The Sub‐picomolar Cu2+ Dissociation Constant of Human Serum Albumin

The apparent affinity of human serum albumin (HSA) for divalent copper has long been the subject of great interest, due to its presumed role as the major Cu2+‐binding ligand in blood and cerebrospinal fluid. Using a combination of electronic absorption, circular dichroism and room‐temperature electron paramagnetic resonance spectroscopies, together with potentiometric titrations, we competed the tripeptide GGH against HSA to reveal a conditional binding constant of log cKCuCu(HSA) =13.02±0.05 at pH 7.4. This rigorously determined value of the Cu2+ affinity has important implications for understanding the extracellular distribution of copper. HSA is a physiological copper carrier, yet its Cu2+ affinity has remained uncertain. Using the tripeptide GGH as an accurate reference, we have eliminated the confounding effects of ternary complex formation with buffer molecules and other competing ligands to reveal a conditional affinity of log cKCuCu(HSA) =13.02 at pH 7.4.