Spin-labeling study on the depth of the active site of papain

Abstract Papain is alkylated with a series of haloacetamide spin labels with varying distance between the haloacetamide residue and the nitroxide portion. The dependence of τ c on the distance and temperature revealed that the active site has a narrow neck and one auxiliary binding site around the neck. The depth of the active site is estimated about 10 A.