The cellular form of prion protein is a precursor of the infectious isoform, which causes fatal neurodegenerative diseases through intermolecular association. One of the characteristics of the prion protein is a high affinity for Cu(II) ions. The site of Cu(II) binding is considered to be the N-terminal region, where the octapeptide sequence PHGGGWGQ repeats 4 times in tandem. We have examined the Cu(II) binding mode of the octapeptide motif and its pH dependence by Raman and absorption spectroscopy. At neutral and basic pH, the single octapeptide PHGGGWGQ forms a 1:1 complex with Cu(II) by coordinating via the imidazole N pi atom of histidine together with two deprotonated main-chain amide nitrogens in the triglycine segment. A similar 1:1 complex is formed by each octapeptide unit in (PHGGGWGQ)2 and (PHGGGWGQ)4. Under weakly acidic conditions (pH approximately 6), however, the Cu(II)-amide- linkages are broken and the metal binding site of histidine switches from N pi to N tau to share a Cu(II) ion between two histidine residues of different peptide chains. The drastic change of the Cu(II) binding mode on going from neutral to weakly acidic conditions suggests that the micro-environmental pH in the brain cell regulates the Cu(II) affinity of the prion protein, which is supposed to undergo pH changes in the pathway from the cell surface to endosomes. The intermolecular His(N tau)-Cu(II)-His(N tau) bridge may be related to the aggregation of prion protein in the pathogenic form.

Raman spectra were measured for metal complexes of amino acid histidine and histidine‐containing peptides. The C4=C5 stretching band of the histidyl imidazole ring appears in the 1606–1594 cm‐1 region when the Nτ atom of the imidazole ring acts as a ligand of the metal coordination and the Nπ atom is protonated, whereas it appears in a lower wavenumber region, 1588–1573 cm‐1, when the metal coordination takes place via Nπ with Nτ being protonated. These wavenumbers are generally higher than those of the corresponding Nπ‐ and Nτ‐protonated tautomeric forms of metal‐free histidine. Upon N‐deuteration, the C4=C5 stretching mode shows downshifts of about 25 and 10 cm‐1 for Nτ– and Nπ–metal complexes, respectively. Raman bands at ca. 1430 and ca. 1275 cm‐1 (ca. 1350 and ca. 1265 cm‐1 for N‐deuterated histidine) gain intensity upon binding of metal to Nτ and Nπ, respectively, and they may also serve as markers of the metal coordination site. In the Raman spectrum of a 27mer zinc finger peptide that contains two histidine residues, the C4=C5 stretching band is identified at ca. 1606 cm‐1 in H2O solution and an intense band is observed at 1342 cm‐1 in D2O solution. These observations are consistent with the zinc coordination by the histidyl Nτ atom in the zinc finger. The histidyl Raman marker bands of metal coordination found here may be useful in analyzing metal–histidine interactions in peptides and proteins. © 1998 John Wiley & Sons, Ltd.

Prion diseases share a common feature in that the normal cellular prion .protein (PrP c) converts to a proteaseresistant isoform PrP ~c. The c¢-helix-rich C-terminal half of PrP c is partly converted into []-sheet in PrP so. We have examined by Raman spectroscopy the structure of an octapeptide PHGGGWGQ that appears in the N-terminal region of PrP c and a longer peptide containing the octapeptide region. The peptides do not assume any regular structure without divalent metal ions, whereas Cu(II) binding to the HGGG segment induces formation of co-helical structure on the C-terminal side of the peptide chain. The N-terminal octapeptide of prion protein may be a novel structural motif that acts as a promoter of c~-helix formation.