Comparison of Cupric Ion-binding Sites in Myoglobin Derivatives and Serum Albumin.

Investigations of the interaction of cupric and zinc ions with sperm whale metmyoglobin (1) suggest that these ions interact at neutral pH with imidazole side chains and induce a reversible molecular denaturation not unlike that caused by acid (2). The binding affinity of sperm whale metmyoglobin for Cu(I1) ions and the number of protons released per mole of bound metal ion, moreover, suggest that each of the four strongest Cu(I1) ion-binding sites is chelate in nature and most probably involves both an imidazole and a peptide bond at pH values as low as 5. The pattern of binding to the metmyoglobin is best interpreted in terms of preferential binding by the denatured protein. In contrast, binding of Zn(I1) ions by serum albumin at neutral pH has been shown to occur through simple complex formation with imidazole side chains (3), and both by analogy to Zn(I1) ions and from polarographic and binding studies (4, 5), the imidazoles of serum albumin have also been implicated as major binding sites of Cu(I1) ions. However, although existent Cu(I1) ion-serum albumin spectra (5, 6) seem compatible with the thesis that at neutral pH the principal mode of interaction of Cu(I1) ion with serum albumin is by simple imidazole complex formation, evidence in support of Cu(I1) ion-chelate formation with serum albumin has also been advanced (5). Accordingly, it seemed of interest to examine the relative behavior of serum albumin and myoglobin derivatives to Cu(I1) ions. In particular, a principal aim of the present study is to present further data showing that the chelate nature of the first four metmyoglobin Cu(I1) ion-binding sites is indeed manifest in the denatured protein as well as in the derivative globin, and to compare the result,ant Cu(II)-globin spectra wit,h spectra arising from the interaction of Cu(II) ion with model ligands and with bovine serum albumin.