 The Cys215 residue is essential for the enzymatic activity of PTP1B and similar cysteine residues are required for the activity of other members of the Class I PTP family.The thiolate anion form is needed for nucleophilic activity but it is susceptible to oxidation by reactive oxygen species (ROS) in the cell which would render the enzyme non-functional.This cysteine residue has been shown to oxidize under increased cellular concentrations of hydrogen peroxide (H2O2), produced in response to EGF and insulin signaling.