Binding of copper (II) ion to an Alzheimer's tau peptide as revealed by MALDI-TOF MS, CD, and NMR.
The tau protein plays an important role in some neurodegenerative diseases including Alzheimer's disease (AD). Neurofibrillary tangles (NFTs), a biological marker for AD, are aggregates of bundles of paired helical filaments (PHFs). In general, the alpha-sheet structure favors aberrant protein aggregates. However, some reports have shown that the alpha-helix structure is capable of triggering the formation of aberrant tau protein aggregates and PHFs have a high alpha-helix content. In addition, the third repeat fragment in the four-repeat microtubule-binding domain of the tau protein (residues 306-336: VQIVYKPVDLSKVTSKCGSLGNIHHKPGGGQ, according to the longest tau protein) adopts a helical structure in trifluoroethanol (TFE) and may be a self-assembly model in the tau protein. In the human brain, there is a very small quantity of copper, which performs an important function. In our study, by means of matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS), circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopy, the binding properties of copper (II) ion to the R3 peptide derived from the third repeat fragment (residues 318-335: VTSKCGSLGNIHHKPGGG) have been investigated. The results show that copper ions bind to the R3 peptide. CD spectra, ultraviolet (UV)-visible absorption spectra, and MALDI-TOF MS show pH dependence and stoichiometry of Cu2+ binding. Furthermore, CD spectra and NMR spectroscopy elucidate the copper binding sites located in the R3 peptide. Finally, CD spectra reveal that the R3 peptide adopts a mixture structure of random structures, alpha-helices, and beta-turns in aqueous solutions at physiological pH. At pH 7.5, the addition of 0.25 mol eq of Cu2+ induces the conformational change from the mixture mentioned above to a monomeric helical structure, and a beta-sheet structure forms in the presence of 1 mol eq of Cu2+. As alpha-helix and beta-sheet structures are responsible for the formation of PHFs, it is hypothesized that Cu2+ is an inducer of self-assembly of the R3 peptide and makes the R3 peptide form a structure like PHF. Hence, it is postulated that Cu2+ plays an important role in the aggregation of the R3 peptide and tau protein and that copper (II) binding may be another possible involvement in AD.
Ma
Qing-Feng
QF
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, 100084 Beijing, China.
Li
Yan-Mei
YM
Du
Jin-Tang
JT
Kanazawa
Kenji
K
Nemoto
Tadashi
T
Nakanishi
Hiroshi
H
Zhao
Yu-Fen
YF
D000544:Alzheimer Disease / Q000378:metabolism; D000595:Amino Acid Sequence; D002942:Circular Dichroism; D003300:Copper / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D019906:Nuclear Magnetic Resonance, Biomolecular; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D016875:tau Proteins
C054369:MAPT protein, human; D009419:Nerve Tissue Proteins; D010446:Peptide Fragments; D016875:tau Proteins; D003300:Copper