Electrochemical detection of protein post-translational modifications: Phosphorylation and nitration of amyloid-beta (1–16)

Abstract

Post-translational modifications (PTM) such as O-phosphorylation of tyrosine (Tyr) and serine (Ser) and 3-nitration of Tyr amino acid residues of proteinaceous molecules were successfully detected by electrochemistry with the amyloid-beta (1–16) peptide (Aβ16) as an example. The detection strategy was based on the effects of Tyr or Ser O-phosphorylation and Tyr 3-nitration on electrochemical activity of the peptide molecule. Square wave voltammograms of the oxidation and reduction of Aβ16 with the unmodified residues, O-phosphorylated Tyr-10 or Ser-8 (Aβ16-pTyr10 or Aβ16-pS8) and 3-nitrated Tyr-10 (Aβ16-nTyr10) were registered on carbon screen printed electrodes within the potential ranges of 0 to 1.7 V and −0.4 to −1.0 V (vs. Ag/AgCl), respectively. The Aβ16 Tyr-10 oxidation which proceeds via the hydroxyl group of benzene ring manifested as a peak at the potential maximum of about 0.6 V. O-phosphorylation of Aβ16 Tyr-10 dramatically reduced the peak at 0.6 V and produced a new oxidation wave at potentials of 1.1–1.7 V. The oxidation process registered at these elevated positive potentials has been attributed to oxidation of the phosphoryl group of benzene ring of Aβ16 Tyr-10. The role of benzene ring in the phosphoryl group oxidation was evaluated by comparing the electrochemical behavior of Aβ16-pTyr10 and Aβ16-pSer8 peptides. The addition of nitro group at the 3rd position of benzene ring of Tyr-10 caused a shift of the oxidation peak potential maximum from 0.6 to 0.75 V. Moreover, the reduction of nitro group of Aβ16-nTyr10 leading to the amino group formation was registered at −0.75 V. The results obtained for unmodified and modified Aβ16 peptides were confirmed by the data on electrochemical behavior of the corresponding free amino acids, viz. Tyr, nTyr, pTyr, and pSer. Each PTM studied was found to strongly influence the electrochemical properties of amino acids and peptides. Electrochemistry allowing to detect O-phosphorylation of Tyr and Ser and 3-nitration of Tyr residues of peptide molecules seems very promising for the study of these and, likely, many other PTM of interest in diverse in vitro systems.