Effects of metal ion binding on structural dynamics of human hemopexin

Hemopexin (Hx) functions as a major heme scavenging protein in blood plasma and as such circulates without heme bound. In recent work, we have demonstrated that Hx binds metal ions in vitro in a manner that varies from one metal ion to another and that changes with heme binding. The structural consequences of metal ion binding to the form of Hx that dominates in plasma have now been evaluated by monitoring metal ion-linked changes in tertiary structure of the protein as reflected by changes in the near-UV CD spectrum and the ultraviolet absorption spectrum as a function of temperature. As part of this analysis we have developed thermally induced difference absorption maps (TIDAMs) to afford efficient visualization of temperature-dependent changes in the UV spectrum of Hx that are induced by binding of metal ions. The results are interpreted in terms of recent models proposed for metal ion binding sites on Hx and have implications for the possible modulation of heme binding to Hx by metal ions in vivo.