Copper-peptide complex structure and reactivity when found in conserved His-X(aa)-His sequences

Abstract

Oxygen-activating copper proteins may possess His-X(aa)-His chelating sequences at their active sites and additionally exhibit imidiazole group δN vs εN tautomeric preferences. As shown here, such variations strongly affect copper ion's coordination geometry, redox behavior, and oxidative reactivity. Copper(I) complexes bound to either δ-HGH or ε-HGH tripeptides were synthesized and characterized. Structural investigations using X-ray absorption spectroscopy, density functional theory calculations, and solution conductivity measurements reveal that δ-HGH forms the Cu(I) dimer complex [{Cu(I)(δ-HGH)}2](2+) (1) while ε-HGH binds Cu(I) to give the monomeric complex [Cu(I)(ε-HGH)](+) (2). Only 2 exhibits any reactivity, forming a strong CO adduct, [Cu(I)(ε-HGH)(CO)](+), with properties closely matching those of the copper monooxygenase PHM. Also, 2 is reactive toward O2 or H2O2, giving a new type of O2-adduct or Cu(II)-OOH complex, respectively.

Figure 1

His-X_aa-His sequences present in the copper active site of (a) PHM (PDB entry 1SDW), (b) SOD5 (4N3T), and (c) MCO (1ZPU).

Chart 1

Figure 2

EXAFS and XANES spectroscopic data for (a) [{Cu^I(δ-HGH)}₂]²⁺ (1) [data (black), fit (red)] and (b) [Cu^I(ε-HGH)]⁺ (2) [data (blue), fit (red)]. Overlapped spectra for comparison are shown in the Supporting Information.

Figure 3

DFT-optimized geometries (RB3LYP/6-311G**) for (a) dimer 1 and (b) mononuclear 2 (∠εN–Cu−εN = 160.2°). The calculations were carried out with the protecting groups on the non-copper-coordinating N_His atom and N-/C-termini replaced with H atoms.

Figure 4

(a) UV–vis spectra of [Cu^II(ε-HGH)(OOH)]⁺ generated by addition of 1.5 equiv of H₂O₂ to a 3.5 mM solution of 2 in acetone at 193 K. (b) EPR spectrum of [Cu^II(ε-HGH)(OOH)]⁺ at 77 K (g_∥ = 2.25, g_⊥ = 2.05, A_∥ = 192 G, A_⊥ = 15 G).

Scheme 1