Ternary Cu2+ Complexes of Human Serum Albumin and Glycyl-l-histidyl-l-lysine.

Human serum albumin (HSA) and the growth factor glycyl-l-histidyl-l-lysine (GHK) bind Cu2+ as part of their normal functions. GHK is found at its highest concentration in the albumin-rich fraction of plasma, leading to speculation that HSA and GHK form a ternary Cu2+ complex. Although preliminary evidence was presented 40 years ago, the structure and stability of such a complex have remained elusive. Here, we show that two ternary Cu(GHK)NImHSA complexes are formed between GHK and the imino nitrogen (NIm) of His side chains of HSA. We identified His3 as one site of ternary complex formation (conditional binding constant cKCu(GHK)NImHis3Cu(GHK) = 2900 M-1 at pH 7.4), with the second site (cKCu(GHK)NImHisXCu(GHK) = 1700 M-1) likely being supplied by either His128 or His510. Together with the established role of HSA as a molecular shuttle in the blood, these complexes may aid the transport of the exchangeable Cu2+ pool and the functional form of GHK.