Figure 2.

Schematic illustration for the label-free detection of tyrosine-kinase catalysed peptide phosphorylation. The peptides that are conjugated with a magnetic bead (MB) contain a single phosphorylation site such as tyrosine (Tyr). Since Tyr has intrinsic electro-activity, the current response from its voltammetric oxidation is monitored. Under optimized conditions, Tyr residue is phosphorylated in the presence of a tyrosine kinase and ATP. During phosphorylation, the phosphate group at the γ-position of ATP is transferred to the hydroxyl group of Tyr. The intrinsic electro-activity of Tyr is lost upon phosphorylation and the current response decays with the increasing concentration of the tyrosine kinase.