Intrinsic Stoichiometric Equilibrium Constants for the Binding of Zinc(II) and Copper(II) to the High Affinity Site of Serum Albumin

Intrinsic stoichiometric equilibrium constants were determined for zinc(II) and copper(II) binding to bovine and human serum albumin. Data were obtained from equilibrium dialysis experiments. Metals were presented to apoprotein as metal chelates in order to avoid metal hydrolysis and to minimize nonspecific metal-protein interactions. Scatchard analysis of the binding data indicated that the high affinity class for both zinc and copper was comprised of one site. Results of binding experiments done at several pH values suggested that while both histidyl and carboxyl groups appear to be involved in copper binding, histidyl residues alone were sufficient for zinc binding. These amino acid residues were used in combination to model several binding sites used in the formulation of equilibria expressions from which stoichiometric constants were calculated. The log10K for bovine serum albumin were calculated to be 7.28 for Zn(II) and 11.12 for Cu(II). Those for human serum albumin were determined to be 7.53 and 11.18 for Zn(II) and Cu(II), respectively. These constants were used in equilibria to simulate speciation of metal-albumin and metal-chelator and to illustrate relative binding affinities. This comparison of binding strengths was possible only through the calculation of an intrinsic stoichiometric binding constant.