-NH2, respectively) constitute good models for the strong copper-binding site of the protein. The behaviour of the three ligands is very similar: complex formation is started by the two His residues, subsequently involving up to three amido nitrogens, as pH increases. The coordination of the two histydyl imidazoles promotes amide ionization in the physiological pH range and this can explain SPARC binding to the Cu(II) ion.