Coordination properties of Cu(II) ions towards a phosphorylated fragment from the R1 domain of the tau protein and the effect of Ser phosphorylation on Cu(II) binding affinity.

Tau hyperphosphorylation plays a key role in Alzheimer's disease, mediating tau protein aggregation and deposition as neurofibrillary tangles in the intracellular space of neurons. The potential implications of Cu(II) ions on the disease have been studied intensively, focusing both on Cu(II)-amyloid and on Cu(II)-tau fragment interactions. Nevertheless, there is still a lack of information concerning the metal binding properties and the affinity of phosphorylated fragments of tau. In this work, the coordination properties of Cu(II) ions toward the peptides Ac-GSTENLKH-NH2 (R1) and Ac-GS(P)TENLKH-NH2 (R1P) (Ser phosphorylated analogue) were studied using potentiometric and spectroscopic (vis-EPR) methods. The above sequence belongs to the 261-268 segment of the R1 pseudorepeat of the microtubule-binding domain of the longest tau isoform. It includes both the required metal anchor site (His268) and the residue Ser262, a well-known tau phosphorylation site. There was no surprise in the coordination process of both peptides, which form 1 : 1 metal : ligand complexes and follow the already well-known binding trend (i.e., His imidazole as an anchor site followed by the deprotonation and subsequent coordination of 1-3 amide donors). The additional participation of the Glu-γ-COOH in the equatorial plane was evident for the species detected in acidic solutions, while in the pH range of 7-10, the same group occupies an axial position. The comparison of the Cu(II) binding affinities, revealed that the phosphorylated peptide (R1P) is a more effective ligand than R1 in acidic media. Above pH ∼ 7.9, the order is reversed, highlighting the role of the phosphate inter- or intra-molecular interactions in Cu(II) binding.