Mechanism of hydrogen peroxide production by copper-bound amyloid beta peptide: a theoretical study

Reaction rates and mechanism of the ascorbic acid oxidation by molecular oxygen facilitated by Cu(II)-containing amyloid-beta complexes and aggregates

A forefront of the research on Alzheimer's disease (AD) is the interaction of amyloid beta (Abeta) peptides with redox metal ions (e.g., Cu(II), Fe(III), and Fe(II)) and the biological relevance of the Abeta-metal complexes to neuronal cell loss and homeostasis of essential metals and other cellular species. This work is concerned with the kinetic and mechanistic studies of the ascorbic acid oxidation reaction by molecular oxygen that is facilitated by Cu(II) complexes with Abeta(1-16), Abeta(1-42), and aggregates of Abeta(1-42). The reaction rate was found to linearly increase with the concentrations of Abeta-Cu(II) and dissolved oxygen and be invariant with high ascorbic acid concentrations. The rate constants were measured to be 117.2 +/- 15.4 and 15.8 +/- 2.8 M(-1) s(-1) at low (<100 muM) and high AA concentrations, respectively. Unlike free Cu(II), in the presence of AA, Abeta-Cu(II) complexes facilitate the reduction of oxygen by producing H(2)O(2) as a major product. Such a conclusion is drawn on the basis that the reaction stoichiometry between AA and O(2) is 1:1 when the Abeta concentration is kept at a much greater value than that of Cu(II). A mechanism is proposed for the AA oxidation in which the oxidation states of the copper center in the Abeta complex alternates between 2+ and 1+. The catalytic activity of Cu(II) toward O(2) reduction was found to decrease in the order of free Cu(II) > Abeta(1-16)-Cu(II) > Abeta(1-42)-Cu(II) > Cu(II) complexed by the Abeta oligomer/fibril mixture > Cu(II) in Abeta fibrils. The finding that Cu(II) in oligomeric and fibrous Abeta aggregates possesses considerable activity toward H(2)O(2) generation is particularly significant, since in senile plaques of AD patients the coexisting copper and Abeta aggregates have been suggested to inflict oxidative stress through the production of reactive oxygen species (ROS). Although Cu(II) bound to oligomeric and fibrous Abeta aggregates is less effective than free Cu(II) and the monomeric Abeta-Cu(II) complex in producing ROS, in vivo the Cu(II)-containing Abeta oligomers and fibrils might be more biologically relevant given their stronger association with cell membranes and the closer proximity of ROS to cell membranes.

Modelling Copper Binding to the Amyloid-β Peptide in Alzheimer

Oxidative modification due to reactive oxygen species generated by Cu2+ bound to the amyloid-β peptide may be one of the sources of neurodegeneration observed in Alzheimer’s disease. Understanding the structure and function of the copper binding site can assist in the design of effective therapeutics. This paper highlights some of the most significant recent developments in computational modelling studies of the structure of the binding site and reaction mechanisms of reactive oxygen species generation.

Neighboring amide participation in thioether oxidation: relevance to biological oxidation

To investigate neighboring amide participation in thioether oxidation, which may be relevant to brain oxidative stress accompanying beta-amyloid peptide aggregation, conformationally constrained methylthionorbornyl derivatives with amido moieties were synthesized and characterized, including an X-ray crystallographic study of one of them. Electrochemical oxidation of these compounds, studied by cyclic voltammetry, revealed that their oxidation peak potentials were less positive for those compounds in which neighboring group participation was geometrically possible. Pulse radiolysis studies provided evidence for bond formation between the amide moiety and sulfur on one-electron oxidation in cases where the moieties are juxtaposed. Furthermore, molecular constraints in spiro analogues revealed that S-O bonds are formed on one-electron oxidation. DFT calculations suggest that isomeric sigma*(SO) radicals are formed in these systems.

Ternary complexes of iron, amyloid-beta, and nitrilotriacetic acid: binding affinities, redox properties, and relevance to iron-induced oxidative stress in Alzheimer's disease

The interaction of amyloid-beta (Abeta) and redox-active metals, two important biomarkers present in the senile plaques of Alzheimer's disease (AD) brain, has been suggested to enhance the Abeta aggregation or facilitate the generation of reactive oxygen species (ROS). This study investigates the nature of the interaction between the metal-binding domain of Abeta, viz., Abeta(1-16), and the Fe(III) or Fe(II) complex with nitrilotriacetic acid (NTA). Using electrospray ionization mass spectrometry (ESI-MS), the formation of a ternary complex of Abeta(1-16), Fe(III), and NTA with a stoichiometry of 1:1:1 was identified. MS also revealed that the NTA moiety can be detached via collision-induced dissociation. The cumulative dissociation constants of both Abeta-Fe(III)-NTA and Abeta-Fe(II)-NTA complexes were deduced to be 6.3 x 10(-21) and 5.0 x 10(-12) M(2), respectively, via measurement of the fluorescence quenching of the sole tyrosine residue on Abeta upon formation of the complex. The redox properties of these two complexes were investigated by cyclic voltammetry. The redox potential of the Abeta-Fe(III)-NTA complex was found to be 0.03 V versus Ag/AgCl, which is negatively shifted by 0.54 V when compared to the redox potential of free Fe(III)/Fe(II). Despite such a large potential modulation, the redox potential of the Abeta-Fe(III)-NTA complex is still sufficiently high for a range of redox reactions with cellular species to occur. The Abeta-Fe(II)-NTA complex electrogenerated from the Abeta-Fe(III)-NTA complex was also found to catalyze the reduction of oxygen to produce H(2)O(2). These findings provide significant insight into the role of iron and Abeta in the development of AD. The binding of iron by Abeta modulates the redox potential to a level at which its redox cycling occurs. In the presence of a biological reductant (antioxidant), redox cycling of iron could disrupt the redox balance within the cellular milieu. As a consequence, not only is ROS continuously produced, but oxygen and biological reductants can also be depleted. A cascade of biological processes can therefore be affected. In addition, the strong binding affinity of Abeta toward Fe(III) and Fe(II) indicates Abeta could compete for iron against other iron-containing proteins. In particular, its strong affinity for Fe(II), which is 8 orders of magnitude stronger than that of transferrin, would greatly interfere with iron homeostasis.