title: Probing kinase activities by electrochemistry, contact-angle measurements, and molecular-force interactions.
abstract: Three different methods to investigate the activity of a protein kinase (casein kinase, CK2) are described. The phosphorylation of the sequence-specific peptide (1) by CK2 was monitored by electrochemical impedance spectroscopy (EIS). Phosphorylation of the peptide monolayer assembled on a Au electrode yields a negatively charged surface that electrostatically repels the negatively charged redox label [Fe(CN)6]3-/4-, thus increasing the interfacial electron-transfer resistance. The phosphorylation process by CK2 is further amplified by the association of the anti-phosphorylated peptide antibody to the monolayer. Binding of the antibody insulates the electrode surface, thus increasing the interfacial electron-transfer resistance in the presence of the redox label. This method enabled the quantitative analysis of the concentration of CK2 with a detection limit of ten units. The second method employed involved contact-angle measurements. Although the peptide 1-functionalized electrode revealed a contact angle of 67.5 degrees , phosphorylation of the peptide yielded a surface with enhanced hydrophilicity, 36.8 degrees. The biocatalyzed cleavage of the phosphate units with alkaline phosphatase regenerates the hydrophobic peptide monolayer, contact angle 55.3 degrees . The third method to characterize the CK2 system involved chemical force measurements between the phosphorylated peptide monolayer associated with the Au surface and a Au tip functionalized with the anti-phosphorylated peptide antibody. Although no significant rupture forces existed between the modified tip and the 1-functionalized surface (6+/-2 pN), significant rupture forces (multiples of 120+/-20 pN) were observed between the phosphorylated monolayer-modified surface and the antibody-functionalized tip. This rupture force is attributed to the dissociation of a simple binding event between the phosphorylated peptide and the fluorescent antibody (Fab) binding region.
authors.affiliation: Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel.
authors.forename: Ofer I
authors.initials: OI
authors.lastname: Wilner
authors.forename: Claudio
authors.initials: C
authors.lastname: Guidotti
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Wieckowska
authors.forename: Ron
authors.initials: R
authors.lastname: Gill
authors.forename: Itamar
authors.initials: I
authors.lastname: Willner
mesh_terms: D047390:Casein Kinase II / Q000737:chemistry* / Q000378:metabolism*; D004563:Electrochemistry; D015394:Molecular Structure; D010770:Phosphotransferases; D013057:Spectrum Analysis
chemical_list: D010770:Phosphotransferases; D047390:Casein Kinase II