Copper binding to the octarepeats of the prion protein - Affinity, specificity, folding, and cooperativity: Insights from circular dichroism

The prion protein (PrP) is a Cu2+ binding cell surface glycoprotein. There is increasing evidence that PrP functions as a copper transporter. In addition, strains of prion disease have been linked with copper binding. We present here CD spectroscopic studies of Cu2+ binding to various fragments of the octarepeat region of the prion protein. We show that glycine and L-histidine will successfully compete for all Cu2+ ions bound to the PrP octapeptide region, suggesting Cu2+ coordinates with a lower affinity for PrP than the f(M) dissociation constant reported previously. We show that each of the octarepeats do not form an isolated Cu2+ binding motif but fold up cooperatively within multiple repeats. In addition to the coordinating histidine side chain residues, we show that the glycine residues and the proline within each octarepeat are also necessary to maintain the coordination geometry. The highly conserved octarepeat region in mammals is a hexarepeat in birds that also binds copper but with different coordination geometry. Finally, in contrast to other reports, we show that Mn2+ does not bind to the octarepeat region of PrP.