The role of active site residues in the oxidant specificity of the Orp1 thiol peroxidase

► Identification of a mutation that affects reactivity of the Orp1 thiol peroxidase towards non-polar peroxides, but not hydrogen peroxide. ► Identification of two conserved amino acids that when mutated, decrease the peroxidase activity of the Orp1 thiol peroxidase. ► Formation of a cysteine sulfenic acid intermediate on the Orp1 thiol peroxidase in response to cumene and tert-butyl hydroperoxide.