A Cu-bis(imidazole) Substrate Intermediate Is the Catalytically Competent Center for Catechol Oxidase Activity of Copper Amyloid-β.

Chiara Bacchella,Luigi Casella,Enrico Monzani,Simone Dell’Acqua,Stefania Nicolis

doi:10.1021/acs.inorgchem.0c02243

Abstract

Interaction of copper ions with Aβ peptides alters the redox activity of the metal ion and can be associated with neurodegeneration. Many studies deal with the characterization of the copper binding mode responsible for the reactivity. Oxidation experiments of dopamine and related catechols by copper(II) complexes with the N-terminal amyloid-β peptides Aβ16 and Aβ9, and the Aβ16[H6A] and Aβ16[H13A] mutant forms, both in their free amine and N-acetylated forms show that efficient reactivity requires the oxygenation of a CuI-bis(imidazole) complex with a bound substrate. Therefore, the active intermediate for catechol oxidation differs from the proposed “in-between state” for the catalytic oxidation of ascorbate. During the catechol oxidation process, hydrogen peroxide and superoxide anion are formed but give only a minor contribution to the reaction.

Highlights

The reaction data for the catechol oxidation by the copper(II)-Aβ peptides here observed indicate that the rate determining step of the mechanism is the reaction with molecular oxygen which occurs after substrate binding to the copper(I) complex
Hydrogen peroxide has almost no effect in the first seconds of the reaction, involving catechol oxidation by copper(II), while it increases the rate of the second phase, suggesting it reacts with copper(I), perhaps by oxidizing the metal ion to the more reactive +2 form and/or forming an additional reactive CuIIhydroperoxo intermediate
The N-terminally confined intermediate is a poor catalyst of catechol oxidation because the coordination set comprising the hard O(carboxylate) and NH2 ligands makes the CuI species unsuitable for efficient binding and activation of dioxygen.[36]

Summary

Introduction

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by the deposition of amyloid-β (Aβ) into the extracellular “senile plaques”,1 and by the presence of intracellular neurofibrillary tangles of β-folded tau.[2,3] In addition to amyloidosis, the dyshomeostasis of redox-active metals promotes the fast disease progression; in particular, higher levels of extracellular labile copper compared to a normal brain have been observed in AD brains.[4,5] A strong correlation between the binding of metal ions with Aβ and the cascade of events resulting in neuronal damage was suggested.[6,7] In particular, copper redox activity seems to play an important role in this process.[8−10]Aβ peptides are generated by proteolytic cleavage of the amyloid precursor protein, the major species being those containing 40 and 42 residues.[11]. The substrate saturation behavior observed in the MC oxidation indicates that the coordination of catechol to the CuI-peptide species is required for efficient reaction with molecular oxygen.

Results

Conclusion