Spectroscopic and Theoretical Study of Cu^I Binding to His111 in the Human Prion Protein Fragment 106–115

The ability of the cellular prion protein (PrP^C) to bind copper in vivo points to a physiological role for PrP^C in copper transport. Six copper binding sites have been identified in the nonstructured N-terminal region of human PrP^C. Among these sites, the His111 site is unique in that it contains a MKHM motif that would confer interesting Cu^I and Cu^II binding properties. We have evaluated Cu^I coordination to the PrP(106–115) fragment of the human PrP protein, using NMR and X-ray absorption spectroscopies and electronic structure calculations. We find that Met109 and Met112 play an important role in anchoring this metal ion. Cu^I coordination to His111 is pH-dependent: at pH >8, 2N1O1S species are formed with one Met ligand; in the range of pH 5–8, both methionine (Met) residues bind to Cu^I, forming a 1N1O2S species, where N is from His111 and O is from a backbone carbonyl or a water molecule; at pH <5, only the two Met residues remain coordinated. Thus, even upon drastic changes in the chemical environment, such as those occurring during endocytosis of PrP^C (decreased pH and a reducing potential), the two Met residues in the MKHM motif enable PrP^C to maintain the bound Cu^I ions, consistent with a copper transport function for this protein. We also find that the physiologically relevant Cu^I-1N1O2S species activates dioxygen via an inner-sphere mechanism, likely involving the formation of a copper­(II) superoxide complex. In this process, the Met residues are partially oxidized to sulfoxide; this ability to scavenge superoxide may play a role in the proposed antioxidant properties of PrP^C. This study provides further insight into the Cu^I coordination properties of His111 in human PrP^C and the molecular mechanism of oxygen activation by this site