The Rich Electrochemistry and Redox Reactions of the Copper Sites in the Cellular Prion Protein

Abstract

This paper reviews recent electrochemical studies of the copper complexes of prion protein (PrP) and its related peptides, and correlates their redox behavior to chemical and biologically relevant reactions. Particular emphasis is placed on the difference in redox properties between copper in the octarepeat (OR) and the non-OR domains of PrP, as well as differences between the high and low copper occupancy states in the OR domain. Several discrepancies in literature concerning these differences are discussed and reconciled. The PrP copper complexes, in comparison to copper complexes of other amyloidogenic proteins/peptides, display a more diverse and richer redox chemistry. The specific protocols and caveats that need to be considered in studying the electrochemistry and redox reactions of copper complexes of PrP, PrP-derived peptides, and other related amyloidogenic proteins are summarized.

Figure 1

Structures of OR–Cu(II) (low Cu(II) occupancy), OR–Cu(II)₄ (high Cu(II) occupancy) and non-OR–Cu(II). OR = (PHGGGWGQ)₄ and the Cu(II) binding in OR–Cu(II)₄ is equivalent to OP–Cu(II) where OP = PHGGGWGQ. The non-OR copper binding segment has a sequence of GGGTH.

Figure 2

(A) Simulated cyclic voltammograms (CVs) for a reversible reduction of O to R at 0.01 (black curve) and 0.04 V/s (red curve). Other parameters for the simulations were given in the text. (B) CV for a reversible electron transfer (black curve) and overlaid with that followed by a catalytic reaction (red curve) as shown in equation 4.

Figure 3

Cyclic voltammograms of Wt-PrP fully loaded with Cu(II) and immobilized onto a boron-doped diamond electrode acquired at (i) 0.05, (ii) 0.01, and (iii) 0.001 mV/s. The dashed line represents the background voltammogram without PrP immobilized. Panels (iv) and (v) indicate the plots (in logarithmic scale) of scan-rate dependences of the peak currents and potentials, respectively. (from Ref [16] with copyright permission from American Chemical Society).

Figure 4

Cyclic voltammograms of (A) OR–Cu(II), (B) OR–Cu(II)₄, (C) non-OR–Cu(II) and (D) free Cu(II) in N₂-saturated (black curve) and O₂-purged solutions (red curve), respectively. [Cu(II)] in all cases was 90 μM, while the OR and non-OR concentrations were 100, 25, and 100 μM in panels (A), (B) and (C), respectively. The dotted line curve in (A) corresponds to the CV of OR. The scan rate was 5 mV/s (from [18] with copyright permission from American Chemical Society).

Figure 5

Cyclic voltammograms of (A) 200 μM Aβ(1–16) (thick line), 100 μM Cu(II) (dashed curve), a mixture containing Aβ(1–16) and 200 μM Cu(II) (thin solid curve), and an O₂-saturated mixture of Aβ(1–16) and Cu(II) (dash-dot-dash curve) and (B) 200 μM α-syn and 50 μM Cu(II) (solid line curve), 100 μM α-syn(1–19) and 50 μM Cu(II) (dashed line curve), and 50 μM free Cu(II) (dotted line curve). All solutions were prepared with 5 mM phosphate buffer (pH 7.4) containing 0.1 M Na₂SO₄. The scan rate was 20 mV/s for (A) and 5 mV/s for (B) and the horizontal arrows indicate the initial scan direction. A voltammogram from a 50 μM tyrosine solution is shown in the inset of (A). A voltammogram of 100 μM α-syn only is shown in the inset of panel (B) with the vertical arrow indicating the irreversible tyrosine oxidation peak. (Adopted from Reference [39] for panel (A) and Reference [45] for panel (B) with permission from American Chemical Society).

Figure 6

Suppression of Cu(II)-catalyzed ascorbate oxidation and production of OH• by the OR peptide. Ascorbate (500 μM) was allowed to react with varying concentrations of Cu(II) (5–80 μM) in 20 mM Mes buffer (pH 7.5) in the presence (solid circles) and absence (open circles) of 20 μM OR peptide, at 25 °C for 2 min, and the concentrations of remaining ascorbate (A) were measured spectrophotometrically. OH• formed during the Cu(II)-catalyzed oxidation of ascorbate (B) were measured with coumarin-3-carboxylic acid (CCA), which forms the product 7-OHCCA whose fluorescence emission can be measured at 450 nm. Adopted from Reference [26] with copyright permission).

Figure 7

(A) Time-dependence of H₂O₂ generation in solutions of: OR–Cu(II) (black), OR–Cu(II)₄ (red), non-OR–Cu(II) (blue), Cu(II)/glutamine (green) and free Cu(II) (magenta). All solutions contained 5 μM Cu(II) and 200 μM AA and the ligand concentration was 100 μM except for the solution of OR–Cu(II)₄ wherein [OR] = 1.25 μM. (B) Variations of [H₂O₂] generated by OR–Cu(II) in solutions of different pH values: 7.4 (black), 6.5 (red), 6.0 (green) and 5.5 (blue) (from Ref [18] with copyright permission from American Chemical Society).