Ternary Coordination Complex between Human Serum Albumin, Copper (II), and L-Histidine*

Physicochemical studies of the human serum albuminCu(II)-L-histidine (HSA-Cu(II)-L-His) ternary complex at pH 7.5 indicate several interesting features. 1. The dissociation constants of the HSA-Cu(II)-L-His ternary complex and the HSA-Cu(I1) binary complex are 1.38 X 1O-22 and 6.61 x lOen, respectively. 2. The absorption spectrum of the ternary complex system has characteristics similar to those of the NHz-terminal peptide (1-24) of bovine serum albumin in the presence of Cu(I1) and L-His. The computed spectrum of HSA-Cu(II)-L-His has a X,,, at 540 nm, a shift of 15 nm toward red from that of HSA-Cu(I1) (X,,,, = 525 nm). 3. The calculated moles of protons liberated in the formation of HSA-Cu(II)-L-His from HSA and Cu(II)-L-His2 are 0.28. While this may originate from an a-amino group, the number is much less than what would be expected if a peptide nitrogen of HSA were involved in the binding. 4. Cu(I1) in the ternary complex is most likely bound to both HSA and L-His. The possible binding mode changes from all nitrogen, as shown in HSA-Cu(II), to either a mixture of nitrogen and oxygen or an additional involvement of imidazole nitrogen. 5. Equilibria, existing in the ternary complex system, suggest that the ternary complex may play an important role in the biological transport of Cu(I1).