The role of metal ions in the thermal aggregation of Bovine Serum Albumine.

In recent years, the study of the interaction between metal ions and proteins has provoked an increasing interest. One of the reasons is the involvement of metal ions in the development of some degenerative pathology, such as Alzheimer disease (AD), and Parkinson's disease. Several evidences indicate their implication in the onset of pathogenic disease: for example, the fibrils formation by beta amyloid peptide (i.e. the main component of the plaques) and the formation of soluble oligomeric aggregates, whose greater toxicity has been speculated, result to be markedly accelerated by zinc and copper ions presence. Nevertheless, has been reported that metal ions may act as inhibitors or promoters in aggregation processes in dependence on thè metal/ protein concentration. We report on thermal aggregation processes of bovine serum albumin (BSA) in presence of Cu(II) or Zn(II) ions. Aim of this work was to delineate the role of metal ions in the early stages of proteins aggregation kinetics. The growth of the aggregates was followed by Dynamic Light Scattering (DLS) measurements whereas the conformational changes occurring in the protein structure were monitored by infrared and Raman spectroscopy. Both in absence and in presence of metal ions, heating treatment induced a partial unfolding of the prevalently a-helix conformation of BSA, giving rise to a prevalent B-sheet structure. The DLS data indicated that the temperature of protein unfolding is not sensitively affected by the presence of Zn(II) or Cu(II) ions. At the contrary, oligomeric aggregates of about 20 nm was formed by native BSA and BSA-Cu(II), while bigger species (about 50 and 100 nm) was formed in presence of Zn(II) ions. The different effects played by the two types of ions were highlighted by Raman measurements. In fact, after heating, two different 3D coordination structures of metal-protein systems occurred. One is characterized by the metal coordination to the imidazole NT atom of His which can promote inter-molecular cross-linking; this manly takes piace in the Zn-BSA System. The other involves metal binding to the NT/ Nrc-histidinate anion as well as to main-chain amide nitrogens, probably leading to an intra-molecular chelation in the Cu-BSA System. This different metal coordination also induced differences in local disulfide conformation and in Tyr environment.