Development of a methodology based on metal-catalyzed oxidation reactions and mass spectrometry to determine the metal binding sites in copper metalloproteins

Efforts have been made to develop a method that uses metal-catalyzed oxidation (MCO) reactions and mass spectrometry (MS) to identify the binding site of copper in metalloproteins. This method uses MCO reactions to oxidize the amino acids in the metal-binding site and MS to identify the amino acids that have been oxidized. Several reaction conditions, including Cu(II)/ascorbate/ O₂, Cu(II)/O₂/H₂O₂, and Cu(II)/ascorbate/O₂/H₂O₂, and have been tested at varying concentrations to find the optimum conditions for specific oxidation of only the amino acids bound to copper. For small peptides, such as angiotensin I (Agt I) and [Gln11]-amyloid-β-protein fragment 1-16 (Aβ_1-16), the optimum conditions for specific modification involve the use of Cu(II)/ascorbate/O₂. For a larger protein, azurin, the speed and specificity of the MCO reactions are enhanced by the presence of a relatively high concentration of ascorbate (100 mM) and a small concentration of H₂O₂ (1 mM). Optimized reaction conditions combined with MS/MS and MSⁿ analysis on a quadrupole ion trap mass spectrometer allow the copper-binding sites to be specifically identified. For Agt I and Aβ_1-16, the amino acids bound to copper can be identified without any false positives. For azurin, four of the five amino acids bound to copper are identified with one false positive. This false positive, however, corresponds to the oxidation of Met44, which is probably due to its susceptibility to oxidation and its proximity to the only residue not identified (i.e., Gly45). The results altogether suggest that MCO reactions and MS provide a very promising approach for identifying the amino acid residues bound to copper in metalloproteins.