Cysteine proteases: The S sub(2)P sub(2) hydrogen bond is more important for catalysis than is the analogous S sub(1)P sub(1) bond

High hydrophobicity of the second amino acid N-terminal to the scissile bond (P sub(2) residue) is generally considered to be the major factor in the specificity of the substrates for cysteine proteases of the papain family. To examine the catalytic contribution of the S sub(2)P sub(2) hydrogen bond apparent from X-ray crystallographic studies, the kinetics of Z-Phe-Gly-OEt and its thiono derivative were compared. The results are interpreted in terms of a distorted binding of the thiono substrate.