Modulating uranium binding affinity in engineered calmodulin EF-hand peptides: effect of phosphorylation

To improve our understanding of uranium toxicity, the determinants of uranyl affinity in proteins must be better characterized. In this work, we analyzed the contribution of a phosphoryl group on uranium binding affinity in a protein binding site, using the site 1 EF-hand motif of calmodulin. The recombinant domain 1 of calmodulin from A. thaliana was engineered to impair metal binding at site 2 and was used as a structured template. Threonine at position 9 of the loop was phosphorylated in vitro, using the recombinant catalytic subunit of protein kinase CK2. Hence, the T₉TKE₁₂ sequence was substituted by the CK2 recognition sequence TAAE. A tyrosine was introduced at position 7, so that uranyl and calcium binding affinities could be determined by following tyrosine fluorescence. Phosphorylation was characterized by ESI-MS spectrometry, and the phosphorylated peptide was purified to homogeneity using ion-exchange chromatography. The binding constants for uranyl were determined by competition experiments with iminodiacetate. At pH 6, phosphorylation increased the affinity for uranyl by a factor of ∼5, from K_d = 25±6 nM to K_d = 5±1 nM. The phosphorylated peptide exhibited a much larger affinity at pH 7, with a dissociation constant in the subnanomolar range (K_d = 0.25±0.06 nM). FTIR analyses showed that the phosphothreonine side chain is partly protonated at pH 6, while it is fully deprotonated at pH 7. Moreover, formation of the uranyl-peptide complex at pH 7 resulted in significant frequency shifts of the ν_as(P-O) and ν_s(P-O) IR modes of phosphothreonine, supporting its direct interaction with uranyl. Accordingly, a bathochromic shift in ν_as(UO₂)²⁺ vibration (from 923 cm⁻¹ to 908 cm⁻¹) was observed upon uranyl coordination to the phosphorylated peptide. Together, our data demonstrate that the phosphoryl group plays a determining role in uranyl binding affinity to proteins at physiological pH.

Redox chemistry of copper-amyloid-beta: the generation of hydroxyl radical in the presence of ascorbate is linked to redox-potentials and aggregation state

Aggregation of the beta-amyloid peptide (Abeta) to amyloid plaques is a key event in Alzheimer's disease. According to the amyloid-cascade hypothesis, Abeta aggregates are toxic to neurons through the production of reactive oxygen species (ROS). Copper ions play an important role, because they are able to bind to Abeta and influence its aggregation properties. Moreover, Cu-Abeta is supposed to be directly involved in ROS production. To get a better understanding of these reactions, we measured the production of HO(.) and the redox potential of Cu-Abeta. The results were compared to other biological copper-peptide complexes in order to get an insight into the biological relevance. Cu-Abeta produced more HO(.) than the complex of copper with Asp-Ala-His-Lys (Cu-DAHK), but less than with Gly-His-Lys (Cu-GHK). Cyclic voltammetry revealed that the order for reduction potential is Cu-GHK>Cu-Abeta>Cu-DAHK, but for the oxidation potential the order is reversed. Thus, easier copper redox cycling correlated to higher HO(.) production. The copper complex of the form Abeta1-42 showed a HO(.) production five-times higher than that of the form Abeta1-40. Time-dependence and aggregation studies suggest that an aggregation intermediate is responsible for this increased HO(.) production.

Structural and thermodynamical properties of CuII amyloid-beta16/28 complexes associated with Alzheimer's disease

The aggregation of the peptide amyloid-beta (Abeta) to form amyloid plaques is a key event in Alzheimer's disease. It has been shown that CuII can bind to soluble Abeta and influence its aggregation properties. Three histidines and the N-terminal amine have been proposed to be involved in its coordination. Here, for the first time, we show isothermal titration calorimetry (ITC) measurements of the CuII binding to Abeta16 and Abeta28, models of the soluble Abeta. Moreover, different spectroscopic methods were applied. The studies revealed new insights into these CuII-Abeta complexes: (1) ITC showed two CuII binding sites, with an apparent Kd of 10(-7) and 10(-5) M, respectively; (2) the high-affinity site has a smaller enthalpic contribution but a larger entropic contribution than the low-affinity binding site; (3) azide did not bind to CuII in the higher-affinity binding site, suggesting the absence of a weak, labile ligand; (4) azide could bind to the CuII in the low-affinity binding site in Abeta28 but not in Abeta16; (5) 1H-NMR suggests that the carboxylate of aspartic acid in position 1 is involved in the ligation to CuII in the high-affinity binding site; (6) the pKa of 11.3 of tyrosine in position 10 was not influenced by the binding of 2 equivalents of CuII.

Characterization of the ZnII Binding to the Peptide Amyloid-β1-16 linked to Alzheimer's Disease

Aggregation of the human peptide amyloid-beta (Abeta) is a key event in Alzheimer's disease (AD). Zinc ions play an important role in AD and in Abeta aggregation. In vitro, Zn(II) binds to Abeta and accelerates its aggregation. In this work we have investigated Zn(II) binding to the synthetic peptide Abeta1-16, which contains the metal-binding domain of Abeta. Cd(II) was used to probe the Zn(II) site. Abeta1-16 bound one equivalent of Zn(II) with an apparent dissociation constant (Kd) of 10(-4) M. This Kd value is in the same range as the Zn concentration needed to precipitate Abeta. Circular dichroism and NMR indicated predominantly random-coil secondary structures of apo-Abeta1-16, Zn(II)-Abeta1-16 and Cd(II)-Abeta1-16, which were all highly dynamic and flexible. The three histidines at positions 6, 13 and 14 were suggested to be ligands to Zn(II) and Cd(II). Evidence that the aspartate at position 1 served as a fourth ligand to Zn(II) and Cd(II) was found at pH 8.7. 111Cd(II) NMR showed a resonance at 84 ppm, in line with a mixed oxygen-/nitrogen-ligand environment. The tyrosine at position 10 could be excluded as a ligand.